LEAF LITTER DECOMPOSITION and MACROINVERTEBRATES in a NEOTROPICAL LOWLAND STREAM, Q. NEGRA, COSTA RICA

DIPLOMARBEIT

zur Erlangung des akademischen Grades Magistra rer. nat.

an der Fakultät für Lebenswissenschaften der Universität Wien

ausgeführt am

Department für Limnologie und Hydrobotanik

verfasst und eingereicht von:

Julia Tschelaut

Wien, August 2005

Danksagung

Ich danke O. Prof. Dr. Fritz Schiemer, für die wissenschaftliche Betreuung der

Arbeit, die große Unterstützung, die Zeit, das Engagement und die konstruktive Kritik

– muchissimas gracias.

Ich danke dem gesamten Team der Tropenstation La Gamba, besonders Dr. Anton Weissenhofer für seine große Unterstützung, die zur Verfügung gestellten Photos

und die Zeit, die er diesem Projekt gewidmet hat.

Meinem Kollegen Christian Pichler danke ich für die gute und produktive

Zusammenarbeit, sowie Astrid Riemerth und Maria Gusenleitner für die

vertiefenden Arbeiten und die zur Verfügung gestellten Daten.

Ich danke weiters Dr. Walter Reckendorfer, der viel zur Durchführung dieses

Projektes beigetragen hat, sowie Dr. Irene Zweimüller, die mir ebenfalls bei

statistischen Fragen geholfen hat.

Bedanken möchte ich mich bei Hubert Kraill, der die chemischen Analysen

durchgeführt hat.

Dr. Wolfgang Wanek, Prof. Dr. Roland Albert, Dr. Peter Weish und Dr. Santiago Gaviria-Melo danke ich für die Unterstützung und das Interesse an der Arbeit sowie

Dr. Wolfram Graf, Dr. Ernst Bauernfeind, Dr. Berthold Janecek, und Prof. Dr. Johann Waringer für die Hilfe bei der Bestimmung des Makrozoobenthos.

Für die schöne gemeinsame Zeit auf der Tropenstation La Gamba danke ich:

las dos Efas, Karim, Christian, Toni, Familie Walder, Nina, Mari, Victor, Eduardo,

Luis, Oli, und Angela - Pura vida!

Ein großes Dankeschön gilt meinen Eltern und Onkels für die Unterstützung und

das langjährige Sponsern meiner bisherigen „Karriere“

Ich danke besonders meinen Freunden – Anke, Kaddl, Lisa und Robsi, für ihre

offenen Ohren, die mein ständiges euphorisches Costa Rica Gequassel aushalten

und Caro, dass das Photogeschäft kein unendliches Labyrinth an Ebenen für mich

geblieben ist.

CONTENTS

page

1. INTRODUCTION 1 2. THE RIVERNETWORK in the „RAINFOREST of the AUSTRIANS“ 4 2.1. Material and Methods 9 2.2. Results 13 2.3. Discussion 41 3. QUEBRADA NEGRA 44 3.1. Material and Methods 44 3.2. Results 46 3.3. Discussion 79 4. MACROINVERTEBRATES and LEAF LITTER DECOMPOSITION in the QUEBRADA NEGRA 81 4.1. Material and Methods 84 4.2. Results 86 4.3. Discussion 96 5. MACROINVERTEBRATE DISTRIBUTION and TROPHIC RELATIONS in a NEOTROPICAL LOWLAND STREAM, QUEBRADA NEGRA, COSTA RICA 99 5.1. Material and Methods 100 5.2. Results 101 5.3. Discussion 109 6. RESEARCH NEEDS 112 LITERATURE CITED 114 PLATES 117

1

1. INTRODUCTION

..............is it not the foreign, the new, the unknown which always stimulates the

science?“ (FÜHREDER 1994)

Studies of tropical streams and rivers have a long history, beginning with early

explorers and naturalists who visited these regions and collected some aquatic

specimens e.g. von HUMBOLDT and BONPLAND (1799-1802), von SPIX and von

MARTIUS (1817-1820), BATES (1848-1859). Later, more extensive collections were

made by professional collectors such as H.S. PARISH (ALEXANDER 1959) and F.

WOYTKOWSKI (WOYTKOWSKI 1978) as well as numerous resident and visiting

scientists. There is a growing interest in the study of neotropical lotic systems and the

number of papers published each year that address tropical stream research has

increased markedly over the last two decades and the advances are clearly evident.

But still, a survey about the percentage of information about what is known of e.g.

taxonomy, life history traits and standing crop biomass, shows that much is still to be

learned about tropical streams and rivers (JACKSON, SWEENEY 1995).

Little is known about the rivers and streams of the Piedras Blancas National Park,

Costa Rica. This protected area provides an unique opportunity to study tropical

freshwater ecology and after participating on a tropical ecology field trip of the

University of Vienna in February 2003 we decided to put our focus of interest on this

topic in order to initiate on-going freshwater ecology studies in this area.

One of the primary objectives of the study was to investigate the rivernetwork within

the Rio Esquinas catchment (Fig.2.3. and 2.4.). Rivers were analyzed with regard to

abiotic parameters such as morphology, hydrology, hydrochemistry, sedimentology

and canopy cover by the riparian vegetation. This work discusses differences

between sites according to geological factors and the seasonal hydrologic

characteristics from streams within the catchment. Research was carried out at nine

study streams and rivers ranging from 1st order streams to 5th order rivers that empty

into the Golfo Dulce at the Pacific Ocean. Furthermore, the morphometric-

hydrological conditions of the Quebrada Negra, a 1st order stream, were investigated

in more detail. Streams display a more or less regular alternation between shallow

2

areas of higher velocity and mixed gravel-cobble substrate (riffles) and deeper areas

of slower velocity and finer substrate (pools). Within the morphometric-hydrological

record of the Q. Negra we identified four different habitat types within the stream run

– riffles, slow shallow areas, pools and cascades.

Analysis of the concentration dynamics and export of solutes from catchments

provides insights into major aspects of ecosystem functions. Chemical composition of

streamwater can reflect and thus reveal geologic features of the catchment. The

chemical regime of the water, particularly nutrient concentrations, influences the

productivity and community structure of the aquatic ecosystem. The relationship

between solute concentration and streamflow can help to explain the interplay among

physical, chemical and biological processes occuring within the catchment.

Stream communities in forested catchments are generally dependent on

allochthonous organic matter as a trophic base. The shading effect of riparian

vegetation effectively limits in situ primary production and inputs of reduced carbon

compounds are therefore of primary importance in the energy budgets of forest

streams (CUMMINS et al.1973, FISHER & LIKENS 1973, CUMMINS 1974). Litter fall

(throughfall) from tree canopies extending over the stream channel constitutes a

direct pathway of import for allochthonous organic matter. Rainforest streams are

shadowed to a great extent and have therefore a small primary production.

Consequently, much of the energy demand by consumers is met from allochthonous

sources. It is largely allochthonous material (leaf litter and detritus) of the forest,

which is brought into the stream and determines its nutrient budget. The

decomposition in tropical lotic systems is much faster than in running waters of higher

latitudes. The leaf litter entered into tropical streams is often determined by the

seasonality of the precipitation, which is an important difference to temperate

streams. Furthermore, the amount of leaf litter entered into streams in the tropics is

twice as much as in temperate regions. Leaves fall directly or are windblown into

streams, become wetted, and commence to leach soluble organic and inorganic

constituents. The rate of leaf breakdown is determined by intrinsic differences among

leaves, a number of environmental variables and the feeding activity of detrivores.

The processing of leaf litter in temperate streams has been the subject of numerous

studies but equivalent tropical ecosystems have received little attention, where many

leaf types are varying in their palatability and tannin levels.

3

Streams are highly heterogeneous environments in which habitat characteristics vary

drastically over small distances. Throughout this study we analysed the relationship

between taxonomical composition and functional organization of stream benthic

communities and some environmental variables in riffle and pool sites of the

Quebrada Negra. Local variations of abiotic factors, such as current velocity,

substratum composition and water depth, shape the distribution of invertebrates.

Distribution patterns of macroinvertebrates in a lotic ecosystem are determined to a

great extend by the substrate. Among the first stream ecologists to investigate the

relationship between benthic distribution patterns and the nature of the substrate

were PERCIVAL & WHITEHEAD (1929). They recognized seven basic substrate types

and found that certain animal species were consistently associated with each. Since

then a lot of ecologists have pointed out the importance of substrate types in

determining stream benthos distributions.

The decomposition and macroinvertebrate colonisation of leaf litter from four different

plant species from the riparian vegetation was investigated using litter bags placed in

the Q. Negra over a 28 day period. The plants were chosen after their frequency and

growth strategy (r- and K- strategists). The objective of the present study was to

determine the difference in decay of this four leaf types but also to determine the

taxonomical composition of the colonising macroinvertebrates.

The second aim of this study was to describe the distribution of stream invertebrates

within the riffle-pool sequences of the Q.Negra, and examin the local variations of

density, richness and functional composition in relation to selected environmental

characteristics like current velocity, water depth and composition of the substratum.

Between-site differences are discussed in relation to physical factors.

This investigation represents the first survey of the benthic invertebrate community

from four different choriotops (habitat types) within the Q. Negra.

2. The river-network in the “Rainforest of the Austrians” Adapted version of WEISSENHOFER & HUBER (2001)

This study took place close to the Biological Field Station La Gamba, located in the Bosque

Esquinas “Regenwald der Österreicher”. This forest belongs to the Piedras Blancas National

Park, Puntarenas, southwest of Costa Rica (8° 42' 46" N, 83° 12' 09" W) and covers 148 km2.

Fig. 2.1. Costa Rica and the Golfo Dulce region with the Corcovado National Park (Parque Nacional Corcovado) and the Piedras Blancas National Park ( Parque Nacional Piedras Blancas = Esquinas forest) (from WEBER & al. 2001)

4

5

GEOGRAPHY – Costa Rica is located between 8° 02' 06" - 11° 13' 12" N latitude and 82° 33'

48" - 85° 57' 57" W longitude between the Caribbean Sea and the Pacific Ocean. The country

boarders to Nicaragua in the north and Panama in the south and is part of the Central

American Isthmus. It has an area of 51 100 km2 with a varied and abrupt topography. The

maximum elevation is 3819 m above mean sea level, and much of the country´s land is

above 500 m.

Four main mountain ranges cross the country from northwest to southeast: (1) the Cordillera

de Guanacaste, (2) the Cordillera de Tilaran, (3) the Cordillera Central and (4) the Cordillera

de Talamanca. The Cordillera de Talamanca extends southwards to Panama and divides the

country into two partitions: the Caribbean and the Pacific slope, both with a distinct climate,

fauna and flora.

To the east the rivers drain into the Caribbean Sea and the western ones drain into the

Pacific Ocean. Rivers on the west coast tend to be short.

In the Esquinas forest the altitude ranges from sea level up to 579 m (Cerro Nicuesa). The

whole region is still tectonically active. Up to ten tremors per day have been measured in the

region, and crustal elevations have been observed.

CLIMATE – On the pacific side of Costa Rica a distinct 'dry' (December - March) and 'rainy'

(May - November) season exists throughout the year. The most heavy rainfalls occur in

September, October and November. February and March are the driest months, sometimes it

does not rain within days. During the drier period of the year some trees drop their leaves

completely and a considerable amount of leaf litter accumulates on the forest floor. High

rainfall starts in April and tends to fall mainly in intense cloudbursts in the afternoon. The

Golfo Dulce region is one of the most humid areas in Costa Rica with more than 6000 mm

precipitation per year. The Esquinas lowland forest and its vicinity are influenced by the rain

gradient caused by the mountains of the Fila Cruces range and the adjacent Talamanca

Mountains. ALLAN (1956) noted the outstandingly high rainfall in the Esquinas region.

Since 1998 meterological data has been recorded at the Field Station La Gamba and

complete data sets for precipitation are available for the years 1999 to 2004. Average annual

precipitation at the field station is about 6200 mm.

The climate is also marked by a high mean annual temperature (27,8°C). The average

minimum temperature is about 23,5°C and the average maximum temperature 32,0 °C. A

constantly high relative humidity, averaging 88,3 % at the station and 97,7 % inside the forest

exists throughout the year.

27,9° 5816

23,220,0

38,533,7

Fig. 2.2. Climatic diagram of the Field Station La Gamba

VEGETATION – Tropical forests are complex ecosystems which cover less than 10 % of the

world’s land surface, and yet contain considerably more than half of the world’s living species

(WILSON 1988). Tropical rainforests are distinguished from all other terrestrial ecosystems by

a very high diversity on many levels (species, habitats, life-forms, etc.).

Costa Rica has 12 major life zones (HOLDRIDGE), based on analyzing combinations of

temperature, rainfall and seasonality. Each zone has a distinctive natural vegetation ranging

from tidal mangrove swamps to subalpine paramó.

The rainforests of the Golfo Dulce region in southeast Costa Rica belong to the most

interesting and species-rich forests in Central America. So far, 2,709 species in 935 genera

of 187 families of vascular plants have been recorded in the rainforests around the Golfo

Dulce (HUBER 1996, HERRERA-MCBRYDE & al. 1997, QUESADA & al. 1997, VAUGHAN 1981,

WEBER & al. 2001, WEISSENHOFER 1996), among them about 700 tree species (QUESADA & al.

1997). The Piedras Blancas National Park consists mainly of narrow ridges and steep slopes

covered with primary forest.

6

7

Within our study site four different characteristic types of vegetation occured.

Primary forest: These are forests which have never been severely influenced by human

activities such as logging, farming or any other large-scale destruction of the original

vegetation.

Secondary forest: These are forests which have suffered severe destruction of the original

vegetation. They are inhabited by subsidiary species according to their stage of succession.

The species composition and the structure of the vegetation are both different and less

diverse.

Riverine vegetation: This forest type is found along small rivers with adjacent flat terraces.

Trees are 35-50 m tall and often have massive trunks with large spreading buttresses.

Conspicuous trees of the canopy layer include Luehea seemanii (Tiliaceae), Sloanea sp.

(Elaeocarpaceae) and Virola spp. (Myristicaceae). The mid canopy layer is relatively open

with trees around 25-35 m tall. Common species include Apeiba tibourbou (Tiliaceae),

Castilla tunu (Moraceae) and Spondias mombin (Anacardiaceae). A prominent species of the

understory is Guatteria chiriquiensis (Annonaceae) and Tetrathylacium macrophyllum

(Flacourtiaceae). The well-represented shrub stratum is dominated by Carludovica drudei

(Cyclanthaceae), Calathea spp. (Marantaceae), Costus spp. (Costaceae), Dieffenbachia spp.

(Araceae) Heliconia spp. (Heliconiaceae) and Acalypha diversifolia (Euphorbiaceae). The

ground layer is bare except for Selaginella spp. and tree seedlings. The water level can rise

dramatically during heavy rainfalls and erase all plants along its way.

Mangrove forest: Mangrove forests accompany the sheltered seashores and estuaries of

rivers, where tidal inundations of salt water from the sea occur.

Mangroves are floristically poor, thus representing the opposite extreme of tropical forests

with their rich species diversity. Species like Rhizophora mangle, Rhizophora racemosa

(Rhizophoraceae) and Pelliciera rhizophorae (Theaceae) are common.

8

STUDY SITE – In 1993 the society “Regenwald der Österreicher” established the Biological

Field Station La Gamba on the edge of the Piedras Blancas National Park. The station is

situated next to primary forest and offers 20 students and scientists accommodation and

working space. The location is an excellent base to carry out scientific work on a tropical

sujet. Since then a lot of Austrian scientists but also international students and scientific

teams came to work on different tropical biology topics. But also tourists are welcome to

enjoy this special research atmosphere right in between the tropical rainforest. The study was conducted during the 'dry' season in February till April and the 'rainy' season

in August till September 2004.

A net of rivers, small streams and drainage channels pass through the national park and its

surroundings, which all flow into Rio Esquinas (esquinas means corner and the river is

named after its several meanders). He forms the natural boarder of the Bosque Esquinas in

the North and West of the national park and drains into the Pacific Ocean. Mangrove swamps

next to the mouth of the Rio Esquinas are existing to a great extent. The riverbanks of the two

main rivers passing through the La Gamba valley, the Rio Bonito and the Rio Esquinas, are

covered by farm land and secondary forest at different stages of regrowth. Due to logging till

the last century nearly no primary forest is left over in the lowland, except small spots along

the coast and deep inside the park. However, the upslope areas within the study catchment

show almost no signs of anthropogenic disturbance. Most of the smaller streams lie within the

rain forest.

Most of Costa Rica´s forested area was cleared in the last century. 25 % of Costa Rica is

somehow protected and national parks are established throughout the country. The Viennese

musician professor Michael SCHNITZLER initialized the protection of the Esquinas area and the

project “Regenwald der Österreicher. Due to donations of people from Austria, Canada, USA

and Costa Rican institutions, the protection started to grow and in 1991 the Piedras Blancas

National Park was designated. Meanwhile the Ministry for Environment and Energy of Costa

Rica (MINAE) is responsible for the protection of the Esquinas area. More land is still bought

to be converted into the national park.

9

2.1. MATERIAL AND METHODS

To get an overview of the running waters in the Piedras Blancas National Park and data to

compare we did research on nine study rivers within this area ranging from 1st order streams

to 5th order rivers: Quebrada Mari, Quebrada Negra, Quebrada Gamba, Quebrada Bolsa,

Quebrada Chorro, Quebrada Sardinal, Rio Oro, Rio Bonito, Rio Esquinas.

At each river we chose an easily accessible sampling site where one or two transects were

established by stretching a cord perpendicular to the current. In this cross sections we

measured the stream depth and the current velocity at regular distances of 0,2 – 0,7 m. The

velocity was measured with an Ottflügel, Type C2, in 40 % water depth above streambottom.

Recording to this detailed hydrological and morphological data, it was possible to calculate

the flow (water volume per second) and build up a depth and a current velocity profile.

Furthermore stream width, stream bed width and slope angle of the bank were assessed.

Water temperature and oxygen content were measured with a WTW Oximeter 330. A WTW

pH-meter was used to record the pH-value.

Sediment size was estimated visually. Therefore we used five standard particle size ranges

(< 2 mm, 2 - 6,3 mm, 6,3 - 20 mm, 20 - 63 mm, > 63 mm) and noted the proportion of each

range in the stream bed sediment.

The shadowing of the river by the riverine vegetation was estimated visually aswell and

assigned to one of the following values: 0 %, 5 %, 25 %, 50 %, 75 %, 100 %. At the

Quebrada Sardinal we recorded the riparian vegetation on a species level.

Between February 2004 and February 2005 water samples were taken at least once of each

site and analyzed for conductivity, pH, Alk., Cl-, SO42-, Si-SiO4, P-PO4, P-s, P-t, N-NO3. N-

NO2, N-NH4, N-sKj, N-tKj, Na+, K+, Ca2+, Mg2+ at the laboratory in Austria as soon as

possible. Water samples, which were taken on the 26.02.04, 03.04.04, 27.04.04, 12.02.05,

13.02.05 and 15.02.05 were assigned to the 'dry' season and the samples from 24.06., 03.08.

and 20.09.2004 are counting for the 'rainy' season.

10

Analysis of data

The statistical analysis of data was performed with the software package SPSS. The PCA

(principal components analysis) is a multivariate procedure which rotates that data such that

maximum variabilities are projected onto the axis. Cluster analysis classifies a set of

observations into two or more groups based on combinations of interval variables. This was

performed on the basis of water chemistry data from the 'dry' and 'rainy' season for all studied

streams.

Fig.2.3. Rio Esquinas catchment with nine sampling sites (Q.Negra, Q.Mari, Q.Chorro, Q.G amba, Q.Sardinal, Q.Bolsa, Rio Bonito, Rio Oro, Rio Esquinas)

11

Fig. 2.4. Rio Esquinas catchment, Rio Bonito catchment and Quebrada Negra catchment within the Piedras Blancas National Park and its vicinty

12

2.2. Results The nine streams studied (Fig. 2.3.) are all tributaries of the Rio Esquinas that lie within

the Piedras Blancas National Park and its vicinity. Every stream or river was conducted

twice – once during the 'dry' and once during the 'rainy' season – except of Quebrada

Mari, Quebrada Chorro and Rio Esquinas.

