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Colonization of Exclosures in a CostaRican Stream: Effects of Macrobenthoson Meiobenthos and the NematodeCommunityMartina Duft a , Katharina Fittkau a , Walter Traunspurger b & SeppFittkaua Ludwig-Maximilian-University of Munich Zoological Institute,Aquatic Ecology , Karlstrasse 23-25, Munich, D-80333, Germanyb University of Bielefeld Animal Ecology , Morgenbreede 45,Bielefeld, D-33615, GermanyPublished online: 06 Jan 2011.

To cite this article: Martina Duft , Katharina Fittkau , Walter Traunspurger & Sepp Fittkau(2002) Colonization of Exclosures in a Costa Rican Stream: Effects of Macrobenthos onMeiobenthos and the Nematode Community, Journal of Freshwater Ecology, 17:4, 531-541, DOI:10.1080/02705060.2002.9663931

To link to this article: http://dx.doi.org/10.1080/02705060.2002.9663931

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Colonization of Exclosures in a Costa Rican Stream: Effects of Macrobenthos on Meiobenthos and the Nematode Community

Martina Dufta and Katharina Fittkau Ludwig-Maximilian-University of Munich

Zoological Institute, Aquatic Ecology Karlstrasse 23-25, D-80333 Munich, Germany

and

Walter Traunspurger University of Bielefeld

Animal Ecology Morgenbreede 45, 0-336 15 Bielefeld, Germany

- Dedicated to Sepp Fittknu -

ABSTRACT

Interactions of macro- on meiobenthos were studied in the sediment of a small stream in northwestern Costa Rica. Three different exclosure types (n = 5) were exposed for 2 1 days: (I) treatment macrofauna (2 mm mesh size) allowed meiobenthic plus macrobenthic colonization; (2) treatment meiofauna (500 pm mesh size) allowed meiobenthic colonization but reduced macrobenthic colonization; and (3) plastic pipe exclosures (35 pm mesh size lid) inhibited access of both meio- and macrobenthos. In each cage, standardized hard substrates were provided for algal colonization. We postulated impacts of macrobenthos on (1) meiofaunal abundance, (2) algal biomass, and (3) nematode community composition. Reduction of macrobenthos did not result in increased population densities for any meiobenthic taxonomic group, but gastropods were more abundant in macro treatments. Ambient population densities were not reached inside the exclosures except for ostracods. Algal biomass significantly increased when macrobenthos was reduced, yet the influence of a caging effect during the experiment cannot be ruled out. Bacterivorous nematodes were dominant in both treatments. Species of the genera Eumonhystera and Plectus (colonizers) were predominantly found in meio treatments, while the genera Rhabdolaimus and Dorylaimus/Mesodorylaimus (persisters) were mainly present in macro treatments.

INTRODUCTION

Meiofauna represents more than 95% of all metazoan organisms in most rivers where the groups with the highest abundances are nematodes and rotifers. However, the role of this community in benthic food webs of lotic ecosystems has scarcely been investigated (Palmer and Strayer 1996). In most experiments concerning interactions among benthic organisms or between herbivores and periphyton in freshwaters, meiofauna has been neglected andlor omitted. This may be due to a time-consuming sampling and extraction and to taxonomic difficulties (Coull and Palmer 1984). However, firm conclusions about relations within the food web cannot be drawn when meiobenthos is studied only on the level of orders (Bell 1980).

Meio- and macrobenthic organisms may interact via mechanisms such as competition and predation. They compete for food (e.g., microalgae, bacteria), and meiobenthos serves as food for macrobenthos (Coull 1990, Dahl and Greenberg 1997,

"Present address: International Graduate School, Markt 23, D-02763 Zittau, Germany, mduft@ihi-zittau.de

Journal of Freshwater Ecology. Volume 17. Number 4 - December 2002

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Gee 1989, Kneib 1985, McCall and Fleeger 1995). Relationships between meiobenthos and their algal food resource have hardly been investigated (Borchardt and Bott 1995). Macrobenthos may have direct or indirect effects on the benthic algal community. Gastropods and insect larvae reduce algal biomass directly by grazing, but grazing can also indirectly stimulate algal growth, probably by increased bioturbation (Diehl et al. 2000, Lamberti et al. 1987, McCormick and Stevenson 1989). Direct negative effects of grazers on algae are considered to be more important than indirect positive effects, at least for periods of up to several weeks. The excretion of distinct metabolites (especially excretions containing nitrogen and phosphorus) or mechanical destruction of larger particles can enhance growth rates of algae and other organisms. Such relations have been found between meiofauna and bacteria (Borchardt and Bott 1995) or bacteria and algae (Montagna 1984).

In this study, we assessed both direct and indirect effects of stream macrofauna on the meiofauna. Colonization and interactions between meio- and macrobenthos were investigated with special emphasis on the nematode community. We hypothesized that macrobenthos has an impact on: (1) meiofaunal abundance, (2) algal biomass and (3) meiofaunal and nematode community composition due to algal grazing.

STUDY AREA

The experiments were carried out in the Quebrada Las Pailas, a small second-order stream in Rincon de la Vieja National Park in northwestern Costa Rica (10'50' N, 85O15' W). Average annual temperatures range from 24' to 26'C, and precipitation is from 1,500 to 2,500 mrn per year. The dry season is from January to April; the rainy season is from May to December. The volcano Rincon de la Vieja is highly active, which geothermically influences the Quebrada Las Pailas. This creek originates at 1,160 m altitude at the southwestern side of the Rincon de la Vieja massif, and its temperature is constantly changed by small confluents. Our study site was a 3.5 m x 4.0 m pool of the Quebrada Las Pailas just before its confluence with the Rio Colorado. Water temperature was 24.9" C, velocity was 0.3 rn/s, conductivity was 139 ps/cm3, pH was 7.7 and oxygen content was 8.9 mgll (monthly means).