Quebrada Mari

The 'Quebrada Mari' is an unnamed 1st order stream, which is one out of 3 headwaters

of the Rio Esquinas. The stream has its source (~1100 m above sea level) in the

mountains of Fila Cruces and has a length of 3,6 km. Sampling took place on the 24th

February 2004 and chemical parameters like temperature, conductivity, pH-value and

oxygen content were measured three times along the stream course. The first sampling

site was a few meters up the stream at the bridge leading to San Miguel. Riparian

vegetation is dense and canopy cover is about 25 % at the sampling site 1. Large

boulders and rocks are dominating the stream bed. Rocks with a diameter of ~1 m are

common. The stream is 2 - 2,5 m wide and the mean depth is approximately 0,2 m. Flow

is about 150 ls-1.

13

Tab.:2.1. Data of three sampling sites at the Q.Mari Quebrada Mari date 24.02.2004 24.02.2004 24.02.2004 position - - - sampling site bridge 2 waterfall time 10.00 am 11.00 am 1.00 pm sea level [m] 450 500 580 temperature [°C] 22,1 22,3 22,4 pH-value 8,07 8,06 8,11 O2 [%/mgl-1] 133 / 10,8 98 / 7,9 103 / 8,1 stream width* [m] 2 – 2,5 - - average depth* [m] 0,20 - - average current velocity* [ms-1] 0,40 - - flow* [ls-1] 150 - - shading [%] 25 - - * estimated

Quebrada Negra

The Quebrada Negra is a tropical low land stream with a low altitudinal gradient and has

its source in the primary forest (~180 m above sea level) of the Esquinas rainforest. On

the entire way (~2,7 km) till the stream flows into Quebrada Gamba (~20 m above sea

level) he discharges primary and secondary rainforest and the La Gamba valley. The

stream flows close behind the Field Station La Gamba and the sampling site was about

14

1,3 km from the stream origin. For a detailed study site description an easily accessible

100 m sector next to the Field Station was chosen. Substratum is gravel in riffles with

leaf accumulations and silt in pools. Water temperature is about 25°C. A comparable

study site was chosen 1 km up the stream with large boulders intermixed with smaller

cobble substrate. For a full report and further details see chapter 3.

Tab.2.2. Data of three sampling sites within the Q. Negra Quebrada Negra date 20.02.2004 20.02.2004 20.02.2004position N 08°42,054' N 08°41,806' W 083°12,085' W 083°12,305' sampling site field station Lodge waterfall time 9.00 am 11.00 am 12.00 am sea level [m] 80 100 160 temperature [°C] 24,7°C 24,9°C 25,2°C pH-value 7,73 8,1 O2[%/mgl-1] 92,5 / 7,6 98 / 8,05 10350,00%

Quebrada Chorro

15

The Quebrada Chorro is a stream with a length of 5,3 km and empties into the Rio

Bonito. The source is at an elevation of 220 m above sea level. Sampling took place on

the 22nd of February and on the 19th of August close to the waterfall (~ 200 m

downstream). At the study site the Quebrada Chorro is a 2nd order stream and 50 % of

the river is shaded. Stream width (3,8 m / 5,3 m), mean depth (0,09 m / 0,15 m) and

average current velocity (0,33 ms-1 / 0,51 ms-1) show a distinct increase in August. Flow

is four times as high (410 ls-1) in the 'rainy' season as in the 'dry' season (108 ls-1). The

stream bottom consists of gravel and cobbles.

Tabl.:2.3. Data of two sampling sites at the Q. Chorro Quebrada Chorro date 22.02.2004 19.08.2004 position N 08° 42,425’

W 083° 10,552’N 08° 42,425’ W 083° 10,552’

sampling site waterfall waterfall time 10.00 am 12.00 am sea level [m] 80 80 temperature [°C] 26,3 - pH-value 7,97 - O2 [% / mgl-1] 8,16 / 102,0 - stream width [m] 3,8 5,3 average depth [m] 0,09 0,15 average current velocity [ms-1] 0,33 0,51 flow [ls-1] 108 410 sediment [mm]

Q 25 Q 50 Q 75

- - -

5,3 18,1 61,6

shading [%] 50 50

16

Quebrada Gamba

The Quebrada Gamba has a length of 7,7 km and the stream empties into the Rio

Bonito. The source is at an elevation of 260 m above sea level. At the sampling site, a

few meters downstream from the bridge leading to La Gamba, it is a 2nd order river.

Stream width and average current velocity nearly do not differ from 'dry' season (2nd of

April) to 'rainy' season (1st of August), but the average depth (0,32 m / 0,49 m) increases

as well as the flow (397 ls-1 / 650 ls-1). Canopy cover is at both times of sampling 50 %.

Gravel is the dominating substratum. Tab.:2.4. Data of two samplings at the Q. Gamba Quebrada Gamba date 02.04.2004 01.08.2004 position N 08° 42,072’

W 083° 11,530’ N 08° 42,072’ W 083° 11,530’

sampling site bridge bridge time 9.30 am 11.00 am sea level [m] 20 20 stream width [m] 6,9 6,8 average depth [m] 0,32 0,49 average current velocity [ms-1] 0,19 0,19 flow [ls-1] 397 650

17

sediment [mm] Q 25 Q 50 Q 75

- - -

7,6 24,0 75,9

shading [%] 25 25

Quebrada Bolsa

The Quebrada Bolsa has its source at 320 m above sea level and after 6,5 km it flows

into the Quebrada Gamba. The cross section was established near the bridge leading to

km 37 at the Interamericana on the 2nd of April and on the 1st of August. Stream width

ranges from 9,6 m to 11 m. The mean depth is between 0,128 m and 0,163 m and the

average current velocity between 0,289 ms-1 and 0,391 ms-1. The detected flow is 408 ls-

1 and 682 ls-1. Whereas half of the river is shaded in the 'dry' season, canopy cover

increases to 75 % in the 'rainy' season. The stream bottom is dominated by gravel and

cobbles.

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Tab.:2.5. Data of two samplings at the Q. Bolsa Quebrada Bolsa date 02.04.2004 01.08.2004 position N 08° 42,452’

W 083° 11,137’ N 08° 42,452’ W 083° 11,137’

sampling site bridge bridge time 9.00 am 12.00 am sea level [m] 20 20 stream width [m] 9,6 11,0 average depth [m] 0,13 0,16 average current velocity [ms-1] 0,29 0,39 flow [ls-1] 408 682 sediment [mm]

Q 25 Q 50 Q 75

- - -

9,1 28,9 91,2

shading [%] 50 75

Quebrada Sardinal

The Quebrada Sardinal – a 2nd order stream - is a tributary of the Rio Bonito and has a

length of 7,3 km. The sampling site (N 08° 43,407', W 083° 12,773', 30 m above sea

level) was 250 m upstream before he flows into the Rio Bonito. Riparian vegetation is

dense and canopy cover was in February about 5 % and during the 'rainy' season 25 %.

19

Substratum is dominated by gravel with some stones (diameter: 0,06 m). The stream is

during the 'dry' season about 6 m wide, but gets larger in August. Mean depth is

approximately 0,2 m. The current velocity in February is low – flow is about 200 ls-1, but

reaches during the 'rainy' season 0,75 ms-1 and flow gets up to 1700 ls-1. Rock surfaces

are densely covered with periphyton. Tab.:2.6. Data of three samplings at the Q. Sardinal Quebrada Sardinal date 21.02.2004 21.02.2004 20.08.2004 position N 08° 43,407’ N 08° 43,407’ N 08° 43,407’ W 083° 12,773’ W 083° 12,773’ W 083° 12,773’sampling site transect 1 transect 2 transect 1 time 15.00 16.00 16.00 sea level [m] 30 30 30 temperature [°C] 27,6 27,6 - pH-value 7,63 7,63 - O2[%/mgl-1] 100,6 / 7,86 100,6 / 7,86 - stream width [m] 6,0 5,8 10,0 average depth [m] 0,16 0,32 0,23 average current velocity [ms-1] 0,28 0,12 0,75 flow [ls-1] 260 173 1765 sediment [mm]

Q 25 Q 50 Q 75

- - -

- - -

3,8 12,0 38,1

shading [%] 5 5 25 Tab.:2.7. Riparian vegetation at the Rio Sardinal with type of growth, frequency(++ often, + present, - absent), height [m] and distance to the riverbank [m] of recorded plants Rio Sardinal

taxon type of growth frequencyheight

[m] distance to the riverbank [m]

MONOCOTYLEDONS Cyclanthaceae Cyclanthus bipartitus giant herb + - - Carludovica drudei giant herb + - - Heliconiaceae Heliconia latispatha herb ++ - 0 Marantaceae Calathea lutea giant herb ++ - - Poaceae Dendrocalamus giganteous + - - Zingiberaceae Hedychium coronarium herb + - -

20

DICOTYLEDONS Anacardiaceae Anacardium excelsum tree + 9 - Spondias mombin tree ++ 32 0 Annonaceae Guatteria amplifolia tree + 12 - Bombacaceae Ceiba pentandra canopy tree + 35 - Cucurbitaceae Gurania macoyana liana + - - Euphorbiaceae Acalypha diversifolia shrub/tree ++ - - Alchornea costaricensis tree + 17 - Fabaceae - Faboideae Lonchocarpus sp. tree + 10 - Calopogonium sp. vine + - - Lauraceae Ocotea sp. tree ++ 20 0 Marcgraviaceae

Souroubea sp. hemiepiphytic

shrub + - - Melastomataceae

Topobea maurofernandeziana hemiepiphytic

tree + - - Sterculiaceae Theobroma cacao tree ++ - - Tiliaceae Luehea seemannii canopy tree ++ 25 0 Apeiba tibourbou canopy tree ++ - - Trichospermum grewiifolium canopy tree ++ - - The abundant riparian vegetation maintains mostly typical species which occur along the

streams and rivers in this region of Costa Rica.

Trees of the canopy layer include Luehea seemannii, Apeiba tibourbou, Trichospermum

grewiifolium (Tiliaceae), Ceiba pentandra (Bombacaceae), and Spondias mombin

(Anacardiaceae) which all reach a height of about 30 m. Common species of the mid

tree layer are Anacardium excelsum (Anacardiaceae), Guatteria amplifolia

(Annonaceae), Alchornea costaricensis (Euphorbiaceae), Lonchocarpus sp. (Fabaceae

– Faboideae), Ocotea sp. (Lauraceae) and Theobroma cacao (Sterculiaceae). The

shrub stratum contains species like Cyclanthus bipartitus, Carludovica drudei

(Cyclanthaceae), Calathea lutea (Marantaceae) and Acalypha diversifolia

(Euphorbiacea)

21

Rio Oro

Rio Oro is at the study site a 3rd order stream, which flows after 6,2 km into Rio Bonito

next to Villa Briceno – km 37 at the Interamericana. The sampling site (N 08°43,004', W

083°10,033', 20 m above sea level) was at the bridge, close before Villa Briceno – km

37. Canopy cover is low – about 5 % and the substrate is dominated by gravel. The

stream width is about 14 m in February and 19 m in August and the depth varies

between 0,08 m and 0,33 m. Mean current velocity ranges from 0,23 ms-1 in the 'dry'

season to 0,29 ms-1 in the 'rainy' season. The flow comes in February to 187 ls-1 and in

August to 1622 ls-1. Tab.:2.8. Data of two samplings at the Rio Oro Rio Oro date 23.02.2004 19.08.2004 position N 08° 43,004' N 08° 43,004' W 083° 10,033' W 083° 10,033'sampling site bridge bridge time 8.45 am 10.00 am sea level [m] 20 20 temperature [°C] 26,8°C -

22

pH-value 7,75 - O2 [%/mgl-1] 109,2 / 8,64 - stream width [m] 14,5 19 average depth [m] -0,08 -0,33 average current velocity [ms-1] 0,23 0,29 flow [ls-1] 187 1622 sediment [mm]

Q 25 Q 50 Q 75

- - -

1,5 6,5 28,5

shading [%] 5 5

Rio Bonito

The Rio Bonito (20,1 km) is one of the two largest rivers within the Piedras Blancas

National Park. The river discharges from dense primary forest (200 m above sea level)

where its canopy cover is about 100 %. At the study site, upstream of the village La

Gamba, it is a 3rd order river with low canopy cover – about 5 %. The Rio Bonito passes

through the anthropogen influenced valley of La Gamba until it drains as a 4th order river

into the Rio Esquinas. Two cross sections with a distance of 10 m were build up at the

study site. Transect 1 shows a pool area, while transect 2 is characterised by a constant

current velocity and depth. The stream width is 3,8 and 7,2 m in February and 6,3 and

23

8,6 m in August and the depth is 0,34 m and 0,12 m during the 'dry' season and 0,37 m

and 0,21 min the 'rainy' season. Mean current velocity ranges from 0,24 and 0,60 ms-1 in

the 'dry' season to 0,52 ms-1 and 0,82 ms-1 in the 'rainy' season. The flow comes in

February to ~540 ls-1 and in August to 1560 ls-1. Tab.:2.9. Data of four samplings at the Rio Bonito Rio Bonito date 02.03.2004 02.03.2004 14.08.2004 14.08.2004 position N 08°43,868' N 08°43,868' N 08°43,868' N 08°43,868' W 83°17,723' W 83°17,723' W 83°17,723' W 83°17,723'sampling site transect 1 transect 2 transect 1 transect 2 time 10.00 am 11.00 am 11.00 am 12.00 am sea level [m] 30 30 30 30 temperature [°C] 28,2 - - - pH-value 8,3 - - - O2 [% / mgl-1] 101 / 8,0 - - - stream width [m] 3,8 7,2 6,3 8,6 average depth [m] 0,34 0,12 0,37 0,21 average current velocity [ms-1] 0,24 0,60 0,52 0,82 sediment [mm]

Q 25 Q 50 Q 75

- - -

- - -

- - -

6,3 18,3 53,3

flow [ls-1] 545 523 1555 1565 shading [%] 5 5 5 5

24

Rio Esquinas

Rio Esquinas is a 5th order river and with a lenght of 42 km the largest one within the

Piedras Blancas National Park. He forms the natural border of the Bosque Esquinas in

the North and West of the national park and drains into the Pacific Ocean. Mangroves

occur in the tidal estuaries next to the mouth of the Rio Esquinas to a great extent.

Mangroves are floristically poor, thus representing the opposite extreme of tropical

forests with their rich species diversity. Species like Rhizophora mangle, Rhizophora

racemosa (Rhizophoraceae) and Pelliciera rhizophorae (Theaceae) are common.

The sampling site (N 08°43,868', W 83°17,723' , 0 m above sea level) was about 4 km

upstream of its mouth into the Pacific Ocean. Canopy cover is 0 % and sediment is

dominated by sand and mud. The stream width is about 30 m and average stream depth

is 1,5 m. The maximum depth reaches 4 m. The estimated current velocity is ~0,6 m and

resulting flow about 20 000 ls-1. Tab.2.10. Data of four samplings at the Rio Esquinas Rio Esquinas date 25.02.2004 position N 08°43,868' W 83°17,723'sampling site mouth time 13.00 am sea level [m] 0

25

temperature [°C] 29,4 pH-value 7,78 O2 [% / mgl-1] 104,4 / 7,94 stream width* [m] 30 average depth* [m] 1,5 average current velocity* [ms-1] 0,6 flow* [ls-1] 20 000 shading [%] 0 * estimated

river length [km]0 5 10 15 20 25 30 35 40 45

altit

ude

[m]

0

20

40

60

80

100

120

140

160

180

200altitude

Gam

baBo

nito

Esqu

inas

Neg

ra

flow

[ls-1

]0

2500

5000

7500

10000

12500

15000

17500

20000

22500

flow

Fig. 2.5. River lenght [km] versus altitude [m] and flow [ls-1] during the 'dry' season of the Q.Negra flowing into Q.Gamba, Rio Bonito and Rio Esquinas.

The altitudinal gradient from the source of the Q.Negra to the mouth of Rio Esquinas,

which is 45 km downstream at the Pacific Ocean, is shown in Fig.2.5. The Q. Negra has

its source at an elevation of 180 m above sea level. After 2,7 km km Q. Negra flows into

the Q. Gamba at an altitude of less than 70 m. 1,2 km downstream the Q. Gamba drains

into Rio Bonito, which flows after 3,1 km into Rio Esquinas. During the 'dry' season

Q.Negra has a flow of 22,5 ls-1, Q.Gamba of 397 ls-1, Rio Bonito of 545 ls-1 and Rio

Esquinas of 20000 ls-1 before he empties into the Golfo Dulce.

26

Chorro - DS

0 2 4 6 8 10 12 14 16

bank

leve

l [m

]

-0,5

0,0

0,5

1,0

Chorro - RS

stream width [m]0 2 4 6 8 10 12 14 16

bank

leve

l [m

]

-0,5

0,0

0,5

1,0

1,60

1,60

Fig.:2.6. Cross section of Quebrada Chorro with stream width [m] and depth [m] during 'dry' [DS] and

'rainy' [RS] season

27

Sardinal - cross section 1 - DS

0 2 4 6 8 10 12 14

bank

leve

l [m

]

-1,0

-0,5

0,0

0,5

1,0

Sardinal - cross section 2 - DS

0 2 4 6 8 10 12 14

bank

leve

l [m

]

-1,0

-0,5

0,0

0,5

1,0

Sardinal - cross section 1 - RS

stream width [m]0 2 4 6 8 10 12 14

bank

leve

l [m

]

-1,0

-0,5

0,0

0,5

1,0

1,20 1,40

2,00

1,35 1,60

Fig.: 2.7. Cross sections of Quebrada Sardinal with stream width [m] and depth [m] during 'dry' [DS] and 'rainy' [RS] season

28

Oro - DS

0 2 4 6 8 10 12 14 16 18 20

bank

leve

l [m

]

-0,5

0,0

0,5

1,0

Oro - RS

stream width [m]0 2 4 6 8 10 12 14 16 18 20

bank

leve

l [m

]

-1,0

-0,5

0,0

0,5

1,0

2,00

2,00

1,40

1,50

Fig.: 2.8. Cross transect of Rio Oro with stream width [m] and depth [m] during 'dry' [DS] and 'rainy' [RS] season A comparison of selected cross sections from Quebrada Chorro, Quebrada Sardinal and

Rio Oro from the 'dry' and 'rainy' season show the increase in stream width and water

depth during the 'rainy' season. Distinct seasonal differences can be seen in Fig.2.9.,

Fig.2.10. and Fig.2.11. Data of cross sections from Quebrada Gamba, Quebrada Bolsa,

Rio Bonito and Rio Esquinas are provided in the Appendix.

The two figures of the cross section at Quebrada Chorro show the increasing stream

width from 3,8 m during the 'dry' season to 5,3 m in the 'rainy' season. Maximum depth

increases from 0,14 m in the 'dry' season to 0,23 m in the 'rainy' season

The cross transect of Quebrada Sardinal within the riffle section was constructed in both

seasons. Stream width (5,6 / 10,5 m) and maximum depth (0,25 / 0,31 m) increases

markedly aswell as the flow (260 / 1765 ls-1) from the 'dry' to the 'rainy' season. Study

site two, which was only analyzed in the 'dry' season, shows a pool section with a

maximum depth of 0,67 m. The flow is 175 ls-1 and the stream width 5,8 m.

29

Rio Oro gets striking larger and deeper during the 'rainy' season. Streamwidth ranges

from 14,4 m during the 'dry' season to 18,9 m in the 'rainy' season. Maximum depth is

0,12 m in February and 0,57 m in August.

riverNegra Chorro Bolsa Gamba Sardinal Oro Bonito

sedi

men

t siz

e [m

m]

0

20

40

60

80

100Q75 Q50 Q25

Fig.: 2.9. Comparison of the sediment size [mm] from the nine study rivers during the 'dry' and the 'rainy' season. Arrangement of the rivers according to their stream order [1st – 5th] Sediment size was only recorded at seven study sites in the 'rainy' season. Sediment

size differs markedly between the study sites. Rio Oro shows the smallest sediment size

whereas Quebrada Bolsa has the largest substrate.

river

Negra Mari Chorro GambaSardinal Bolsa Bonito Oro Esquinas

wid

th [m

]

0

5

10

15

20

25

30

35

40

45

rainy seasondry season

Fig.:2.10. Comparison of the river width [m] from the nine study rivers during the 'dry' and the 'rainy' season. Arrangement of the rivers according to their stream order [1st – 5th]

30

The streamwidth of Q.Negra, Q.Chorro, Q.Sardinal, Rio Oro and Rio Bonito markedly

gets larger from the 'dry' season to the 'rainy' season. Q.Bolsa and Q.Gamba are wider

during the 'dry' season, but get deeper in the 'rainy' season as the transect figures show.

At Q.Mari and Rio Esquinas it was just once, during the 'dry' season, possible to

estimate the stream width. River width ranges from 2,3 m (Q.Mari) to 42 m at the

Esquinas study site.

riverNegra Mari Chorro GambaSardinal Bolsa Bonito Oro Esquinas

dept

h [m

]

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

dry seasonrainy season

Fig.:2.11. Comparison of the average depth [m] with standard deviation from the nine study rivers during the 'dry' and the 'rainy' season. Arrangement of the rivers according to their stream order [1st – 5th] All seven rivers where data is available from both seasons get deeper during the 'rainy'

season, just Q.Mari and Rio Esquinas were only analyzed once in the 'dry' season.

Standard deviation are markedly smaller at cross transects, which were constructed at

riffles to the ones in pools.

31

riverNegra Mari Chorro GambaSardinal Bolsa Bonito Oro Esquinas

curre

nt v

eloc

ity [m

s-1]

0,0

0,2

0,4

0,6

0,8

1,0

1,2

dry seasonrainy season

Fig. 2.12. Comparison of the average current velocity [ms-1] with standard deviation from the nine study rivers during the 'dry' and the 'rainy' season. Arrangement of the rivers according to their stream order [1st – 5th] Current velocity increases at every study site during the 'rainy' season. Data of Q. Mari

and Rio Esquinas is only available for the 'dry' season. In the 'dry' season average water

current velocity is slow, ranging from 0,187 ms-1 (Q. Gamba) to 0,5 ms-1 (Rio Esquinas).

In the 'rainy' season higher speeds were recorded. Whereas Q. Gamba does not show a

distinct difference, average current velocity is increasing intensively in Quebrada

Sardinal (0,285 / 0,749 ms-1). Standard deviation is higher at cross sections within pools.

32

river

Negra Mari Chorro GambaSardinal Bolsa Bonito Oro Esquinas

flow

[m3 s-1

]

0

1

2

10rainy seasondry season

Fig.: 2.13. Comparison of the flow [m3s-1] from the nine study rivers during the 'dry' and the 'rainy' season. Arrangement of the rivers according to their stream order [1st – 5th] Flow increases at all study rivers, but for Q. Mari and Rio Esquinas no data is available

for the 'rainy' season. The rivers show large differences in flow. Quebrada Negra is the

smallest stream with a baseflow of 0,031 m3s-1. Flow of Rio Esquinas is about 300 times

as high (10m3s-1). Streamflow was highly seasonal.

Furthermore all rivers show large fluctuations between 'dry' and 'rainy' season. Water

flow is increasing in the late 'rainy' season (September, October). Therefore differences

in flow are depending on sampling date. Values between 0,094 m3s-1 (Q. Negra) and

1,776 m3s-1 (Q.Sardinal) were recorded. Rio Oro shows the largest fluctuation (0,187 /

1,622 m3s-1).

33

Tab.:2.11. Hydrochemical characteristics of 9 sampled rivers within the Piedras Blancas National Park

river date Lf pH Alk Cl- SO42- Si-SiO4 P-PO4 P-s P-t N-NO3 N-NO2

N-NH4

N-sKj

N-tKj Na+ K+ Ca2+ Mg2+

µS/cm; 25°C

mg/l

mg/l

mg/l

mg/l

µg/l

µg/l

µg/l

µg/l

µg/l

µg/l

µg/l

µg/l

mg/l

mg/l

mg/l

mg/l

Rio Bonito 26.02.04 155 8 104,9 0,5 1,4 21,5

13 14 19 47 0 6 47 55 5,0 0,6 16,7 7,0Rio Oro 26.02.04 372 8,3 141,6 1,9 12 12,2 14 16 22 166 0 1 65 92 7,8 1,3 44,4 2,6Esquinas 26.02.04 299 8,2 167,2 5 9,4 13,2 16 17 40 92 1 0 48 79 9,6 1,1

44,3 4,7

Quebrada Mari 26.02.04 260 8,3 126,9 0,2 2,6 4,4 3 4 11 285 0 0 19 24 1,0 0 34,8 2,9Quebrada Negra 26.02.04 200 8,1 128,1 0,4 1,3 26,2 48 49 50 64 0 0 28 39 6,9 0,8 24,0 5,6Rio Sardinal 26.02.04 132 8,1 101,3 0,4 1,0 22,0 19 21 21 173 0 31 39 39 3,9 0,6 18,3 5,5Quebrada Bolsa 03.04.04 145 8,4 91,5 0,6 1,5 23,4 31 32 34 120 0 20 60 83 6,2 0,7

14,8 4,4

Quebrada Gamba 03.04.04 154 8,4 102,5 0,5 1,1 23,2 20 21 25 62 0 3 44 52 6,2 0 17,3 5,0Quebrada Negra 03.04.04 175 8,4 112,3 0,5 1,4 23,8 26 28 32 95 0 16 49 70 6,1 0,8 20,4 4,8Quebr. Bolsa 27.04.04 158 8,3 102,5 0,5 1,6 23,5 28 40 40 118 1 16 19 24 6,5 0,7 17,0 4,8Quebrada Gamba 27.04.04 161 8,4 108,6 0,5 1,1 23,8 28 39 41 86 1 4 18 32 6,3 0,7 18,1 5,0Quebrada Negra 27.04.04 185 8,4 117,2 0,4 1,4 25,0 45 45 46 66 1 9 18 39 6,4 0,8 20,8 4,9Quebrada Negra 24.06.04 197 7,9 129,4 0,5 1,1 29,7 40 42 46 37 0 0 16 33 7,9 0,4 26,9 6,4Quebrada Negra 03.08.04 196 8,5 129,4 0,6 1,1 30,5 46 46 49 69 1 4 54 67 7,3 0,7

26,5 6,4

Quebrada Bolsa 20.09.04 157 8,4 106,2 0,5 1,2 25,9 76 77 87 21 4 34 134 154

7,1 0 18,8 5,5Rio Bonito 20.09.04 150 8,4 100,1 0,4 0,9 22,2 22 23 25 34 1 6 44 51 4,6 0 16,4 7,3Quebrada Chorro 20.09.04 128 8,2 81,8 0,4 0,6 27,9 42 44 45 9 1 25 32 64 5,8 0,6

14,2 4,0

Quebrada Gamba 20.09.04 160 8,3 106,2 0,4 0,7 27,0 55 58 76 23 1 69 100

167

6,8 0 18,4 5,6Quebrada Negra 20.09.04 177 8,5 114,7 0,4 0,9 13,3 47 47 53 143 1 48 77 88 6,3 0,6 22,6 5,4Rio Oro 20.09.04 314 8,4 178,2 0,7 14,2 26,9 36 36 75 269 0 13 101 120 7,3 0,9 52,4 2,4Quebrada Sardinal 20.09.04 126 8,2 83 0,3 0,7 22,5 19 20 22 25 1 12 34 39 3,7 0 12,3 6,1Quebrada Negra 12.02.05 205 8,4 125,7 1,4 1,3 26,8 39 39 50 36 0,3 27 44 83 7,5 0,8 26,2 6,0Quebrada Negra 13.02.05 204 8,2 125,7 1,2 1,3 26,7 31 31 42 110 3,1 4 27 86 7,3 0,9 26,0 6,0Quebrada Negra 15.02.05* 152 8,1 92,7 1 1,2 22,3 31 32 45 86 1 4 69 109 5,7 0,8 19,0 4,4Quebrada Negra 15.02.05** 168 8,2 103,7 0,6 1,1 22,0 36 38 56 82 1 4 64 138 5,7 0,8 22,5 4,5Rio Esquinas 17.02.05 278 8,3 156,2 1,3 12,9 7,4 8 8 18 195 0 25 55 79 7,8 0,8 46,0 3,5Quebrada Gamba 17.02.05 169 8,3 102,5 1,1 1,3 23,7 29 30 34 23 1,2 32 36 47 6,9 0,6 19,9 5,6Rio Oro 17.02.05 367 8,4 197,7 1,6 22,9 12,5 1 2 68 78 1,6 21 43 147 9,1 1,1 65,2 2,8

34

river date Na+ K+ Ca2+ Mg2+ Alk Cl- SO4

2- Ionenbilanz skat san an% mval

mval

mval

mval

mval

mval

mval

diffkat%

mval

mval

Rio Bonito

26.02.04 0,2 0 0,8 0,6 1,7 0 0 -7,3 1,6 1,8 107,3Rio Oro 26.02.04 0,3 0 2,2 0,2 2,3 0,1 0,3 6,2 2,8 2,6 93,8Esquinas 26.02.04 0,4 0 2,2 0,4 2,7 0,1 0,2 -1,2 3,0 3,1 101,2Quebrada Mari 26.02.04 0 0 1,7 0,2 2,1 0 0 -6,1 2,0 2,1 106,1Quebrada Negra

26.02.04 0,3 0 1,2 0,5 2,1 0 0 -8,3 2,0 2,1 108,3

Rio Sardinal 26.02.04 0,2 0 0,9 0,5 1,7 0 0 -8,9 1,6 1,7 108,9Quebrada Bolsa 03.04.04 0,3 0 0,7 0,4 1,5 0 0 -11,4 1,4 1,5 111,4Quebrada Gamba 03.04.04 0,3 0 0,9 0,4 1,7 0 0 -10,9 1,5 1,7 110,9Quebrada Negra

03.04.04 0,3 0 1,0 0,4 1,8 0 0 -10,5 1,7 1,9 110,5

Quebr. Bolsa 27.04.04 0,3 0 0,8 0,4 1,7 0 0 -11,7 1,5 1,7 111,7Quebrada Gamba 27.04.04 0,3 0 0,9 0,4 1,8 0 0 -12,9 1,6 1,8 112,9Quebrada Negra 27.04.04 0,3 0 1,0 0,4 1,9 0 0 -12,7 1,7 2,0 112,7Quebrada Negra 24.06.04 0,3 0 1,3 0,5 2,1 0 0 3,0 2,2 2,2 97,0Quebrada Negra 03.08.04 0,3 0 1,1 0,4 2,1 0 0 0,9 2,2 2,2 99,1Quebrada Bolsa

20.09.04 0,3 0 0,9 0,5 1,7 0 0 -4,8 1,7 1,8 104,8

Rio Bonito 20.09.04 0,2 0 0,8 0,6 1,6 0 0 -3,3 1,6 1,7 103,3Quebrada Chorro 20.09.04 0,3 0 0,7 0,3 1,3 0 0 -4,3 1,3 1,4 104,3Quebrada Gamba 20.09.04 0,3 0 0,9 0,5 1,7 0 0 -5,4 1,7 1,8 105,4Quebrada Negra

20.09.04 0,3 0 1,1 0,4 1,9 0 0 -2,6 1,9 1,9 102,6

Rio Oro 20.09.04 0,3 0 2,2 0,2 2,9 0 0,3 -2,5 3,2 3,2 102,5Quebrada Sardinal

20.09.04 0,2 0 0,6 0,5 1,4 0 0 -8,3 1,3 1,4 108,3

Quebrada Negra 12.02.05 0,3 0 1,3 0,5 2,1 0 0 0,9 2,1 2,1 99,1Quebrada Negra 13.02.05 0,3 0 1,3 0,5 2,1 0 0 0,6 2,1 2,1 99,4Quebrada Negra 15.02.05* 0,2 0 0,9 0,4 1,5 0 0 0,6 1,6 1,6 99,4Quebrada Negra

15.02.05** 0,2 0 1,1 0,4 1,7 0 0 1,1 1,8 1,7 98,9

Rio Esquinas 17.02.05 0,3 0 2,3 0,3 2,6 0 0,3 2,8 2,9 2,9 97,2Quebrada Gamba 17.02.05 0,3 0 1,0 0,5 1,7 0 0 1,7 1,8 1,7 98,3Rio Oro 17.02.05 0,4 0 3,3 0,2 3,2 0 0,5 3,9 3,9 3,8 96,1(* 5.00 pm, ** 7.30 pm)

35

A summery description of the chemical characteristics of the sampling sites is provided

in Table 2.11. The Q. Mari (260 µScm-1), the Rio Oro ( 372 / 314 µScm-1) and the Rio

Esquinas (299 µScm-1) show in comparison to the Q. Negra, Chorro, Gamba, Bolsa,

Sardinal and Bonito a distinctly higher conductivity. Seasonal differences are slight.

The pH – value of all studies rivers is about 8 in the 'dry' season aswell as in the 'rainy'

season.

Rio Oro and Rio Esquinas have the highest alkalinity. The differences do not show a

clear trend between 'dry' and 'rainy' season.

Rio Oro and Rio Esquinas show distinctly higher values of chlorid. The differences

between the two seasons in general, except at Rio Oro, are slight.

The Rio Oro (12,0 / 14,2 mgl-1) and the Rio Esquinas (9,4 mgl-1) have high

concentrations of SO42-. The values in the Q. Negra, Mari, Chorro, Bolsa, Sardinal and

Rio Bonito are between 0,6 and 2,6 mgl-1. The are almost no differences between 'dry'

and 'rainy' season.

During the 'dry' season the S-SiO4 concentration of Q. Mari, Rio Oro and Rio Esquinas

is distinctly lower than in the Q. Negra, Gamba, Bolsa, Sardinal and Rio Bonito. Clear

seasonal differences are only apparent in the samples of Rio Oro.

The Q. Mari has a very low concentration of P-PO4. During the 'rainy' season all streams

have higher values of phosphorus concentration than in the 'dry' season. The Q. Mari

shows a very low value of soluble phosporus. Q. Negra, Chorro, Gamba and Bolsa have

the highest concentrations. Most of the streams show higher values in the 'rainy' season.

Q. Gamba (76 µgl-1), Q.Bolsa (87 µgl-1) and Rio Oro (75 µgl-1) show the highest values

of total phosphorus concentration during the 'rainy' season.

Q. Mari (285 µgl-1) has the highest concentration of N-NO3 during the 'dry' season and

the Rio Oro (269 µgl-1) during the 'rainy' season.

N-NO2 concentrations are higher at all study streams during the 'rainy' season. Seasonal

differences are obvious. Q. Bolsa has the highest concentration (4 µgl-1) of all rivers

during the 'rainy' season. Q. Gamba (69 µgl-1) has the highest concentration of N-NH4

during the 'rainy' season. No clear trend between the two seasons and between the

study rivers is perceptible.

In the 'dry' season Quebrada Mari has the lowest and Quebrada Oro the highest values

of N-sKj. In the 'rainy' season Q. Negra, Gamba, Bolsa and Oro show a distinct increase

in the concentration of N-sKj. In the 'dry' season Quebrada Mari has the lowest and

36

Quebrada Oro the highest values. In the 'rainy' season Q. Negra, Gamba, Bolsa and

Oro show a distinct increase in the concentration of N-tKj.

The Q. Mari (1,0 mgl-1) has the lowest, the Rio Esquinas (9,6 mgl-1) the highest content

of sodium in the 'rainy' season. Seasonal differences are not present.

Kalium concentrations are in general higher during the 'dry' season. Rio Oro and Rio

Esquinas are the only two rivers which have a Kalium concentration higher than 1 mgl-1

during the 'dry' season.

The content of calcium in the Q. Negra, Chorro, Gamba, Sardinal, Bolsa and the Rio

Bonito is between 14,2 and 25,3 mgl-1. Mari (34,8 mgl-1), Oro (44,4 / 52,4 mgl-1) and

Esquinas (44,3 mgl-1) show clearly higher concentrations. Streams do not show a

distinct seasonal difference.

The Q. Negra, Chorro, Gamba, Sardinal, Bolsa, Rio Bonito and Rio Esquinas show

values between 4,0 mgl-1 and 7,3 mgl-1 of magnesium. The Q. Mari (2,9 mgl-1) and the

Q. Oro (2,6 / 2,4 mgl-1) have about half of the amount of magnesium as the other

mentioned rivers. The differences between 'dry' and 'rainy' season are slight.

river

mva

l

0

1

2

3

4

CaMgNaAlkSO4

Negra Mari Chorro Gamba Sardinal Bolsa Bonito Oro Esquinas

Fig.:2.14. Ion balances based on concentrations of Ca2+, Mg2+, Na+, alkalinity and SO42- [mval] for the nine

study streams Differences in solute concentrations among the nine streams seem largely a function of

area specific runoff.

37

34,64FAC 1

geochemical paramters

-2 -1 0 1 2 3 4

nutri

ents

FAC

227

,07

-3

-2

-1

0

1

2

3BonitoOroEsquinasMariNegraSardinalBolsaGambaChorro

low high

low

high

Fig.:2.15. Ordination biplot resulting from a PCA (principal component analysis) according to the hydrochemical characteristics of the nine study streams during the 'dry' and 'rainy' season

The results of the PCA (principal component analysis) show the distribution of the study

streams in relation to the first two axis. The PCA describes 61,64 % of the total

physicochemical variability during the 'dry' and 'rainy' season of the streams. “Axis 1”

accounted for 34,64 % of the variance. “Axis 2” explains about 27,07 %. In Fig. 2.15. the

study sites are lined up along axis 1. Full black signs are standing for the sampling site

during the 'dry' season wheras the white signs stand for the 'rainy' season. The principal

components are the nutrients like nitrogen and phosphor and the geochemical

parameters. Axis 1 can be interpreted as the axis for the nutrients and Axis 2 stands for

the geochemical parameters

38

C A S E 0 5 10 15 20 25 Label Num +---------+---------+---------+---------+---------+ Gamba3 11 Negra3 12

1 = 26.02.04 2 = 03.04.04 3 = 27.04.04 4 = 24. 06.04 5 = 03.08.04 6 = 20.09.04 7 = 12.02.05 8 = 13.02.05 9 = 15.02.05, 17:00 10 = 15.02.05; 19:30 11 = 17.02.05

Gamba11 27 Negra5 14 Negra7 22 Bolsa2 7 Negra2 9 Gamba2 8 Bolsa3 10 Chorro6 17 Negra9 24 Negra10 25 Negra8 23 Negra1 5 Negra4 13 Bonito6 16 Sardinal6 21 Bonito1 1 Sardina1 6 Negra6 19 Bolsa6 15

Gamba6 18 Mari1 4

Oro1 2 Esquina11 26 Esquina1 3

Oro6 20 Oro11 28 Fig.:2.16.The dendrogram is showing relationships between the nine rivers based on chemical parameters. Dendrogram using Average Linkage (Between Groups), Rescaled Distance Cluster Combine

The river dendrogram (Fig.2.8.) reveals two large river clusters. Cluster one is the

largest and consists of six rivers. Cluster two consists of the Rio Oro and the Rio

Esquinas. The Quebrada Mari cannot be clearly assigned to one of the two clusters.

The clusters can be assigned to two different catchments according to their geochemical

parameters. The first is the subcatchment of Rio Bonito and the second one are the

rivers, which drain from Fila Cruces (Rio Oro, Rio Esquinas)

39

Tab.:2.12. Results of a discriminant function analysis with Eigenvalue, % of Variance, Cumulative %, Canonical Correlation, Chi-square and Significance (p < 0,05) Function Eigenvalue % of Variance Cumulative % Canonical Correlation

1 12,705 100 100 0,963 Test of function(s) Wilks-Lambda Chi-square df Significance

1 0,073 39,267 22 0,013 A discriminant function analysis according to the hydrochemical parameters of the rivers

shows a significant (p<0,05) difference of the streams between the 'dry' and 'rainy'

season.

40

41

2.3. DISCUSSION

The physical and chemical data reported from some selected streams of the Rio

Esquinas catchment provide a limited picture of the variable conditions existing at the

sampling sites. However, the data show the differences between the sites and their

changing conditions during the 'dry' and 'rainy' season. High water temperatures (>

25°C, except of Q.Mari with 22,1°C) is a permanent feature of the rivers in the

Piedras Blancas National Park and its surroundings, as it is of many other rivers in

Central America.

The results of the morphometric-hydrological measurements show the expected

trends. The two first order streams, Quebrada Negra and Quebrada Mari, have the

lowest values of stream width and flow. In general, stream width and flow gets larger

with increasing stream order. Mean current velocities and mean depths do not show

these tendencies. This might be due to randomly chosen study sites. The cross

sections were sometimes constructed within pool areas and sometimes within riffle

sections, therefore these two parameters are not really comparable within the study

sites.

No trends from 1st to 3rd order river are clearly visible within the sediment size.

Sediment size was just visually estimated once during the 'rainy' season, therefore

the records at the sampling sites only show a small picture and are not really

comparable. Sediment size is not only correlating with the streamorder; it is

dependent upon different physical parameters (current velocity, incline) of the stream.

Mean current velocities and mean depths increase in the 'rainy' season as well as the

recorded flow and are also dependend on the sampling time and sampling site. The

differences in mean current velocities between 'dry' and 'rainy' season are larger

within riffle sections, whereas differences of mean depths are larger within pool

sections. Standard deviation of both parameters is higher in the 'rainy' season which

means a higher heterogeneity.

42

The streams in this seasonal ('dry' and 'rainy' season) environment exhibit a high

annual variation in discharge and turbidity. In the 'rainy' season frequent (almost

daily) storms can cause bankfull conditions and in some cases flooding. Results of

the recorded flow show in all rivers a distinct increase from the 'dry' to the 'rainy'

season. The study catchments are characterized by strong sesaonal variations in

runoff. In the 'dry' season, streamflow exhibits a continous recession and is highly

predictable. In the 'rainy' season there is both a predictable component (the increase

in baseflow) as well as an unpredictable component involving the timing and

magnitude of individual storms. The differences in flow become more apparent when

samples where taken in the late 'rainy' season. The rivers and streams are

characterized by extreme short-term variability in flow. Most Central American rivers

are short in length and do not sustain flood condition for long periods between

successive downpours.

Tropical rivers in general have low concentrations of chemical constituents compared

with temperate rivers. This is because tropical rivers are for the most part

precipitation-dominated, but local geology can also be important. Geological

processes are probably largely responsible for the observed physical and chemical

differences between the studied rivers. Quebrada Mari, one of the headstreams of

Rio Esquinas, Rio Oro and Rio Esquinas have its source in the Fila Cruces while all

the others arise from the Fila Gamba and Fila Golfito. Geology plays a greater role

than land use in determining the nutrient concentrations of unpolluted waters. The

chemical data of the water samples indicate the same source of Quebrada Mari and

Rio Oro. Conductivity, calcium concentration and sulfate concentration is higher, and

the phosphorus concentration is lower than in the six other studied rivers, which have

their source in the Golfo Dulce area. The catchment area of Q.Mari and Rio Oro

seems to exist predominately from calcium sulphate (gypsum) – therefore high

concentrations of CaSO4. Differences in solute concentrations among the six streams

seem largely a function of area specific runoff.

Nitrate values typically are elevated by runoff of agricultural fertilizer and phosphorus

values by sewage effluent. Quebrada Bolsa, which flows close to the village La

Gamba and the Quebrada Negra next to the Esquinas Rainforest Lodge, show high

values of total phosphorus. The lowest amount of total phosphorus was recorded in

the Quebrada Mari, which is surrounded by an anthropogenic unaffected region.

43

The reported concentrations of nitrate, phosphate, total dissolved nitrogen and total

dissolved phosphorus, exhibit a clear relationship with land use. Streams like

Quebrada Gamba, Quebrada Bolsa, Rio Bonito and Rio Esquinas are draining

agricultural land, which has a strong influence on the river chemistry. These rivers

are next to plantations (cacao, oilpalms and other land use) and have higher nutrient

concentrations than those draining forested land. Lowlands are more heavily farmed

and settled than upland regions.

Streams within the Piedras Blancas National Park and its vicinity show significant

differences in their chemistry between the 'dry' and 'rainy' season. If the water

samples of Q.Negra, Q.Bolsa and Q.Gamba from the 27.04.2005 are assigned to the

'rainy' season, the differences become clearer.

Sites in pasture streams like Rio Bonito, Rio Oro, Quebrada Sardinal and Rio

Esquinas were less shaded than those of the forest stream.

Diverse assemblages of macroconsumers (i.e. fish and shrimps) characterize the

streams and rivers of Bosque Esquinas.

44

3. Quebrada Negra

3.1. MATERIAL AND METHODS

For a detailed study site description of the Quebrada Negra, a 100 m stream sector

next to the Biological Field Station was chosen. Transects were established at

regular intervals (5 m apart) by stretching a cord perpendicular to the current. In this

perpendicular stripes water depth and stream current velocity were measured every

0,2 m. The velocity was assessed with an Ottflügel, Type C2, in 40 % water depth

above streambottom. Flow and water volume were calculated to build up a depth and

current velocity profile. Furthermore stream width, stream bed width and slope angle

of the bank were measured. The 100 m sector was analyzed twice – once during the

'dry' season from 14th -18th February 2004 and once in the 'rainy' season from 6th - 8th

August 2004. A comparable study site was chosen 1 km upstream, next to the

Esquinas Rainforest Lodge, and three cross sections were constructed. Abiotic

parameters were recorded as mentioned above.

In the studied section of the Q. Negra four different habitat types (choriotopes) were

identified. Choriotopes are classified by a certain type of structure and differ in their

current velocity, depth and substrate. Four choriotop types are recognized in the

stream: riffles, shallow and slow sites, pools and cascades. Their position within the

100 m was recorded.

To start the hydrological measurement a water level gauge was installed. The water

level was recorded daily at 10 am, together with the precipitation and the minimum

and maximum temperature of the previous day. Precipitation was monitored in a rain

gauge in an open area at the La Gamba Field Station. A detailed short term

chronology of the water level fluctuations and precipitation during rainfall was

recorded on August 14th to show the effects of high rainfall. We estimated peak flows

(when wading was impossible) as the product of the cross-sectional area and a

single estimate of velocity.

45

Water temperature and oxygen content were measured with an WTW Oximeter 330.

A WTW pH-meter was used to record the pH-value. These abiotic parameters were

recorded three times within the stream course of the Q.Negra - see chapter 2.

Between February 2004 and February 2005 water samples were taken ten times and

analyzed for conductivity, pH, Alk., Cl-, SO42-, Si-SiO4, P-PO4, P-s, P-t, N-NO3. N-

NO2, N-NH4, N-sKj, N-tKj, Na+, K+, Ca2+, Mg2+ at the laboratory in Austria as soon as

possible.

The riparian vegetation was recorded to describe the ecotone, an intermediate zone

between the forest and the aquatic site, more detailed. We chose two sites of 10 m

width within the 100 m transect and another one at the comparable site for an

assessement of site specific trees, shrubs, herbs and the ground layer together with

their height, crown diameter and their distance to the bank.

To quantify the leaf litter input of the riverine vegetation 20 samples within the 100 m

section were taken at random. The leaves from an area of 0,108 m2 were dried and

weighted. This examination has been carried out twice in the 'dry' season (on the 5th

March and after heavy rain on the 29th March) and once in the 'rainy' season on the

8th August. At the same time we counted the number of leaf accumulations (> 0,025

m2) within the 100 m section.

Analysis of data

The statistical analysis of data was performed with the software package SPSS. The

PCA (principal components analysis) is a multivariate procedure which rotates that

data such that maximum variabilities are projected onto the axis. This was performed

on the basis of water chemistry data from the 'dry' and 'rainy' season.

3.2. RESULTS

cross section - 0 m - DS

0 2 4 6 8

bank

leve

l [m

]

-0,5

0,0

0,5

1,0

cross section - 0 m - RS

0 2 4 6 8

-0,5

0,0

0,5

1,0

1,68 1,57

cross section - 5 m - DS

0 2 4 6 8

bank

leve

l [m

]

-0,5

0,0

0,5

1,0

cross section - 5 m - RS

0 2 4 6 8-0,5

0,0

0,5

1,0

1,421,58

cross section - 10 m - DS

0 2 4 6 8

bank

leve

l [m

]

-0,5

0,0

0,5

1,0

cross section - 10 m - RS

0 2 4 6 8-0,5

0,0

0,5

1,0

1,50 1,40

cross section - 15 m - DS

stream width [m]0 2 4 6 8

bank

leve

l [m

]

-0,5

0,0

0,5

1,0

cross section - 15 m - RS

stream width [m]0 2 4 6 8

-0,5

0,0

0,5

1,0

2,26 2,20

46

cross section - 45 m - DS

0 2 4 6 8

bank

leve

l [m

]

-0,5

0,0

0,5

1,0

cross section - 45 m - RS

0 2 4 6 8-0,5

0,0

0,5

1,0

1,73 1,90 2,00 1,80

cross section - 50 m- DS

0 2 4 6 8

bank

leve

l [m

]

-0,5

0,0

0,5

1,0

cross section - 50 m - RS

0 2 4 6 8-0,5

0,0

0,5

1,0

1,50 1,67 1,70 1,90

cross section - 55 m - DS

stream width [m]0 2 4 6 8

bank

leve

l [m

]

-0,5

0,0

0,5

1,0

cross section - 55 m - RS

stream width [m]0 2 4 6 8

-0,5

0,0

0,5

1,0

1,78 3,80 1,75 3,60

47

cross section - 85 m - DS

0 2 4 6 8

bank

leve

l [m

]

-0,5

0,0

0,5

1,0

cross section - 85 m - RS

0 2 4 6 8-0,5

0,0

0,5

1,0

1,68 1,71 1,91 1,77

cross section - 90 m - RS

0 2 4 6 8

Y D

ata

-0,5

0,0

0,5

1,0

cross section - 90 m - DS

0 2 4 6 8

bank

leve

l [m

]

-0,5

0,0

0,5

1,0

1,79 1,78 2,05 1,70

cross section - 95 m - DS

0 2 4 6 8

bank

leve

l [m

]

-0,5

0,0

0,5

1,0

cross section - 95 m - RS

0 2 4 6 8-0,5

0,0

0,5

1,0

2,00 1,572,00 1,61

cross section - 100 m - DS

stream width [m]0 2 4 6 8

bank

leve

l [m

]

-0,5

0,0

0,5

1,0

cross section - 100 m - RS

stream width [m]0 2 4 6 8

-0,5

0,0

0,5

1,0

2,25 1,40 2,20 1,35

Fig. 3.1. Selected cross sections of riffles and pools (0 - 15 m, 45 - 55 m, 85 - 100 m) within the 100 m sector, showing stream bed width [m] and depth [m]

48

49

This comparison of the selected cross sections (Fig. 3.1.) within the 100 m sector of

the Q.Negra show the distinct increase of stream width and water depth from the 'dry'

to the 'rainy' season and its heterogeneity with alternating riffles and pools. The

course of the stream in both seasons and all the single transects within the 100 m

section can be seen in Figure 3.2 and 3.3. The position of the field station and the

type of riparian vegetation are indicated.

A series of figures compares the width, depth, current velocity and flow data of the

Q.Negra from the 'dry' and 'rainy' season. Average stream width (Fig.3.4.) shows a

distinct difference between the two seasons. The variance within the 100 m section is

higher during the 'dry' season than in the 'rainy' season. Mean depth (Fig. 3.5.) is

increasing from 0,089 m on the 13.02.2004 to 0,141 m on the 06.08.2004. Average

current velocity was 0,207 ms-1 in February and 0,297 ms-1 in August. This is

accompanied by a distinct increase in mean flow (Fig. 3.7.). On the 06.08.2004 (18,5

ls-1) mean flow is five times higher as in the 'dry' season on the 13.02.2004 (94,2 ls-1).

secondary forest

botanical garden

0 m

50 m

100 m

0 m 30 m

Fig.3.2. Course of the Q.Negra during the 'dry' season within the 100 m section with bank level [m] and water depth [m]. Position of the sampling points, type of riparian vegetation and the Field station are indicated.

50

secondary forest

botanical garden

0 m

50 m

100 m

0 m 30 m

Fig 3.3. Course of the Q.Negra during the 'rainy' season within the 100 m section with bank level [m] and water depth [m]. Position of the sampling points, type of riparian vegetation and Field station are indicated

51

wid

th [m

]

0

1

2

3

4

5

dry season13.02.2004

rainy season06.08.2004

Fig. 3.4. Stream width [m] of the Q. Negra in the 'dry' and 'rainy' season

dept

h [m

]

0,0

0,1

0,2

0,3

0,4

0,5

0,6

dry season13.02.2004

rainy season06.08.2004

Fig. 3.5. Stream depth [m] of Q. Negra during the 'dry' and 'rainy' season

52

curre

nt v

eloc

ity [m

s-1]

0,0

0,2

0,4

0,6

0,8

1,0

1,2

dry season13.02.2004

rainy season06.08.2004

Fig. 3.6. Current velocity [ms-1] of the Q.Negra in the 'dry' and 'rainy' season

flow

[ls-1

]

0

20

40

60

80

100

120

140

160

180

dry season13.02.2004

rainy season06.08.2004

Fig. 3.7. Flow [ls-1] of the Q.Negra during the 'dry' and 'rainy' season

53

longitudinal transect [m]

0 10 20 30 40 50 60 70 80 90 100

dept

h [m

]

0,0

0,1

0,2

0,3

0,4

0,5

0,6 riffle pool riffle pool rifflepoolriffle

longitudinal transect [m]

0 10 20 30 40 50 60 70 80 90 100

dept

h [m

]

0,0

0,1

0,2

0,3

0,4

0,5

0,6

riffle pool riffle pool riffleriffle pool

Fig. 3.8. Mean depth [m] and pool - riffle - alternation of the Q.Negra during the 'dry' and 'rainy' season within the longitudinal 100 m section and standard deviation

54

The two graphs(Fig.3.8.) show the natural depth profile of the 100 m section. The

longitudinal transect is characterized by four riffle and three pool sequences.

At the beginning of the 'rainy' season the Q. Negra started to erode the riverbank and

one of the houses and four trees of the botanical garden were endangered. Due to a

bank regulation at meter 20 till 35 the morphological regime in the 'rainy' season

differs from the one during the 'dry' season (Fig. 3.9).

longitudinal transect [m]0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

dept

h [m

]

0,0

0,1

0,2

0,3

0,4

0,5dry seasonrainy season

bank regulation

Fig. 3.9. Mean depth [m] of the Q.Negra during the 'dry' and 'rainy' season within the longitudinal 100 m section and standard deviation. Marked bank regulation site from the 'rainy' season.

55

longitudinal transect [m]

0 10 20 30 40 50 60 70 80 90 100

curr

ent v

eloc

ity [m

s-1]

0,0

0,2

0,4

0,6

0,8

1,0

1,2riffle pool riffle pool rifflepoolriffle

longitudinal transect [m]

0 10 20 30 40 50 60 70 80 90 100

curr

ent v

eloc

ity [

ms-1

]

0,0

0,2

0,4

0,6

0,8

1,0

1,2

riffle pool riffle pool riffleriffle pool

Fig. 3.10. Mean current velocity [ms-1] during the 'dry' and 'rainy' season within the 100 m sector and standard deviation

56

The two graphs (Fig. 3.13.) show the natural current velocity profile of the 100 m

section. The longitudinal transect is characterized by four riffle and three pool

sequences. Mean current velocities are higher in the 'rainy' season. – in pools speed

reaches 0,2 ms-1 and in riffles 0,6 ms-1. The highest recorded current velocity is 1,06

ms-1. During the 'dry' season current velocities in pools are about 0,1 ms-1 and 0,4

ms-1 in riffle sections. The highest recorded current velocity is 0,80 ms-1.

Due to a bank regulation at meter 20 till 35 the morphological regime in the 'rainy'

season differs from the one during the 'dry' season (Fig. 3.11.).

longitudinal transect [m]0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

curre

nt v

eloc

ity [m

s-1]

0,0

0,2

0,4

0,6

0,8

1,0

dry seasonrainy season

bank regulation

Fig. 3.11. Mean current velocity [ms-1] of the Q.Negra in the 'dry' and 'rainy' season within the 100 m section and standard deviation. Marked bank regulation site from the 'rainy' season.

57

longitudinal transect [m]0 10 20 30 40 50 60 70 80 90 100

dept

h [m

]

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,0

0,1

0,2

0,3

0,4

0,5

0,6

mean current velocity DS mean depth DS

curre

nt v

eloc

ity [m

s-1 ]

Fig. 3.12. Mean current velocity [ms-1] and mean depth [m] in the 'dry' season (DS) within the longitudinal 100 m section This graph shows now both – mean depth and mean current velocity during the 'dry'

season. It corresponds to the natural regime of a not regulated stream. Whereas

current velocities decrease at pool sequences, riffle sequences are characterized by

higher current velocities.

longitudinal transect [m]0 10 20 30 40 50 60 70 80 90 100

dept

h [m

]

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7average current velocity RSaverage depth RS

curre

nt v

eloc

ity [m

s-1]

Fig. 3.13. Mean current velocity [ms-1] and mean depth [m] in the 'rainy' season (RS) within the longitudinal 100 m section of the Q.Negra

58

This figure (3.13.) shows now both – mean water depth and the mean current velocity

during the 'rainy' season of the Q.Negra. Differences of mean depth and mean

current velocity between the two seasons is not only due to the natural dynamics but

also caused by a regulation of the riverbank, which was carried out between the 20

and 35 m cross transect.

Sites where high velocities were recorded, show low average depths. Contrary low

velocities occured at deep sections. A correlation of mean depth versus mean current

velocity from the 'dry' and 'rainy' season is shown in figure 3.14.

mean current velocity [ms-1]

0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35

mea

n de

pth

[m]

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

rainy seasonregression line RSdry seasonregression line DS

Fig. 3.14. The mean depth [m] plotted against the mean current velocity [ms-1] in the 'dry' and 'rainy' season. The lines are the regression for each sampling date.

59

longitudinal transect [m]0 10 20 30 40 50 60 70 80 90 100

flow

[ls-1

]

0

20

40

60

80

100

120

140

160

180

dry seasonrainy season

Fig. 3.15. Flow [ls-1] during the 'dry' (DS) and 'rainy' season (RS) within the longitudinal 100 m section of the Q.Negra Flow during the 'dry' season is quite steady with small deviations within the 100 m

longitudinal transect. Flow ranges from about 30 to 40 ls-1. Not much water seems to

disappear into the hyporheic zone. During the 'rainy' season flow gets a way larger

and ranges from about 90 to 100 ls-1. One large fluctuation to 150 ls-1 at meter 5

might be due to a measurement mistake.

60

date1.3. 1.4. 1.5. 1.6. 1.7. 1.8. 1.9. 1.10. 1.11. 1.12. 1.1. 1.2. 1.3. 1.4. 1.5. 1.6. 1.7.

wat

er le

vel [

m]

0,00

0,05

0,10

0,15

0,20

0,25

prec

ipita

tion

[m]

0,00

0,05

0,10

0,15

0,20

0,25

water levelprecipitation

2004 2005

Fig. 3.16. Water level of the Q.Negra and precipitation at the tropical field station La Gamba from 23rd February 2004 to 1st July 2005. Water level on 23.02.2004 was defined as a baseline of 0 to provide a measure of

range and fluctuation in water levels over the observation period. Water level cero

shows the baseflow of 0,031m3s-1. Within the sampling time it is the lowest observed

water level. The highest water level which lasted for several hours was 0,19 m. Water

levels rise and drop quickly. Sudden changes in water level were the rule throughout

the months we observed the water level of Q.Negra in 2004. High water levels

occured even in the 'dry' season after a rain, but the highest levels were noted in

months of heavy and continuous rain when the soil was soaked and runoff heavy.

Frequent afternoon and early evening rains with more than 20 mm precipitation

caused about an average raise of 1,25 cm in water level, which returned to near

normal during the night. The average decrease of the water level on days with no

precipitation is about 0,42 cm. Distinct fluctuations in water level between the two

seasons can be seen in figure 3.16.

61

time 1:00 2:00 3:00 4:00 5:00

wat

er le

vel [

cm]

0

10

20

30

40

50

60

70

80

water level

prec

ipita

tion

[mm

]

0

5

10

15

20

precipitation

Fig. 3.17. Short term fluctuation and detailed recording of the waterlevel and precipitation from 13:00 till 17:00 on the14.08.2004 at the Q.Negra The highest recorded water level from the Q. Negra was 65 cm during a heavy rain

on the 14th of August, but this lasted only for a few minutes. Two peaks of the rising

water level are shown in Figure 3.17. Rain started at 1.02 pm and the water level

started to rise slowly. The first water level peak of 50 cm at 2.30 pm was followed by

a drop to 21 cm 15 minutes later. Heavy rainfall started again at 3.15 pm and the

water level followed to the highest peak of 65 cm at 3.50 pm. Turbidity generally

increased during flooding but cleared quite quickly when rain stopped.

62

water level [m]0,00 0,05 0,10 0,15 0,20

time

[d]

0

50

100

150

200

250

300

350

Fig 3.18. Water level [m] of the Q.Negra at a certain amount of days [d]

This graph shows on how many days per year (366 days) a certain water level was

reached. Data is available from 23rd February 2004 till 22nd February 2005. A water

level of 0,01 m at the defined water gauge was reached 121 times, wheras a water

level of more than 0,10 m was recorded only 36 times. The highest assessed water

level was 0,19 m.

63

water level [m]0,00 0,02 0,04 0,06 0,08 0,10 0,12

flow

[ls-1

]

0

20

40

60

80

100

120

140

rainy season 06.08.2004

dry season 13.02.2004

y = 20,816e18,477x

R2 = 0,98

mean flow rainy season

mean flow dry season

Fig. 3.19. Measured flow [ls-1] of five different water levels [m]. Accounted mean flow [ls-1] of the Q. Negra during the 'dry' and 'rainy' season (according to mean water level of three month) and mean flow [ls-1] of 06.08 and 13.02.04, when the 100 m sector was constructed.

Flow for five different water levels was calculated based on current velocity and depth

profiles at the 55 m cross transect within the 100 m sector. At water levels above

0,10 m wading was impossible and it was not able to measure the current velocity.

Flow was calculated for five different water levels, where it was possible to measure

the current velocity. At a water level above 0,10 m we were not able to measure the

velocity anymore. To calculate the mean flow within the two seasons we took the

average water level of three months of each season. Streamflow was highly

seasonal. Average water level in the 'dry' season (23.02 - 29.02.2004 / 01.12.04 -

23.02.2005) is 0,41 cm and 2,72 cm in the 'rainy' (01.08 - 30.10.2004) season. This

includes average flows of 22,5 and 58,9 m3s-1. The relative magnitude of high flow

episodes generallly corresponded to measured precipitation.

64

mean depth [m]0,04 0,06 0,08 0,10 0,12 0,14 0,16 0,18 0,20 0,22

mea

n cu

rrent

vel

ocity

[ms-1

]

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

BADEAB

Fig. 3.20. Classified choriotop types (A - riffles, B - shallow sites with low current velocity, D - pools, E - cascades) according to mean depth [m] and mean current velocity [ms-1] within the Q.Negra The Q. Negra displays a more or less regular alternation between shallow areas of

higher velocity and mixed gravel-cobble substrate (riffles) and deeper areas of slower

velocity and finer substrate (pools). The four classified choriotop types within the Q.

Negra are based on their different morphological and hydrological properties. At the

most general level of resolution, we divide channel units into fast- and slow-water

categories that approximately correspond to the commonly used terms 'riffle' (A) and

'pool' (D). Within the slow-water categories, we also identified shallow habitats (B),

where waterdepth was not more than 5 cm. At the comparable study site to the 100

m section, we identified another choriotop type (E). Cascades are marked by a steep

incline and rocks with a diameter of 20-50 cm. Water depth is in general shallow and

mean current velocity is 0,18 ms-1. In the 'rainy' season choriotop B was gone and

therefore a fifth choriotop type (AB) was classified. AB could not either be assigned to

A or D. AB is not as deep as pools (D), not as shallow as B and has a lower current

velocity than riffles (A).

65

66

Tab. 3.1. Hydrochemical characteristics of the Q.Negra at ten sampling times between February 2004 and February 2005

26.0

2.04

03.0

4.04

27.0

4.04

24.0

6.04

03.0

8.04

20.0

9.04

12.0

2.05

13.0

2.05

15.0

2.05

*

15.0

2.05

**

Lf [µScm-1; 25°C] 200 175 185 197 196 177 205 204 152 168 pH 8,1 8,4 8,4 7,9 8,5 8,5 8,4 8,2 8,1 8,2 Alk. [mgl-1] 128,1 112,3 117,2 129,4 129,4 114,7 125,7 125,7 92,7 103,7 Cl- [mgl-1] 0,4 0,5 0,4 0,5 0,6 0,4 1,4 1,2 1 0,6 SO4

2- [mgl-1] 1,3 1,4 1,4 1,1 1,1 0,9 1,3 1,3 1,2 1,1

Si-SiO4 [mgl-1] 26,2 23,8 25 29,7 30,5 13,3 26,8 26,7 22,3 22 P-PO4 [µgl-1] 48 26 45 40 46 47 39 31 31 36 P-s [µgl-1] 49 28 45 42 46 47 39 31 32 38 P-t [µgl-1] 50 32 46 46 49 53 50 42 45 56 N-NO3 [µgl-1] 64 95 66 37 69 143 36 110 86 82 N-NO2 [µgl-1] 0 0 1 0 1 1 0,3 3,1 1 1 N-NH4 [µgl-1] 0 16 9 0 4 48 27 4 4 4 N-sKj [µgl-1] 28 49 18 16 54 77 44 27 69 64 N-tKj [µgl-1] 39 70 39 33 67 88 83 86 109 138 Na+ [mgl-1] 6,9 6,1 6,4 7,9 7,3 6,3 7,5 7,3 5,7 5,7 K+ [mgl-1] 0,8 0,8 0,8 0,4 0,7 0,6 0,8 0,9 0,8 0,8 Ca2+ [mgl-1] 24 20,4 20,8 26,9 26,5 22,6 26,2 26 19 22,5 Mg2+ [mgl-1] 5,6 4,8 4,9 6,4 6,4 5,4 6 6 4,4 4,5 Na+ [mval] 0,3 0,3 0,3 0,3 0,3 0,3 0,3 0,3 0,2 0,2 K+ [mval] 0 0 0 0 0 0 0 0 0 0 Ca2+ [mval] 1,2 1,0 1,0 1,3 1,1 1,1 1,3 1,3 0,9 1,1 Mg2+ [mval] 0,5 0,4 0,4 0,5 0,4 0,4 0,5 0,5 0,4 0,4 A [mval] 2,1 1,8 1,9 2,1 2,1 1,9 2,1 2,1 1,5 1,7 Cl- [mval] 0 0 0 0 0 0 0 0 0 0 SO4

2- [mval] 0 0 0 0 0 0 0 0 0 0

san [mval] 2,0 1,7 1,7 2,2 2,2 1,9 2,1 2,1 1,6 1,8 skat [mval] 2,1 1,9 2,0 2,2 2,2 1,9 2,1 2,1 1,6 1,7 an% 108,3 110,5 112,7 97,0 99,1 102,6 99,1 99,4 99,4 98,9 Ion balance [diffkat%] -8,3 -10,5 -12,7 3,0 0,9 -2,6 0,9 0,6 0,6 1,1 (* 5 pm, ** 7.30 pm)

A summery description of the chemical characteristics of the Q.Negra is provided in

Table 3.1. Conductivity ranges between 175 and 200 µScm-1 and was relatively

constant seasonally such as the pH-value and the chlorid concentration. The pH

ranges from 7,9 to 8 and the chlorid concentration shows values between 0,4 mgl-1

and 1,4 mgl-1.

Sulfide concentrations showed a distinct seasonal pattern. Concentrations are higher

during the 'dry' than in the 'rainy' season; in February and April values range from 1,1

to 1,4 mgl-1 whereas values during the 'rainy' season range from 0,9 to 1,1 mgl-1.

67

The stream has its lowest phosporus (26 µgl-1), soluble phosphorus (28 µgl-1) and

total phosphorus (32 µgl-1) concentration on the 3rd April 2004 and shows no distinct

seasonal differences such as the concentration of S-SiO4, where values are ranging

from 13,3 mgl-1 in September to 30,5 mgl-1 in August.

No clear trend between the 'dry' and 'rainy' season is also perceptible according to

the nitrate, nitrit and ammonium concentrations. Nitrit concentrations are low. Values

of 0 µgl-1 were measured three times (20.02./03.04./24.06.2004) and concentrations

of 1 µgl-1 were measured five times within the sampling period. The highest recorded

nitrit concentration of 3,1 µgl-1 was on the 13th February 2005. No ammonium was

measured on the 20th February and 24th June 2004. The highest ammonium

concentration was 48 µgl-1 in September 2005.

Concentrations of N-sKj, N-tKj show no seasonal pattern. On the 24th June, the

stream has its lowest (16 µgl-1) concentration of N-sKj. The highest value of 77 µgl-1

occurs in September. On the 24th June, Quebrada Negra has the lowest (33 µgl-1)

concentration of N-tKj. The highest value of 138 µgl-1 occurs on the 15th February

2005 after rain.

Sodium concentrations range between 6,1 mgl-1 in April 2004 and 7,9 mgl-1 in June

2004. Seasonal differences are slight.

Potassium and calcium concentrations showed a distinct seasonal pattern. The

potassium content is in general higher during the 'dry' season than in the 'rainy'

season. Samples in the 'dry' season have a value of 0,8 mgl-1. The lowest content of

potassium (0,4 mgl-1) was measured in June 2004. During the 'rainy' season the

content of calcium is higher than in the 'dry' season. The lowest content of calcium

(20,4 mgl-1) was measured in April 2004.

Magnesium concentrations range between 4,4 mgl-1 in February 2005 and 6,4 mgl-1

in June and August 2004. Seasonal differences are not perceptible.

34,67FAC 1

geochemical parameters

-2 -1 0 1 2

phos

phor

usFA

C 2

25,1

5

-3

-2

-1

0

1

2

dry seasonrainy season

low high

low

high

Fig. 3.21. Ordination biplot resulting from a PCA (principal component analysis) of the chemical analyses of ten water samples during the 'dry' and 'rainy' season in the Q.Negra

The results of the PCA (principal component analysis) show the distribution pattern of

the sampling dates in relation to the first two axis. The PCA describes 59,82 % of the

total physicochemical variability during the 'dry' and 'rainy' season of the Q.Negra.

“Axis 1” accounts for 34,67 % of the variance. “Axis 2” explains 25,15 %. In Figure

3.21. the sampling dates are lined up along axis 1. Full black signs stand for the 'dry'

season and the white signs for the 'rainy' season. The principal components are the

geochemical parameters (Na+, K+, Ca2+, Mg2+) – axis 1 and the phosphorus

concentration – axis 2. Except of two sampling dates within the 'dry' season, it seems

that watersamples from the 'rainy' season have higher concentration of geochemical

parameters.

68

69

Tab. 3.2. Riparian vegetation at the Q. Negra with type of growth, frequency (++ often, + present, - absent), height [m] and distance to the riverbank [m] of recorded plants.

type of growth frequency height [m] distance to the riverbank [m]

taxon 45-55 m /85-95 m / Lodge MONOCOTYLEDONS Araceae Dieffenbachia oerstedii herb - / - / + - / - / - - / - / - Homalomena wendlandii herb + / - / + - / - / - - / - / - Philodendron sp. herb + / - / - - / - / - - / - / - Spathiphyllum silvicola herb + / - / - - / - / - - / - / - Arecaceae Bactris glandulosa palm - / - / + - / - / - - / - / - Prestoea decurrens palm - / - / + - / - / - - / - / - Welfia regia palm - / - / + - / - / 15 - / - / - Costaceae Costus pulverulentus herb + / + / - 0,6 / - / - - / - / - Cyclanthaceae Asplundia pittieri herb - / - / + - / - / - - / - / - Carludovica drudei giant herb + / + / + - / - / - - / - / - Cyclanthus bipartitus giant herb - / - / + - / - / - - / - / - Heliconiaceae Heliconia danielsiana herb - / - / + - / - / - - / - / - Heliconia imbricata herb - / - / + - / - / - - / - / - Heliconia latispatha herb ++ / + / - 2 / - / - - / - / - Heliconia trichocarpa herb - / - / + - / - / - - / - / - Heliconia sp. herb ++ / + / - - / - / - - / - / - Marantaceae Calathea lutea giant herb - / + / - - / - / - - / - / - Calathea crotalifera giant herb - / - / ++ - / - / - - / - / - DICOTYLEDONS Annonaceae Guatteria chiriquiensis tree + / + / + 8 / 25 / - 6 / 3 / - Anacardiaceae Spondias mombin tree - / + / - - / 27 / - - / - / - Boraginaceae Cordia cymosa tree - / - / + - / - / 28 - / - / 14 Cecropiaceae Cecropia obtusifolia tree - / - / ++ - / - / 22 - / - / - Cecropia insignis tree - / - / + - / - / 10 - / - / - Convolvulaceae Dicranostyles sp. liana + / - / - - / - / - - / - / - Dilleniaceae Doliocarpus hispidus liana + /- / - - / - / - - / - / - Elaeocarpaceae Sloanea sp. tree - / - / + - / - / 6 - / - / - Euphorbiaceae Acalypha diversifolia shrub/tree ++ / + / + 6 / - / 6 0 / 0 / 0 Hyeronima alchorneoides tree + / - / - 20 / - / - - / - / - Fabaceae-Faboideae Lonchocarpus sp. shrub/tree - / + / + - / 22 / 12 - / - / - Mucuna sp. liana + / - / - - / - / - - / - / - Fabaceae-Caesalpinioideae Bauhinia bahiachalensis liana + / - / - - / - / - - / - / - Flacourtiaceae

70

Tetrathylacium macrophyllum tree - / - / + - / - / 15 - / - / - Gesneriaceae Episcia lilacina herb + / - / - - / - / - - / - / - Lecythidaceae Grias cauliflora tree - / - / + - / - / - - / - / - Lythraceae Lagerstroemia c.f. tree + / - / - 13 / - / - 5 / - / - Melastomataceae Clidemia dentata shrub + / - / - - / - / - - / - / -

Blakea gracilis epiphytic

shrub/tree - / - / + - / - / - - / - / - Meliaceae Carapa guianensis tree/treelet - / - / + - / - / 25 - / - / - Moraceae Artocarpus altilis tree - / + / - - / 25 / - - / 0,5 / - Castilla tunu tree - / + / + - / - / 18 - / - / - Ficus tonduzii tree - / - / + - / - / - - / - / - Myristicaceae Virola koschnyi tree + / + / - 3 / 10 / - - / 1 / - Virola guatemalensis tree - / - / + - / - / 14 - / - / - Piperaceae Peperomia saintpauliella herb - / - / + - / - / - - / - / - Piper auritum shrub/tree ++ / + / - 2 / - / - 0 / - / - Rubiaceae Pentagonia wendlandii shrub/treelet - / - / + - / - / - - / - / - Pentagonia tinajita treelet - / - / + - / - / - - / - / - Psychotria elata shrub + / - / - 4 / - / - - / - / - Psychotria solitudinum shrub + / - / - 3 / - / - - / - / - Simira maxonii tree + / - / - - / - / - - / - / - Sapindaceae Cupania livida shrub/tree - / + / - - / 17 / - - / - / - Sterculiaceae Theobroma cacao tree ++ / + / - 1,5 / 7 / - - / - / - Tiliaceae Apeiba tibourbou tree - / - / + - / - / - - / - / - Luehea seemannii canopy tree ++ / + / + 25 / 23 / - 1,5 / 1 / - Trichospermum grewiifolium canopy tree ++ / - / - 20 / - / - - / - / - Urticaceae Myriocarpa longipes shrub/tree + / - / ++ 3 / - / - 0 / - / - Urera elata c.f. shrub/tree + / - / - - / - / - - / - / - Violaceae Rinorea dasyadena c.f. tree - / - / + - / - / - - / - / - PTEROPHYTES Oleandraceae Nephrolepis sp. fern + / + / - - / - / - 4 / 0 / - Cyatheaceae Alsophila firma treefern - / - / + - / - / 5 - / - / - LYCOPHYTES Selaginellaceae Selaginella sp. ground layer + / + / + - / - / - 2 / 2 / 1

71

The abundant riparian vegetation at the Quebrada Negra maintains mostly typical

species which occur along the streams and rivers in this region of Costa Rica. The

riverine vegetation was recorded three times along the streamrun. Twice within the

100 m sector (at the 45 - 55 m and 85 – 95 m transect) and once at the Lodge site.

Trees of the canopy layer include Guatteria chiriquiensis (Annonaceae), Spondias

mombin (Anacardiaceae), Cordia cymosa (Boraginaceae), Cecropia obtusifolia

(Cecropiaceae), Sloanea medusula (Elaeocarpaceae), Hyeronima alchorneoides

(Euphorbiaceae), Lonchocarpus sp. (Fabaceae – Faboideae), Carapa guianensis

(Meliaceae), Artocarpus altilis, Castilla tunu (Moraceae), Cupania livida

(Sapindaceae), Apeiba tibourbou, Luehea seemannii, Trichospermum grewiifolium

(Tiliaceae).

Common species of the mid tree layer are Welfia regia (Arecaceae), Cecropia

insignis (Cecropiaceae), Tetrathylacium macrophyllum (Flacourtiaceae),

Lagerstroemia c.f. (Lythraceae), Virola koschnyi and Virola guatemalensis

(Myristicaceae) and Theobroma cacao (Sterculiaceae).

The shrub stratum contains species like Dieffenbachia oerstedii, Homalomena

wendlandii, Philodendron sp., Spathiphyllum silvicola (Araceae), Costus

pulverulentus (Costaceae), Carludovica drudei, Cyclanthus bipartitus

(Cyclanthaceae), Heliconiaceae, Calathea lutea (Marantaceae), Piper auritum

(Piperaceae), Pentagonia wendlandii, Psychotria elata (Rubiaceae), Myriocarpa

longipes, Urera elata (Urticaceae) and Nephrolepis sp. (Oleandraceae).

The ground layer is bare except for Episcia lilacina (Gesneriaceae) and Selaginella

sp.

Fig. 3.22. Riparian vegetation of the Q.Negra between 45 m and 55 m of the 100 m section

72

Fig. 3.23. Riparian vegetation of the Q.Negra between 85 m and 95 m of the 100 m section

73

Fig. 3.24. Riparian vegetation of the Q.Negra at the comparable study site next to the Esquinas Lodge

74

75

Tab. 3.3. Number of leaf accumulations (within the stream) of three sampling dates (twice in the 'dry' season and once in the 'rainy' season) within each 5 m transect of the 100 m section at the Q.Negra amount of leaf accumulations transect [m] 05.03.2004 29.03.2004 08.08.20040-5 m 1 1 3 5-10 m 3 3 3 10-15 m 4 2 2 15-20 m 5 1 3 20-25 m 2 2 0 25-30 m 2 1 2 30-35 m 6 2 2 35-40 m 8 1 1 40-45 m 3 1 3 45-50 m 2 2 2 50-55 m 1 0 1 55-60 m 1 2 0 60-65 m 1 0 1 65-70 m 1 1 2 70-75 m 1 0 0 75-80 m 0 0 1 80-85 m 0 1 1 85-90 m 1 0 0 90-95 m 4 2 2 95-100 m 2 1 0 sum 48 23 29

The highest number of leaf accumulations (48) within the 100 m sector of the Q.

Negra were found during the 'dry' season on the 05.03.3004. The debris record of the

29.03.2004 after a flood event shows a distinct decrease of leaf accumulations, only

23 debris dams were present. During the 'rainy' season on the 08.08.2004 only 29

debris dams were found.

longitudinal transect [m]

0-15

5-20

10-2

5

15-3

0

20-3

5

25-4

0

30-4

5

35-5

0

40-5

5

45-6

0

50-6

5

55-7

0

60-7

5

65-8

0

70-8

5

75-9

0

80-9

5

85-1

00

debr

is [g

m-2

]

0

50

100

150

200

250

debris DS 05.03.2004debris DS 29.03.2004debris RS 08.08.2004

dept

h [m

]

0,04

0,06

0,08

0,10

0,12

0,14

0,16

0,18

0,20

0,22

depth DSdepth RS

Fig. 3.25. Amount of debris [gm-2] of three pooled transects within the 100 m sector – Q.Negra The mean amount of debris and mean water depth of the Q.Negra within three

pooled transects is shown in Figure 3.25. Debris is higher in the 'dry' season than in

the 'rainy' season. Values of more than 100 g per squaremeter were often recorded

on the 5th of March. The highest value in the 'rainy' season was 17,8 g. Moreover the

graph shows a distinct decrease in debris on the 29th of march, the day after flooding.

In comparison to the 5th of march at most of the sites debris is lower. At riffle sections

more leaf litter was found than in pool sections.

76

dry

wei

ght [

gm-2

]

0

50

100

150

200

250

dry season 05.03.2004

dry season29.03.2004

rainy season08.08.2004

Fig. 3.26. Mean amount [gm-2] of collected leaf litter on three sampling dates (05.03. / 29.03. / 08.08.04) within the 100 m transect during 'dry' and 'rainy' season The graph shows the huge difference between the two seasons. More leaf litter was

found during the 'dry' season in the Q. Negra. Reasons are the higher leaf fall during

the 'dry' season and during the 'rainy' season most of the leaf litter gets transported

downstream due to the floodpulses and the higher flow. After a flood event on the

29th of March leaf litter was transported downstream, but still more debris than in the

'rainy' season was found. In August just 4,61 gm-2 were recorded.

77

depth [m]0,04 0,06 0,08 0,10 0,12 0,14 0,16

dry

wei

ght [

gm-2

]

0

50

100

150

200

250

Fig. 3.27. The leaf litter dry weight [gm-2] plotted against the stream depth [m] in the 'dry' season (05.03.2004). The line is the regression.

The mean amount of debris within three pooled transects plotted against the stream

depth is shown in Figure 3.27. Within the 100 m sector of the Q.Negra the dry weight

of the debris during the 'dry' season on the 05.03.2004 correlates with the stream

depth. Clear differences between riffles and pools according to their amount of debris

dams are visible. At riffle sections more leaf litter was found than in pool sections,

because at shallow sites more leaves are accumulating in front of the stones of the

Q.Negra.

78

79

3.3. DISCUSSION The natural dynamics of the stream are expressed in the alternating riffle and pool

sections. The 100 m transect of the Q. Negra represents the expected trend of high

heterogeneity. Current velocity, stream depth, stream bed width and sediment size

fluctuate within the longitudinal 100 m section. Geomorphic changes between the

'dry' and 'rainy' season of the 100 m logitudinal transect are slight (Fig. 3.9.). Riffle –

pool – alternation of the 'dry' season corrsponds to the 'rainy' season, except

between the 20 - 35 m transect, which is caused by a regulation of the riverbank.

In the year 2004 'rainy' season started at the end of march. This was two to three

weeks earlier in comparison to the recorded precipitation data of former years at the

Field Station La Gamba. Streams in this seasonal ('dry' and 'rainy' season)

environment exhibit a high annual variation in discharge and turbidity. In the 'rainy'

season, frequent (almost daily) storms can cause bankfull conditions and in some

cases flooding. Water level fluctuations are following the precipitation. The 'dry' and

'rainy' season can be distinguished at first sight, but the difference in mean water

level is slight. The seasonal differences in baseflow discharge, however, show a

distinct increase from the 'dry' to the 'rainy' period. The mean flow in the 'rainy' period

is about 60 ls-1 and therefore 3 times as high as in the 'dry' season.

Like most Central American rivers (BUSSING 1993), which do not sustain flood

conditions for long periods between successive downpours, the short term

fluctuations of the Q. Negra are very typical and play a more important role than the

long term fluctuations. The water level rises quickly after the beginning of rain

because of the soil. Water discharges above ground and not much water disappears

in the hyporheique interstitial, which is shown by a comparison of flow values at

different cross transects (Fig. 3.15.). Flow during flood conditions could not be

measured but was estimated for 16000 ls-1 at a water level of 110 cm. Water levels

drop very quickly, which can be explained by the high resilience of the rainforest.

Precipitation and discharge have enormous effects on various morphological and

hydrological parameters of the stream. The results of the morphometric-hydrological

measurements at the Q. Negra show a clear picture of a natural, dynamic and

unregulated streamcourse. The four classified habitat types in the 'dry' season, which

differ in their current velocities and mean water depths also show the heterogeneous

80

run of the stream. In the 'rainy' season a fifth habitat type (choriotop AB) was

recorded.

The chemical data reported from our study stream consists of ten spot-

measurements (three in the 'rainy' and seven in the 'dry' season), indicating the

variable conditions during the seasons.

Tropical rivers in general have low concentrations of chemical constituents compared

with temperate rivers. This is because tropical rivers are for the most part

precipitation-dominated. Local geology can also be important. Geology plays a great

role together with land use in determining the nutrient concentrations of unpolluted

waters. The chemical data of the water samples from the Q. Negra indicate its source

in the Fila Gamba, Golfo Dulce area.

Magnesium and Calcium originate almost entirely from the weathering of rocks,

particularly magnesium-silicate minerals. Conductivity, magnesium, calcium and

sulfate concentration is lower and the phosphorus concentration is higher than in

other rivers from the region, which are sourced from the Fila Cruces. The Quebrada

Negra, located next to the Esquinas Rainforest Lodge, shows high values of total

phosphorus. Phosphorus values are typically elevated by sewage water.

The riparian vegetation of the Quebrada Negra is dense and consists of typical

species which occur along the streams and rivers in this region of Costa Rica. Higher

leaf fall during the 'dry' season is responsible for the higher leaf litter input in the

stream, and during the 'rainy' season most of the leaf litter gets transported

downstream due to the floodpulses and the higher flow.

4. Macroinvertebrates and leaf litter decomposition in the Quebrada Negra

The decomposition and macroinvertebrate colonisation of exposed leaf litter was

investigated using litter bags placed over a 28 day period in the Quebrada Negra.

Leaf decay rates and macroinvertebrate densities on leaf packs of Cecropia

obtusifolia (Cecropiaceae), Acalypha diversifolia (Euphorbiaceae), Tetrathylacium

macrophyllum (Flacourtiaceae) and Sloanea medusula (Elaeocarpaceae) were

compared. The abundance and taxonomic composition of invertebrates colonizing

the leaves were recorded. Anatomical cuts of the leaf material show the varying

nature of the leaf structure and support the results of the differences in decay (in

prep. GUSENLEITNER & RIEMERTH 2005).

The survey of the riverine vegetation (see chapter 3) provided a basis for the

selection of the four plant species. The trees were selected according to life history

strategies and abundance. All four chosen plant species are common and typical for

the riparian vegetation in this area of Costa Rica.

Tab. 4.1. Plant species of the riparian vegetation and their life history strategy used for the litter bag experiment taxon life history strategy Cecropia obtusifolia (Cecropiaceae) r Acalypha diversifolia (Euphorbiaceae) features of r and K Tetrathylacium macrophyllum (Flacourtiaceae) features of r and K Sloanea medusula (Elaeocarpaceae) K ACALYPHA DIVERSIFOLIA (Euphorbiaceae) – Plate 1a Acalypha diversifolia is a monoecious shrub or small tree up to 5(-15)m tall, which is

widely distributed in tropical America. It is a very common species of the Central

American lowlands in evergreen and deciduous forests from southern Mexico to

Peru, Bolivia and Brazil and one of the most abundant shrubs of the region. The plant

exhibits r and K features; light demanding, often present in the understory along

streamruns, anemophily.

A. diversifolia (Jacq). Leaves chartaceous, narrowly elliptic to ovate-elliptic or oblong-

lanceolate, 6-20 cm long, 2-8 cm wide, base obtuse to cuneate, margin dentate,

81

sparsely pubescent above, pubescent along the veins beneath; inflorescences

axillary, spicate, 2-10cm long, mostly bisexual, with a few female flowers at base;

male flowers minute, calyx cupular, petals absent, stamens (4-)8(-16); female flowers

minute, 2-3, subtended by a foliaceous, 3-4 mm long bract, petals absent, ovary (2-3)

locular; fruits small capsules with 3 bivalved cocci, ca. 2,5 mm in diameter.

CECROPIA OBTUSIFOLIA (Cecropiaceae) – Plate 1c Common name (Costa Rica): guarumo (BURGER 1977, HOLDRIDGE et al. 1997)

Cecropia obtusifolia is a shortliving pionieer species (r- strategist) with prominent stilt

roots. Trees are up to 23 (20) m tall and reach a DBH of 25,3 cm. They are common

species on wet sites in clearings and secondary forest, from southern Mexico to

Ecuador. Cecropia obtusifolia is a light demanding species, grows fast and often

appears at early successions as a monoculture.

Leaves are deeply lobed, lobes usually 11-13, stipules fully amplexicaul, 5-12 cm

long; staminate inflorescences pedunculate clusters of 12-18 spikes, these 8-22 cm

long, subsessile or up to 5 mm stipitate, pistillate inflorescences pedunculate clusters

of usually 4 spikes, these 18-50 cm long, sessile or shortly stipitate, accrescent in

fruit; fruits small achenes, 2 mm long, 1,2 mm wide, usually flattened.

Cecropia obtusifolia lives in close association with the heavily stinging Azteca ants.

These animals inhabit the hollow stems of the trees, which are subdivided by thin

transverse walls at each node (Zizka 1990). Due to the ants no epiphytic growth on

Cecropia is possible. Large trunks can be used as water gutter.

SLOANEA MEDUSULA (Elaeocarpaceae) – Plate 1b Sloanea medusula is a large tree with long-lived leaves of primary rainforests from

Guatemala to Colombia. Trees are up to 40 m tall; leaves alternate, coriaceous,

glabrate to puberulent, margin undulate to irregular; inflorescences 4-18 cm long;

flowers 10-20 mm in diameter, usually pinkish; fruits 4-valved capsules, to 4,5 cm

long, subglobose, seed 1. Sloanea medusula is a K- strategist; rare, tall, canopy

cover, hardwood, fruits are dispersed by birds.

82

TETRATHYLACIUM MACROPHYLLUM (Flacourtiaceae) – Plate 1d The genus Tetrathylacium belongs to the pan-tropical family Flacourtiaceae. Two

species are known: T. macrophyllum POEPP. & ENDL. (Synonyms: T. Costaricense

STANDL., T. Nutans Sleum., T. pacificum STANDL., Edmondstonia pacifica Seem.) and

T. Johanseni Standl., which both occur on the south pacific part of Costa Rica. In

contrast to T. macrophyllum, T. Johanseni has no myrmecophytic traits.

T. macrophyllum is a treelet growing mainly in the forest understorey. It is found

preferentially on steep slopes near rivers and creeks in primary forest (JANZEN 1983).

The average height is 8 m but it may reach a maximum height of 15-20 meters.

leaves oblong, entire to serrate with caducous stipules; inflorescences usually

axillary, pendent with numerous spike-like secondary axes; flowers reddish-purple or

maroon, spaced along spikes; fruits subglobose, redpurple, up to 2,5 cm in diameter.

The distribution area of T. macrophyllum is mostly in primary and secondary lowland

rain forest, from Costa Rica to Amazonian Peru and Brazil. The altitudinal distribution

ranges from 0 up to 1500 m. The plant exhibits r and K features; often present in the

understory along streamruns or at gaps, rapid growth, seeds are dispersed by birds.

83

4.1. MATERIAL AND METHODS Preparation of litter bags

Litter bags were made out of nylon mesh (10 × 10 mm) and cotton binding. Leaves

were collected from trees in the riparian vegetation at the Quebrada Negra. Only old

leaves, prior to abscission, were used. Small stones were added to each litter bag to

ensure that contents were in contact with the stream bed. 16 litter bags of each

species (8 g of leaves) were tied on strings and placed in the stream parallel to the

current. The experimental sites were located in riffles of moderate depth and velocity

(approximately 0,08 m deep and 0,2 ms-1).

Collection and processing of litter bags

The plants were exposed over a 28 day period in the stream. Two bags of each

species were collected after 4, 8, 14, 21 and 28 days by placing a net (200 µm) under

each so as not to lose colonizing invertebrates. After cutting the attachment string the

litter bags were transferred to a plastic tube and transported to the laboratory.

Invertebrates were washed from the leaves, sorted and preserved in 70 % ethanol.

Macroinvertebrates were later counted and determined to family level. Insects were

assigned to functional feeding groups following MERRITT and CUMMINS (1996).

Leaves were dried to constant weight at 70°C and weighed. The experiment ran from

28.02.2004 to 27.03.2004. The study came to a premature close owing to a high

water and loss of the remaining litter bags.

Anatomical leaf cuts were made of Acalypha diversifolia (Euphorbiaceae), Cecropia

obtusifolia (Cecropiaceae), Sloanea medusula (Elaeocarpaceae) and Tetrathylacium

macrophyllum (Flacourtiaceae) (GUSENLEITNER & RIEMERTH 2005).

Analysis of data

The loss of leaf mass over time is approximately log-linear, although some data have

been interpreted as linear or as consisting of two or more distinct stages. WEBSTER

and BENFIELD (1986) argue that a simple exponential model provides a general and

utilitarian description of the breakdown process. The exponential model Wt = Wo e –kt

(where Wt is the amount of leaf litter remaining after time, t, of the initial amount Wo,

and k is the processing coefficient) was used to calculate processing coefficients.

The exponent k (in units days-1), which is the slope of the plot of loge of leaf mass

84

versus time, provides a single measure of breakdown rate. This model assumes that

there is a constant fractional loss of material present at any given time.

The statistical analysis of data was performed with the software packages SPSS and

CANOCO. The influence of plant taxon and time of exposition within the stream on

the macroinvertebrate colonization was tested by a univariate analysis of variance. A

one way ANOVA also tested if there is a signifikant difference of the

macroinvertebrate colonization within the four plant species. Only species, who

appeared in more than three samples were used in the analysis.

The DCA (detrended correspondence analysis) is a procedure of analysis of

gradients. This was performed on the basis of the macroinvertebrate distribution in

context with the time of exposition of the leaf material. The results are represented as

a Biplot.

85

4.2. RESULTS

exposure time [d]0 5 10 15 20 25 30

dry

wei

ght [

g]

0

1

2

3

4

5

6 Acalypha diversifolia (Euphorbiaceae)Cecropia obtusifolia (Cecropiaceae)Tetrathylacium macrophyllum (Flacourtiaceae)Sloanea medusula (Elaeocarpaceae)

Fig. 4.1. Single values and mean loss of weight from four different types of leaf packs (Acalypha diversifolia, Cecropia obtusifolia, Tetrathylacium macrophyllum, Sloanea medusula) exposed in the Q. Negra for 28 days.

exposure time [d]0 5 10 15 20 25 30

dry

wei

ght [

%]

0

20

40

60

80

100

Acalypha diversifolia (Euphorbiaceae) Cecropia obtusifolia (Cecropiaceae) Tetrathylacium macrophyllum (Flacourtiaceae) Sloanea medusula (Elaeocarpaceae)

Fig. 4.2. Single values of percentage loss [%] and mean percentage [%] loss of weight from four different types of leaf packs (Acalypha diversifolia, Cecropia obtusifolia, Tetrathylacium macrophyllum, Sloanea medusula) exposed in the Q. Negra for 28 days.

86

The differences of the four plant species in decay and loss of weight within the

exposure time are shown in Figure 4.1. and Figure 4.2. Weight loss from leaf packs

was affected dramatically by litter type. Sloanea medusula (Elaeocarpaceae), the K-

strategist does not show the rapid loss of weight as the between r- and K-strategist,

Tetrathylacium macrophyllum (Flacourtiaceae). By day 28, Tetrathylacium

macrophyllum packs had lost 72,28 % of their initial weight, Acalypha diversifolia had

lost 66,93 %, Cecropia obtusifolia had lost 54,66 %, but leaves of Sloanea medusula

had lost only 20,13 % on day 24. The weight loss of Tetrathylacium macrophyllum

(e.g. 47,24 % loss after 14 days) was consistently faster than that of the Sloanea

medusula (e.g. 13,60 % loss after 17 days). Tetrathylacium was after 28 days of

exposure almost grazed down to the leaf nervation.

Tab. 4.2. Values of daily litter decomposition rate (k) from four different types of leaf packs (Acalypha diversifolia, Cecropia obtusifolia, Tetrathylacium macrophyllum, Sloanea medusula) exposed in the Q. Negra for 28 days, obtained from the slopes of regression equations, between log Wt and t taxon W0 Wt t k Acalypha diversifolia (Euphorbiaceae) 3,16 1,045 28 0,0395198Cecropia obtusifolia (Cecropiaceae) 3,295 1,71 28 0,0234255Tetrathylacium macrophyllum (Flacourtiaceae) 3,175 0,88 28 0,0458265Sloanea medusula (Elaeocarpaceae) 4,67 3,73 24 0,0093646 The relationship between the leaf litter remaining (Wt) of each plant species and their

values for daily litter decomposition rates (k) is presented in Table 4.2.

According to the “hierarchy of species along a processing continuum” of PETERSEN

and CUMMINS (1974) and the known decomposition rates, the four exposed plant

species can be either put into the “fast” or “slow” group. Acalypha diversifolia (0,04),

Cecropia obtusifolia (0,0234), and Tetrathylacium macrophyllum (0,0458) can be

assigned to the “fast” group. Sloanea medusula (0,0094) can be placed into the slow

group.

87

Anatomical cuts

Sloanea medusula (Elaeocarpaceae) – K-strategist

88

Tetrathylacium macrophyllum (Flacourtiaceae) - between r- and K-strategist

89

The histological analysis (GUSENLEITNER & RIEMERTH 2005) of the exposed plant

species show the varying nature of the leaf structure. Sloanea medusula

(Elaeocarpaceae) has a larger middle rib than Tetrathylacium macrophyllum

(Flacourtiaceae); the leafblade is almost the same.

The red coloured cells are the lignified cells and the blue coloured cells are cellulose

– the non-lignified cell walls. Sloanea medusula is higher lignified and has a higher

content of tannins than Tetrathylacium macrophyllum, which makes T. macrophyllum

more palatable for the macroinvertebrates. These anatomical cuts support the results

of the differences in decay of the two plant species.

Macroinvertebrate colonization

exposure time [d]0 5 10 15 20 25 30

mea

n nu

mbe

r of m

acro

inve

rtebr

ates

0

50

100

150

200

Acalypha diversifolia (Euphorbiaceae)Cecropia obtusifolia (Cecropiaceae)Tetrathylacium macrophyllum (Flacourtiaceae) Sloanea medusula (Elaeocarpaceae)

Fig. 4.3. Mean number of macroinvertebrates colonizing four different types of leaf packs (Acalypha diversifolia, Cecropia obtusifolia, Tetrathylacium macrophyllum, Sloanea medusula) within exposure time [d]

90

A clear trend to colonize the leaves from day one of the exposure within the stream is

evident. Macroinvertebrate colonisation first increases on all four plant species but

abundance at Acalypha diversifolia, Tetrathylacium macrophyllum and Cecropia

obtusifolia declined at the end of the study, when most of the plant material is broken

down and little of the leaves – just the lignified, not palatable parts - remained in the

bags. The number of species present in the litter bags increased steadily in the first

14 days of the study (Fig. 4.3) at all plant species except at Sloanea medusula. By

day 28 species richness has declined. The K-strategist Sloanea medusula does not

show the rapid increase of colonization like the 3 other plants.

A total of 3339 individuals of macroinvertebrate morphospecies were collected. The

invertebrate community was dominated by insect groups. 13 orders and about 39

families colonised the litter bags during the course of study (see Tab. 4.3.) The

assemblage was dominated by Chironomidae (41,39 %), mostly Orthocladiinae

(23,06 %) and Ephemeroptera (39,41 % of the total fauna), comprising Baetidae

(6,83 %), Caenidae(0,6 %), Leptohyphidae (27,73 %) and Leptophlebiidae (4,25 %).

Trichoptera (8,12 %), mostly Leptoceridae (2,73 %) and Hydropsychidae (2,40 %)

were also abundant, while Plecoptera (Perlidae) constituted only 0,03 % of total

colonizers. Diptera (44,65 %) were also quite abundant. Odonata, Coleoptera and

Turbelaria were the only other order that made up more than 1 % of the total

assemblage. Numerically the groups of Hydrachnellae, Mollusca and Hydra were of

little importance. Colonizer densities (total individuals and abundance of major taxa)

were highest on Cecropia obtusifolia and Tetrathylacium macrophyllum leaves and

lowest on Sloanea medusula.

91

Tab. 4.3. Total number and percent composition [%] of the litter bag macroinvertebrate fauna and FFG (Fi-filterer, Pr-predator, Co – collector-gatherer, Sh-shredder, Pi-piercer, Sc-scraper)

taxon total percent composition Functional

Feeding Group EPHEMEROPTERA 1316 39,41 Baetidae 228 6,83 Co Baetidae_sp1. 113 3,38 Co Baetidae_sp2. 113 3,38 Co Baetidae_sp3. 2 0,06 Co Caenidae 20 0,60 Co Caenidae sp. 20 0,60 Co Leptohyphidae 926 27,73 Co Leptohyphes_sp. 10 0,30 Co Leptohyphidae_sp1. 503 15,06 Co Leptohyphidae_sp2. 103 3,08 Co Leptohyphidae_sp3. 249 7,46 Co Leptohyphidae_sp4. 8 0,24 Co Leptohyphidae_sp5. 43 1,29 Co Leptohyphidae_sp6. 10 0,30 Co Leptophlebiidae 142 4,25 Co Leptophlebiidae_sp1. 129 3,86 Co Leptophlebiidae_sp2. 4 0,12 Co Leptophlebiidae_sp3. 2 0,06 Co Leptophlebiidae_sp4. 7 0,21 Co TRICHOPTERA 271 8,12 Hydropsychidae sp. 80 2,40 Co-Fi Hydroptilidae 12 0,36 Pi Oxyethira_sp. 3 0,09 Pi Hydroptila_sp. 9 0,27 Pi Leptoceridae 91 2,73 Sh Atanatolica_sp. 91 2,73 Sh Philopotamidae 65 1,95 Co-Fi Wormaldia_sp. 65 1,95 Co-Fi Calamoceratidae 18 0,54 Sh Phylloicus_sp. 18 0,54 Sh new-unknown 5 0,15 PLECOPTERA 1 0,03 Perlidae 1 0,03 Pr Anacroneuria_sp. 1 0,03 Pr DIPTERA 1491 44,65 Chironomidae 1382 41,39 Chironomini_sp. 77 2,31 Co Orthocladiinae_sp. 770 23,06 Co Tanypodinae_sp. 452 13,54 Pr Tanytarsini_sp. 83 2,49 Co-Fi Ceratopogonidae 18 0,54 Pr Dixidae 4 0,12 Co Athericidae 7 0,21 Pr

92

Simuliidae 50 1,50 Co-Fi Stratiomyidae 1 0,03 Co Thaumaleidae 21 0,63 Sc Culicidae 1 0,03 Co Tipulidae 7 0,21 Sh COLEOPTERA 39 1,17 Elmidae 21 0,63 Co Elmidae_sp. 21 0,63 Co Hydrophilidae 8 0,24 Pr Hydrophilidae_sp. 8 0,24 Pr Dytiscidae 4 0,12 Pr Dytiscidae_sp. 3 0,09 Pr Dytiscidae_sp2. 1 0,03 Pr Hydroscaphidae 2 0,06 Sc Hydroscaphidae_sp. 2 0,06 Sc Lampyridae 2 0,06 Lampyridae_sp. 2 0,06 Ptilodactylidae 1 0,03 Sh Anchytarsus_sp. 1 0,03 Sh Staphylinidae 1 0,03 Pr Staphylinidae_sp. 1 0,03 Pr HETEROPTERA 29 0,87 Naucoridae 25 0,75 Pr Limnocoris_sp. 25 0,75 Pr Hebridae 1 0,03 Pr Hebrus major 1 0,03 Pr Veliidae 3 0,09 Pr Microvelia_sp. 1 0,03 Pr Rhagovelia_sp. 2 0,06 Pr ODONATA 61 1,83 Anisoptera 8 0,24 Pr Zygoptera 53 1,59 Pr HYDRACHNELLAE 12 0,36 TURBELARIA 41 1,23 MOLLUSCA 4 0,12 Co-Fi HYDRA 1 0,03 REST 73 2,19 Total individuals 3339

93

Detrended correspondence analysis

DCA Axis 122,5

time axis

0 1 2 3

DC

A A

xis

29,

2

0

1

2

4

44

48

814

14

14

12124

28

284

4

4 4

8 88 814

14

14

17

17

21

22121

21

24

28

28

28

28

Fig. 4.4 Ordination biplot resulting from a DCA of the sampling scores (exposure times) of four types of leaf packs placed for a 28 day period in the Q. Negra. (blue = Acalypha diversifolia, red = Cecropia obtusifolia, black = Sloanea medusula, green = Tetrathylacium macrophyllum)

DCA Axis 122,5

time axis

-2 -1 0 1 2 3 4 5

DC

A A

xis

29,

2

-3

-2

-1

0

1

2

3

4

CoPr

Co

Co

Fi

FiFi

Fi

CoCo

Co

Co

Fi

CoPr

ShCo

CoPr

Co

Pr

Sc

ShCo

Co

Pr

CoPr

Sh

CoCo

Pr

PrCo Pi

Fig. 4.5. Ordination biplot resulting from a DCA of the fauna collected from four types of leaf packs placed in the Q. Negra. The macroinvertebrates were assigned to functional feeding groups. (Fi-filterer, Pr-predator, Co – collector-gatherer, Sh-shredder, Pi-piercer, Sc-scraper)

94

A detrended correspondence analysis (Fig. 4.4. and 4.5.) shows the patterns of the

sample scores and the macroinvertebrate colonization. Axis 1 can be interpreted as

the time axis. The macroinvertebrates were assigned to feeding groups after MERRITT

and CUMMINS (1996) and the seperation into functional feeding groups revealed

considerable differences between the colonisation dynamics of filterers and

shredders. Colonisation of the plants by collectors and filterers was rapid in the first 8

days but when the leaf material gets broken down to a certain grade, no more

substrate is left over for the filterers. On the other hand the leaf material is then

palatable for the shredders and the shredders appear. In contrast to the filterer and

shredder group, collector species existed throughout the study.

Tab. 4.4. One way ANOVA. The influence of exposition time [d] and leaf material (taxon) on the macroinvertebrate colonization. species day taxon taxon*dayEphemeroptera 0,0498 n.s n.s Trichoptera 0,0299 n.s n.s Plecoptera n.s n.s n.s Diptera n.s n.s n.s Chironomidae 0,0014 n.s n.s Coleoptera n.s n.s n.s Odonata 0,0239 0,0357 n.s Heteroptera n.s n.s n.s The statistical analysis (one way ANOVA - analysis of variance) shows no significant

influence of the plant taxon on the abundance of macroinvertebrate colonisation (p >

0,05). The major macroinvertebrate colonizers like Ephemeroptera, Trichoptera,

Chironomidae and Odonata showed a significant difference in colonisation within

time (p < 0,05). All other species of colonizers showed no significant differences in

colonising the plants within time. Most of the variation in colonizer densities and

species composition was explained by the time of exponation. The effect of time had

more influence on the abundant species on the leaves than the leaf type itself.

95

4.3. DISCUSSION The results reported herin indicate that leaves of plants with different life history

strategies are broken down with varying rates of processing in the stream.

Invertebrate consumers contribute to the decay of organic matter in temperate

streams (KAUSHIK & HYNES 1971, PETERSEN & CUMMINS 1974, WALLACE & WEBSTER

1996). Studies have shown that abundance of shredding invertebrates and decay

rates of leaves are positively related (WALLACE, WEBSTER & CUFFNEY 1982, BENFIELD

& WEBSTER 1985, MALMQVIST 1993).

Species-specific differences in leaf breakdown rates are well documented (PETERSEN

& CUMMINS 1974, PADGETT 1976). PETERSEN and CUMMINS (1974) found a “hierarchy

of species along a processing continuum” and seperated leaf species into three

categories, according to the rate of the breakdown process in the stream. Species

with a processing coefficient, k, of greater than 0,01 day-1 were placed in the “fast”

group. According to this category Sloanea medusula has a slow breakdown rate,

wheras Tetrathylacium macrophyllum leaves lost weight considerably faster. This

matched our initial assumptions about their relative palatibility to macroinvertebrates,

and this was reflected in higher colonizer densities on Tetrathylacium macrophyllum.

The high rates of leaf litter decomposition for Acalypha diversifolia, Cecropia

obtusifolia and Tetrathylacium macrophyllum, in comparison to Sloanea medusula,

may be attributed to the differences in leaf structure, i.e. thickness and presence or

absence of cuticularized cell walls as well as the differences in their chemical

composition, for instance, tannin content, of the four species. Sloanea medusula

leaves are thicker than the leaves of the three other plants. These factors may

explain the relatively slow weight loss of Sloanea medusula observed in this study.

The relationship between leaf lifetime and the type of defense is documented in a

study (COLEY 1985, 1988) for Panamanian trees, which supports the hypothesis that

immobile defenses are more common in longer lived leaves. There are significant

positive relationships between leaf lifetime and all the fiber and condensed tannin

measures.

Colonizer densities on leaf packs increased with time, a trend seen in most studies of

litter breakdown in streams (DUDGEON 1982, WEBSTER & BENFIELD 1986, BENSTEAD

1996). Indeed, time and leaf type explained most of the variation in

macroinvertebrate densities on leaf packs in this study (DUDGEON & WU 1999).

96

A marked feature of the litter bag community was its dominance by insects. The

number of Ephemeroptera, Diptera (mainly Chironomidae), Trichoptera, Coleoptera

and Odonata species was particularly high. By contrast, the Plecoptera were almost

not represented. Faunal composition of the riverbed (chapter 5) was quite similar to

the one reported in this study, both for taxonomical and functional composition.

COVICH (1988) states that the taxonomic composition of neotropical stream

communities is characterised by high endemicity of certain groups coupled with a

paucity of species in others.

A DCA indicated differences in the composition of the macroinvertebrate feeding

group assemblage within time of the leaf exposition. Two of the major groups

(filterers and shredders) showed very different colonization patterns. The response

from the filterer community was initially rapid, but shredders did not appear before

the leaf material was exposed for a certain time. Filterers may need the leaf material

as a substrate and shredders rely on the leaf material, which is more palatable after a

certain time of exposure, as a food resource. In contrast to the filterer and shredder

group, collector species existed throughout the study. The balance of the

macroinvertebrate colonization e.g. immigration and emigration of filterers and

shredders, was therefore strongly dependent on the mass of leaf litter remaining.

The colonisation of the collectors seems to be a more passive process than that of

filterers or shredders and independent of the amount of leaf litter remaining in the

bags. Predatory invertebrates showed also a rapid colonisation of the litter bags. By

the end of the study period the number of predators had declined, possibly as a

result of the decreasing amounts of leaf litter present in the bags. This decrease may

indicate the importance of leaf litter as a microhabitat for predators in the studies

stream.

Potential detrivores present in the stream, such as fish and crab species, are not

recorded in the litter bags because of the mesh size. These groups were assumed to

have partial access to the contents of the litter bags, and their contribution to litter

processing in the stream studied is not known but may have been significant

(WOOTON & OEMKE 1992).

If stream macroinvertebrates are using leaf litter as a source of food, we would

expect shredders to comprise of a significant proportion of leaf-pack colonizers.

However, this functional feeding group constituted only 3,5 % of animals from the

collected colonizer assemblage during the present study. Elsewhere in a Costa Rican

97

stream (BENSTEAD 1996) two species of trichopteran shredders comprised of over 30

% of litter bag colonists (and 98 % of all shredders). The lack of shredders is also

reported in studies about Costa Rican lowland streams (PRINGLE & RAMIREZ 1998).

PRINGLE et al. (1993) suggests due to the lack of insect shredders the hypothesis that

in neotropical streams the decomposition of plant material in fine particulate organic

matter is operated either by macroconsumers, such as crustaceans and fish, or by

enhanced microbial activity. The litter processing could have been a result of the

combined feeding activities of caddis flies, mayflies and chironomids plus microbial

action and the physical fragmentation of leaves.

DUDGEON (1982) concluded that the rapid processing rate in subtropical Hong Kong

streams was largely due to high ambient temperature, and it seems likely that high

temperature, in comparison to temperate streams, was an important contributing

factor in this study. High water temperature would give rise to rapid microbial

conditioning of leaf litter, with a potential, subsequent increase in the rate of

consumption by the shredder community.

Despite the paucity of shredders among leaf colonists, the findings of this

investigation imply strongly that the responses of stream macroinvertebrates to leaf

litter occur because of its role as a source of food.

In any stream where allochthonous inputs consist of leaves of various types, some

will be more palatable than others. The palatable litter will serve mainly as a food

source and support high densities of macroinvertebrates, while lower densities of

animals will be assosiated with less palatable litter which is used mainly as a

substrate. In tropical streams, where many leaf types of varying palatibility and

diverse defensive compounds are present (including a greater proportion of species

with high levels of condensed tannins: STOUT 1989), the patch-specific response of

faunal densities to changes in the total amounts of this mixture can be expected to be

rather weak, and macroinvertebrate abundance is unlikely to correlate closely with

litter biomass (DUDGEON 1999).

98

5. Macroinvertebrate distribution and trophic relations in a neotropical lowland stream, Q.Negra, Costa Rica

The Q. Negra is characterized by a wide diversity of habitats with different current

velocity, depth and substrate. The morphometric-hydrologic conditions, the

hydrochemical characteristics and the riparian vegetation have been analysed.

Detailed observations on physical-chemical characteristics of the Q. Negra are found

in chapter 3. Within this framework the macroinvertebrate community has been

described. The main objective was to examine the composition and trophic structure

of the benthic community. This investigation represents the first survey of the benthic

invertebrate community from four different habitat types within the Q. Negra. Four

basic choriotops (habitat types) have been distinguished in the stream: riffles, shallow

sites with low current velocity, pools and cascades.

99

5.1. MATERIAL AND METHODS The sampling was conducted in March and April 2004 under conditions of low

discharge. Different sampling methods (Surber sampler, kick sampler) were used

depending on the physical characteristics of the sites.

The benthic invertebrate composition of the stream bottom in every choriotop was

assessed by using a standard Surber sampler (mesh size: 200 µm) and a qualitative

kick sampler (250 µm). Two random Surber samples and one kick sample were

taken from each site (choriotop). Samples were collected by disturbing the substrate

manually to a depth of about five cm for approximately five minutes.

Riffle environments (Choriotop A) were selected in relatively shallow turbulent areas

(0,1 m deep) where the water flow was high (usually 0,47 ms-1). Choreotop B were

sites with shallow areas and low water flow. Pools (Choriotop D), 0,24 m deep and

with low flow (0,15 ms-1) were sampled. Cascades (Choriotop E) were only

conducted with the kick sampling net. Quantitative sampling was not possible

because of the large size of boulders which dominated its substrate.

Invertebrate samples were sorted under a binocular and placed in 70 % ethanol. In

the laboratory, macroinvertebrates from each sample were hand sorted, identified to

taxonomic units and counted. Identification of the taxa was performed according to

MERRITT & CUMMINS (1996) and aquatic insects were identified in most cases on the

level of families. Invertebrates were assigned to functional feeding groups (FFG),

according to MERRITT & CUMMINS (1996): collector-gatherers (Co), filterers (F),

predators (P), scrapers (Sc) piercers (Pi) and shredders (Sh).

Analysis of data The statistical analysis of data was performed with the software package SPSS.

A one way ANOVA (analysis of variance) tested if there is a signifikant difference of

the macroinvertebrate colonisation within the four habitat types.

100

5.2. RESULTS Habitat types Tab. 5.1. Four different choriotop types within the Q.Negra, distinguished by the mean values of physical characteristics such as velocity and depth

choriotop type mean velocity [ms-1] mean depth [m] A riffles 0,47 0,10 B shallow / slow velocity 0,21 0,05 D pools 0,06 0,24 E cascades 0,15 0,10

Riffle environments (Choriotop A) are characterized by relatively shallow turbulent

areas (0,1 m deep) and a high current velocity (0,47 ms-1). Choriotop B were sites

with shallow areas and low water flow. Pools (Choriotop D) have a mean depth of

0,24 m and a mean current velocity of 0,15 ms-1. Cascades (Choriotop E) are

characterized by a high mean current diversity, low mean depth and large size of

boulders which dominated its substrate. A general comparison of the community

composition and density of the benthic community between the four choriotop types

can be made according to their velocity and depth differences.

Fig. 5.1. Pool and riffle site within the 100 m sector of the Q.Negra

101

Distribution of the macroinvertebrate fauna in the Q.Negra Tab. 5.2. Mean number, standard deviation and percent composition [%] of the macroinvertebrate fauna within four different choriotop types (A-riffles, B-shallow sites, low velocity, D-pools, E-cascades) and FFG (Fi-filterer, Pr-predator, Co – collector-gatherer, Sh-shredder, Pi-piercer, Sc-scraper) A B D E total individuals = 218 total individuals = 26 total individuals = 120 total individuals = 447 n = 3 n = 3 n = 5 n = 2

taxon

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EPHEMEROPTERA 27,00 ± 7,21 36,67 1,00 ± 1,00 11,49 7,60 ± 7,13 31,93 71,50 ± 6,36 31,36 Baetidae 2,00 ± 2,00 2,72 0,33 ± 0,58 3,83 0,20 ± 0,45 0,84 12,50 ± 0,71 5,48 Co Baetidae_sp1. 2,00 ± 2,00 2,72 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 Co Baetidae_sp2. 0,00 0,00 0,33 ± 0,58 3,83 0,00 0,00 0,50 ± 0,71 0,22 Co Baetidae_sp3. 0,00 0,00 0,00 0,00 0,00 0,00 12,00 ± 1,41 5,26 Co Caenidae 0,00 0,00 0,00 0,00 2,6 ± 2,70 10,92 0,00 0,00 Co Caenidae_sp. 0,00 0,00 0,00 0,00 2,6 ± 2,70 10,92 0,00 0,00 Co Leptohyphidae 18,33 ± 3,79 24,90 0,67 ± 1,15 7,66 4,00 ± 4,95 16,81 43,00 ± 7,07 18,86 Co Leptohyphes_sp. 2,67 ± 1,53 3,62 0,00 0,00 0,00 0,00 7,50 ± 4,95 3,29 Co Lepthyphidae_sp1. 15,67 ± 2,52 21,28 0,33 ± 0,58 3,83 0,20 ± 0,45 0,84 24,50 ± 2,12 10,75 Co Leptohyphidae_sp2. 0,00 0,00 0,00 0,00 1,00 ± 1,73 4,20 0,00 0,00 Co Leptohyphidae_sp4. 0,00 0,00 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 Co Leptohyphidae_sp5. 0,00 0,00 0,33 ± 0,58 3,83 2,60 ± 4,77 10,92 11,00 ± 0,00 4,82 Co Leptophlebiidae 6,67 ± 3,79 9,05 0,00 0,00 0,80 ± 0,84 3,36 16,00 ± 0,00 7,02 Co Leptophlebiidae_sp1. 5,67 ± 4,04 7,70 0,00 0,00 0,40 ± 0,55 1,68 11,00 ± 1,41 4,82 Co Leptophlebiidae_sp2. 0,00 0,00 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 Co Leptophlebiidae_sp3. 0,00 0,00 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 Co Leptophlebiidae_sp4. 0,00 0,00 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 Co Leptophlebiidae_sp5. 0,67 ± 0,58 0,91 0,00 0,00 0,60 ± 0,89 2,52 4,50 ± 0,71 1,97 Co Leptophlebiidae_sp6. 0,33 ± 0,58 0,45 0,00 0,00 0,20 ± 0,45 0,84 0,50 ± 0,71 0,22 Co TRICHOPTERA 6,33 ± 1,53 8,60 1,00 ± 1,00 11,49 0,20 ± 0,45 0,84 41,5 ± 13,44 18,20 Glossosomatidae_sp 0,33 ± 0,58 0,45 0,00 0,00 0,00 0,00 0,00 0,00 Sc Hydropsychidae_sp 2,00 ± 1,00 2,72 1,00 ± 1,00 11,49 0,00 0,00 18,50 ± 2,12 8,11 Co-Fi Hydroptilidae 0,33 ± 0,58 0,45 0,00 0,00 0,00 0,00 2,00 ± 0,00 0,88 Pi Hydroptila_sp. 0,33 ± 0,58 0,45 0,00 0,00 0,00 0,00 2,00 ± 0,00 0,88 Pi Philopotamidae 3,33 ± 1,53 4,53 0,00 0,00 0,00 0,00 14,00 ± 9,90 6,14 Co-Fi Wormaldia_sp. 3,33 ± 1,53 4,53 0,00 0,00 0,00 0,00 14,00 ± 9,90 6,14 Co-Fi Polycentropodidae_sp 0,00 0,00 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 Co-Fi Rhyacophilidae_sp 0,33 ± 0,58 0,45 0,00 0,00 0,00 0,00 0,00 0,00 Pr new 0,00 0,00 0,00 0,00 0,00 0,00 7,00 ± 5,66 3,07 PLECOPTERA 0,67 ± 1,15 0,91 0,00 0,00 0,00 0,00 1,00 ± 1,41 0,44 Perlidae 0,67 ± 1,15 0,91 0,00 0,00 0,00 0,00 1,00 ± 1,41 0,44 Pr Anacroneuria_sp. 0,67 ± 1,15 0,91 0,00 0,00 0,00 0,00 1,00 ± 1,41 0,44 Pr DIPTERA 9,30 ± 4,51 12,63 2,70 ± 3,79 31,03 8,60 ± 0,89 36,13 30,50 ± 0,71 13,38 Chironomidae 5,33 ± 1,15 7,24 1,00 ± 1,73 11,49 8,20 ± 6,98 34,45 22,50 ± 2,12 9,87 Chironomini_sp. 0,33 ± 0,58 0,45 0,00 0,00 1,20 ± 0,84 5,04 0,00 0,00 Co Orthocladiinae_sp. 3,00 ± 1,00 4,07 0,33 ± 0,58 3,83 0,20 ± 0,45 0,84 4,50 ± 3,54 1,97 Co Tanypodinae_sp. 2,00 ± 0,00 2,72 0,00 0,00 5,40 ± 8,38 22,69 15,00 ± 2,83 6,58 Pr Tanytarsini_sp. 0,00 0,00 0,67 ± 1,15 7,66 1,40 ± 2,07 5,88 3,00 ± 2,83 1,32 Co-Fi Ceratopogonidae 0,00 0,00 0,00 0,00 0,40 ± 0,89 1,68 1,00 ± 1,41 0,44 Pr Dixidae 0,00 0,00 0,00 0,00 0,00 0,00 0,50 ± 0,71 0,22 Co Empididae 0,00 0,00 0,00 0,00 0,00 0,00 0,50 ± 0,71 0,22 Pr Athericidae 0,00 0,00 0,00 0,00 0,00 0,00 1,50 ± 0,71 0,66 Pr Simuliidae 1,00 ± 0,00 1,36 0,00 0,00 0,00 0,00 2,00 ± 1,41 0,88 Co-Fi Thaumaleidae 0,00 0,00 0,00 0,00 0,00 0,00 0,50 ± 0,71 0,22 Sc Tipulidae 3,00 ± 3,61 4,07 1,67 ± 2,08 19,16 0,00 0,00 2,00 ± 1,41 0,88 Sh

102

MEGALOPTERA 0,33 ± 0,58 0,45 0,00 0,00 0,00 0,00 2,00 ± 2,83 0,88 Corydalidae 0,33 ± 0,58 0,45 0,00 0,00 0,00 0,00 2,00 ± 2,83 0,88 Pr COLEOPTERA 18,00 ± 16,52 24,45 1,00 ± 0,00 11,49 2,80 ± 3,11 11,76 58,50 ± 0,71 25,66 Elmidae 11,00 ± 9,54 14,94 0,67 ± 0,58 7,66 1,80 ± 1,64 7,56 27,50 ± 6,36 12,06 Co Elmidae_sp. 11,00 ± 9,54 14,94 0,67 ± 0,58 7,66 1,80 ± 1,64 7,56 27,50 ± 6,36 12,06 Co Hydrophilidae 0,00 0,00 0,33 ± 0,58 3,83 0,00 0,00 0,00 0,00 Pr Hydrophilidae_sp. 0,00 0,00 0,33 ± 0,58 3,83 0,00 0,00 0,00 0,00 Pr Psephenidae 7,00 ± 7,00 9,51 0,00 0,00 1,00 ± 1,73 4,20 10,00 ± 1,41 4,39 Sc Psephenops_sp. 7,00 ± 7,00 9,51 0,00 0,00 1,00 ± 1,73 4,20 10,00 ± 1,41 4,39 Sc Ptilodactylidae 0,00 0,00 0,00 0,00 0,00 0,00 19,50 ± 2,12 8,55 Sh Anchytarsus_sp. 0,00 0,00 0,00 0,00 0,00 0,00 19,50 ± 2,13 8,55 Sh Staphylinidae 0,00 0,00 0,00 0,00 0,00 0,00 1,00 ± 1,41 0,44 Pr Staphylinidae_sp. 0,00 0,00 0,00 0,00 0,00 0,00 1,00 ± 1,41 0,44 Pr Dryopidae 0,00 0,00 0,00 0,00 0,00 0,00 0,50 ± 0,71 0,22 Sh Dryopidae_sp. 0,00 0,00 0,00 0,00 0,00 0,00 0,50 ± 0,71 0,22 Sh HETEROPTERA 0,33 ± 0,58 0,45 1,67 ± 1,15 19,16 0,40 ± 0,89 1,68 0,50 ± 0,71 0,22 Naucoridae 0,33 ± 0,58 0,45 1,33 ± 1,53 15,33 0,40 ± 0,89 1,68 0,00 0,00 Pr Limnocoris_sp. 0,33 ± 0,58 0,45 1,33 ± 1,53 15,33 0,40 ± 0,89 1,68 0,00 0,00 Pr Veliidae 0,00 0,00 0,33 ± 0,58 3,83 0,00 0,00 0,50 ± 0,71 0,22 Pr Microvelia_sp. 0,00 0,00 0,33 ± 0,58 3,83 0,00 0,00 0,50 ± 0,71 0,22 Pr ODONATA 3,67 ± 1,53 4,98 0,33 ± 0,58 3,83 0,80 ± 1,30 3,36 7,00 ± 2,83 3,07 Anisoptera 0,33 ± 0,58 0,45 0,33 ± 0,58 3,83 0,40 ± 0,55 1,68 3,50 ± 4,95 1,54 Pr Zygoptera 3,33 ± 2,08 4,53 0,00 0,00 0,40 ± 0,89 1,68 3,50 ± 2,12 1,54 Pr LEPIDOPTERA 1,33 ± 1,15 1,81 0,00 0,00 0,00 0,00 0,50 ± 0,71 0,22 Sh ARACHNIDAE 0,00 0,00 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 HYDRACHNELLAE 3,33 ± 4,04 4,53 1,00 ± 1,00 11,49 1,20 ± 1,64 5,04 10,50 ± 0,71 4,61 TURBELARIA 1,33 ± 1,53 1,81 0,00 0,00 0,20 ± 0,45 0,84 0,00 0,00 CRUSTACEAE 0,00 0,00 0,00 0,00 0,00 0,00 1,00 ± 0,00 0,44 REST 2,00 ± 2,00 2,72 0,00 0,00 1,80 ± 3,03 7,56 3,50 ± 0,71 1,54

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riffle

percent composition [%]0 5 10 15 20 25 30

Leptohyphidae

Leptophlebiidae

Baetidae

Caenidae

rheo

philie

pool

percent composition [%]0 5 10 15 20 25 30

Fig. 5.2. Percent composition [%] of Ephemeroptera at riffles and pools, Q.Negra. Ecological arrangement according to their preference for lotic habitats

riffle

percent composition [%]0 1 2 3 4 5

Philopotamidae

Hydropsychidae

Glossosomatidae

Hydroptilidae

Rhyacophilidae

Polycentropod

rheo

philie

pool

percent composition [%]0 1 2 3 4 5

Fig. 5.3. Percent composition [%] of Trichoptera at riffles and pools, Q.Negra. Ecological arrangement according to their preference for lotic habitats

riffle

percent composition [%]0 2 4 6 8 10 12 14 16

Elmidae

Psephenidae

pool

percent composition [%]0 2 4 6 8 10 12 14 16

Fig. 5.4. Percent composition [%] of Coleoptera at riffles and pools, Q.Negra. Ecological arrangement according to their preference for lotic habitats

104

riffle

percent composition [%]0 5 10 15 20 25

Orthocladiinae

Tanypodinae

Chironomini

Tanytarsini

pool

percent composition [%]0 5 10 15 20 25

Fig. 5.5. Percent composition [%] of Chironomidae at riffles and pools, Q.Negra. Ecological arrangement according to their preference for lotic habitats

50 taxa were identified (Tab. 5.2.). Identification keys of stream insects and

invertebrates in Central America on species level are not available. In this study,

aquatic insects represent the dominant component of stream benthos.

At riffle sites (A) Ephemeroptera was the most represented insect order (36,7 %),

and within this group the most abundant family was that of Leptohyphidae (24,9 %).

Coleoptera and Diptera were the following major groups of invertebrates collected in

riffles. Members of Elmidae were the dominant Coleoptera. Chironomidae was the

main family, followed by Tipulidae and Simuliidae of Diptera. Trichoptera was

represented by two caseless filterer families: Hydropsychidae and Philopotamidae.

Odonata was represented by Zygoptera and Anisoptera.

At shallow sites with low current velocity (B) Diptera (31 %) was the most

represented insect order and within this group Tipulidae (19,16 %) were the most

abundant family followed by Chironomidae (11,49 %). Heteroptera (19,16 %),

Ephemeroptera (11,49 %) and Trichoptera (11,49 %) were the following major

groups of invertebrates.

In pools (D) Diptera (36,1 %) was the most represented insect order and within this

group the most abundant family was that of Chironomidae (34,5 %). Ephemeroptera

(31,9 %) was the following major group of invertebrates. Leptohyphidae (16,8 %) was

the main family, followed by Caenidae (10,9 %). Caenidae are not found at the other

sampling sites.

At cascade sampling sites (E) Ephemeroptera, Coleoptera and Trichoptera

collectively contributed about two-thirds of the total number of the taxa found.

Ephemeroptera was the most represented insect order in the samples, and within

this group the most abundant family was that of the Leptohyphidae (18,9 %) followed

105

by Leptophlebiidae (7,0 %). Coleoptera was the second major group of invertebrates

collected in the stream, members of Elmidae and Ptilodactylidae wer dominant.

Ptilodactylidae were not found at the other sampling stations.

Some species like Plecoptera (Anacroneuria sp.) are only found within riffle and

cascade sites. To point out the different macroinvertebrate distribution within the

stream, the composition of the streamfauna at riffle and pool sites is shown in Figure

5.2., 5.3., 5.4. and 5.5. The percent composition of Ephemeroptera, Trichoptera,

Coleoptera and Chironomidae within riffle and pools can be seen. Note the different

scale of the x-axis.

Tab. 5.3. One-way ANOVA of invertebrate abundance within four different choriotop types (A-riffles, B-shallow sites, low velocity, D-pools, E-cascades) Statistical comparisons (ANOVA, Tamhane-T2) between choriotops in number of invertebrates.

Taxon Signifikanz Tamhane Ephemeroptera <0,01 AE; DE Baetidae <0,01 AE; BE Caenidae n.s. - Leptohyphidae <0,01 AB; AD Leptophlebiidae <0,01 BE; DE Trichoptera <0,01 - Glossosomatidae n.s. - Hydropsychidae <0,01 - Hydroptilidae <0,01 BD; BE Philopotamidae <0,01 BD Polycentropodidae n.s. - Rhyacophilidae n.s. - new <0,01 AB; AD; BD Plecoptera n.s. - Diptera <0,05 - Chironomidae <0,05 BE Megaloptera n.s. - Coleoptera <0,01 BE; DE Lepidoptera n.s. - Heteroptera n.s. - Odonata n.s. -

The statistical comparison (one way ANOVA, Tamhane-T2) between

macroinvertebrate abundance and different habitat types of the Q. Negra show a

clear picture of different population densities. There is a significant difference of the

abundance of certain taxa of the invertebrate community within the four different

habitat types of the Q.Negra. The major macroinvertebrates like Ephemeroptera,

106

Trichoptera, Trichoptera, Diptera (Chironomidae) and Coleoptera showed a

significant difference in colonisation within different habitat types (p < 0,05). All other

major taxa such as Megaloptera, Plecoptera, Odonata and Heteroptera of colonizers

showed no significant differences within the choriotop types.

Trophic relationships

The analysis of the trophic structure of stream communities is a tool to understand

the relationships between the macroinvertebrate community and the different organic

matter inputs into the river (CUMMINS 1973).

We examined the trophic structure at the sampling sites using the system of

functional feeding groups (FFG) of CUMMINS (1973) to place each of the species in its

trophic class. The Quebrada Negra invertebrate fauna is in terms of species numbers

mostly made up of collector-gatherers (53,3 %), followed by predators (16,5 %),

filterers (14,5 %), shredders (8,5 %), grazer-scrapers (6,5 %) and piercer (0,7 %).

The most representative family of the collector-gatherers groups are Ephemeroptera

and some Chironomids that are collectors. Trichoptera such as Hydropsychidae

contribute to the filterers. Shredders were poorly represented in the samples with

clear differences between the stations, individuals of Ptilodactylidae and Tipulidae

contribute to this group. Shredders like Leptoceridae and Calamoceratidae were

found within the study “Macroinvertebrates and leaf litter decomposition in the

Q.Negra” – see chapter 4.

A diagram of the major trophic pathways in the Q. Negra is given in Fig. 5.6. and 5.7.

The block diagram is representing the trophic interactions of the typical community of

the Q. Negra and is based on the food habitats of each species. Many species utilize

food from two or more trophic levels. Fish species would almost certainly occupy

several of the functional feeding groups and would probably exert their greatest

impact through predation.

The abundance of macroinvertebrates provides abundant food for many other

predators, including a diverse fish assemblage. Food resources of invertebrate

consumers include periphyton and other surface layer complexes, macrophytes,

detritus and other animals.

The consumption of autumn-shed leaves in woodland streams by various

invertebrates is the most extensively investigated trophic pathway involving CPOM. It

107

is largely allochthonous material (leaf litter and detritus) of the forest, which is

brought into the stream and determines its nutrient budget. Invertebrates that feed on

decaying leaves (shredders) in our study stream include crane fly larvae (Tipulidae)

and several families of trichopterans (Leptoceridae, Calamoceratidae) and

Coleoptera (Ptilodactylidae). Caddisflies in the superfamiliy Hydropsychoidea (which

includes the Philopotamidae, Polycentropodidae and Hydropsychidae) spin silken

nets in a variety of designs. Most are passive filter feeders, constructing nets in

exposed locations. Larvae of black flies (Simuliidae) are highly specialized

suspension feeders. Rainforest streams are shadowed to a great extent and have

therefore a small primary production. Due to the low periphyton production in the

Q.Negra, scrapers and grazers constitute only 6,5 % of the macroinvertebrate

community. Predation, used here to refer to the consumption of animal prey, is a

widespread and potentially important process affecting the biota of running waters.

The classification of the invertebrate consumers of streams into feeding guilds has

demonstrated great utility for description and analysis. Characteristics of a particular

stream or river, including its size, hydrology and the vegetation of the surrounding

landscape significantly influence which pathways predominate.

The trophic level above (the herbivores) controls events in the trophic level below

(the periphyton). In contrast there is evidence that food availability determines

herbivore abundance and distribution. Whether bottom-up or top-down control

prevails, may depend on time, place and environmental circumstances, and they may

not be mutually exclusive.

108

5.3. DISCUSSION

The main objective of the work was to determine the composition and trophic

structure of the macroinvertebrate communties in the Q. Negra. The results show the

effects of current velocity, depth and substrate on the benthic macroinvertebrate

community.

Faunal composition of the riverbed was similar to the one reported in studies about

Costa Rican lowland streams (PRINGLE & RAMIREZ 1998), both for taxonomical and

functional composition. Stream invertebrate assemblages varied within the course of

the Q. Negra. Both invertebrate density and taxonomical richness increased with the

increasing current velocity. Substratum influences invertebrate abundance and

taxonomical richness, with boulders and cobbles richer than sandy habitats.

We found that the number and taxa diversity of stream benthos greatly varied among

different sites within the Q.Negra. In general all the dominant species are gatherer-

collectors or filter-feederer with their associated predators.

As previous stated, current velocity was an important factor in shaping benthic

communities, both in structural and functional composition: higher velocities were

associated with a richer and more abundant invertebrtae assemblage. It is likely that

current is related to water oxygenation and also plays a key role in the functional

feeding of some groups, such a filterers. Cascades also have a turbulent water

regime, although velocity is not as high as at riffle sites. Moreover, it is well

established that micro-flow dynamics play a key role in the small scale distribution of

benthic communities (STATZNER and HOLM 1982).

Macroinvertebrate shredders were nearly absent in the samples while collector-

gatherer and filterer (both feeding on fine particulate matter) were dominant. Not

even taxa of shredders (Calamoceratidae, Leptoceridae) reported in chapter 4

(Macroinvertebrtaes and leaf litter decomposition) were abundant. PRINGLE et al.

(1993) suggests due to the lack of insect shredders the hypothesis that in neotropical

streams the decomposition of plant material in fine particulate organic matter is

operated either by macroconsumers, such as crustaceans and fish, or by enhanced

microbial activity. In this study crustaceans were not recorded and are not shown in

the trophic pathway diagrams.

109

Fig. 5.6. Major trophic pathways of the lotic food web, Q.Negra, Costa Rica

110

Fig.5.7. Major trophic pathways of the lotic food web, Q.Negra, Costa Rica and percent composition [%] of major invertebrates

111

6. Research needs The single survey of the rivers within the Piedras Blancas National Park is a good

initial effort but provides only a coarsely resolved basis for integration of research

results. Further studies of the rivers and streams in the Rainforest of the Austrians

are clearly “warranted”. It remains to be seen therefore, whether the findings reported

herin will apply to other streams, or even to the same streams in a year where

variation in the magnitude of rains alters the duration or intensity of spates.

Further objects of research could be testing ecotone concepts or the river continuum

concept (RCC) in tropical lotic systems according to their global applicability.

Is the RCC applicable to tropical streams? This and other questions can only be

answered by future research.

The seasonal climate ('dry' and 'rainy' season) provides ideal conditions for testing

the food web theory and niche partitioning, in neotropical lotic systems.

Understanding the ecological energetics will require detailed research. We expect

that there are differences in the habitat structure between the 'dry' and 'rainy' season.

This requires a more detailed description of the stream. Further interesting aspects

are biogeochemical aspects of rainforest-river systems, nutrient retention and nutrient

spiralling. Clearly, more information on the requirements and interactions of stream

organism is necessary to understand whether top down effects play an important role

in tropical streams.

Within the Piedras Blancas National Park the freshwater invertebrate fauna is largely

unsurveyed and undescribed. This protected area in the south of Costa Rica offers

an unique opportunity to study the tropical lotic invertebrate community. Identifying

the relative role of seasonal changes in physico-chemical factors and availability and

nature of food resources in explaining temporal pattern in functional organisation of

macroinvertebrate communities might be a particularly interesting line of

investigation.

Understanding community structure and function and their determinants is one of the

main objectives of ecology. In Costa Rica, except for some studies on aquatic insects

(PRINGLE et.al. 1998) benthic macroinvertebrate fauna is poorly known. Results from

such studies can be used to predict taxa distribution at local small-scale and identify

factors that can influence micro-distribution patterns in the lotic system of this area. It

is clear that many questions remain to be resolved regarding the way in which

112

physical, chemical and seasonal factors control benthic macroinvertebrate

community organization. In particular, there is scope for further studies of the

seasonal changes that occur in lotic communities and how they relate to seasonal

cycles in environmental variables.

In this regard, we consider the present study an initial step.

113

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116

c b

a

d

a Acalypha diversifolia (Euphorbiacae)b Sloanea medusula (Elaeocarpacea)c Cecropia obtusifolia (Cecropiaceae)d Tetrathylacium macrophyllum (Flacourtiacae) Plate 1

EPHEMEROPTERA

Baetidae Caenidae

Leptophlebiidae Leptohyphidae

PLECOPTERA

Plate 2Perlidae

TRICHOPTERA

Calamoceratidae Hydropsychidae

Hydroptilidae Leptoceridae

Philopotamidae Plate 3

DIPTERA

Ceratopogonidae Dixidae

SimuliidaeRhagionidae

Stratiomyidae Tipulidae

Plate 4

Culicidae Thaumaleidae

Chironomidae

Plate 5