For our experiments we chose locations in the pool where the biomass ratio of meio- and macrobenthos was approximately 1, to provide equal starting conditions. Sites with this biomass ratio were identified in a preliminary sampling session.

METHODS AND MATERIALS

Experimental design The exclosures were cylindrical cages, 13 cm high and 20 cm in diameter. The

frame was made of stainless steel wire net, connected horizontally at top and bottom by circular flexible steel rings and vertically by aluminum bars (Fig. 1). The bottoms and lids were made of plastic nets (same mesh size) that were sewed to the cage frame with nylon cord or wire. All seams were sealed with silicone.

In order to manipulate the colonization rates of meio- and macrobenthos separately, cages of two different mesh sizes were used. Meiofauna: The mesh size was 500 pm, enabling meio- and microbenthos (bacteria, protozoa, algae and fungi) to colonize these cages, while reducing the colonization rates of macrobenthos and fish by allowing only small and immature stages to enter the cages. Macrofauna: The mesh size was 2 mm, enabling macro-, meio- and microbenthos to colonize these cages, whereas large macroinvertebrates were excluded.

Additionally, cages without mesh were used to quantify the caging effect itself. A plastic pipe served as the frame, while the bottom and the lid consisted of 35 pm mesh netting.

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Aluminum bars Lid of plastic net

\ I Tiles of stoneware /

A

Flexible 8 steel ring Azoic sediment (stainless steel)

20 cm

Figure 1. Schema of exclosures for the treatments, differing in mesh sizes of metal wire and plastic nets.

River sediment was collected next to the sampling site and boiled for 15 min in order to kill the infaunal organisms. Larger stones, leaves, and roots were removed. We filled the exclosures with the homogenized azoic sediment (mean grain size 240 pm) up to a level of 8 cm (Fig. 1). Four quadratic tiles of unglazed stoneware (surface size 5 x 5 cm, height 0.5 cm) that were glued with silicone onto cleaned flat river stones of approximately the same size were placed into each cage for colonization by algae. A total of fifteen exclosures was arranged in five groups of three treatments (one meiofauna, one macrofauna, one without mesh) in sandy areas of the pool. Small holes were dug into the river sediment (water depth 29 cm), and the cages put in carefully so that the upper surface of the sediment of each cage was flush with the surrounding sediment.

Sampling and analysis Samples (n = 5) from the (undisturbed) sediment surrounding the cages were taken

four weeks prior to the onset of the experiment, which was started on July 4, 1998. After 2 1 days, we took five sediment samples from each cage to determine the abundance of meio- and macrobenthos. Sediments were sampled to a depth of 5 cm with acrylic pipe corers (inside diameter 2 cm) following Traunspurger (2000). The samples were kept in formaldehyde (4%) and stained with rose bengal. Extraction of benthic organisms was achieved with the elutriation h e l method of Uhlig et al. (1973). All specimens of nematodes were prepared separately (Seinhorst 1959 and 1962), identified to the species level at lOOOx magnification, and classified into four feeding types (Traunspurger 1997): deposit (bacterial) feeders, epistrate (algal) feeders, chewers (predatory and omnivorous nematodes) and suction feeders. All other organisms were counted and identified at 30x magnification.

We took triplicate shallow (2 cm) cores with the acrylic pipes in each cage and from the surrounding (undisturbed) sediment to determine the chlorophyll a content and we removed all tiles to determine the chlorophyll a content of epilithic periphyton. The aufwuchs was brushed from the tiles and filtered over glass fiber filters. The filters were then extracted with hot ethanol (90%), and the chlorophyll content was measured spectrophotometrically following Nusch (1980). Additionally, the grain size of the sediments in the exclosures was analyzed by wet sieving (Schwoerbel 1994).

Statistics We tested the data for normality using the Kolmogorov-Smirnov test. We analyzed

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all data using a two-way-ANOVA, with mesh size (meio, macro, control) as the treatment factor and location of the cages in the pool as a blocking factor, followed by Tukey post tests to check for differences between the treatments. If the effects of location were not significant (p>0.1), we repeated the analyses as one-way-ANOVAs of treatment effects. Analyses were performed using the software package Prism (Version 2.0 1, GraphPad Software, San Diego, USA) for Windows NT.

RESULTS

Abundances of meio- and macrobenthos The dominant invertebrate taxa were Ostracoda, Rotatoria, Nematoda, Copepoda,

Acari, Gastropoda, Annelida, Chironomidae, and "others" (Table 1). Others comprised abundances of Gastrotricha, Tardigrada, nauplii, Cladocera, Arnphipoda, Turbellaria, Trichoptera, and Coleoptera. Ostracods were the most abundant taxon inside the exclosures.

Table 1. Abundance11 0 cm2 of dominant taxa and total abundance in the treatments without mesh, the meio- and macrofaunal treatments and the outside sediment (mean * SE, n = 5).

Pipe Meio Macro Outside Ostracoda 1.82 * 0.67 91.83 * 34.50 99.26 * 24.86 52.23 * 9.52 Rotatoria Nematoda Copepoda Acari Gastropoda Annelida Chironomidae Others Total 95.98 * 41.24 177.56 i 31.17 265.92 * 19.90 584.07 i 103.35

As expected, cage location in the stream had no significant effect on the abundance of any taxon (two-way-ANOVA, p>0.1). For nematodes, rotifers, copepods, ostracods and annelids there was no significant difference in the colonization of meio and macro cages (Table 1). Gastropod density was, however, significantly higher in macro compared to meio cages (one-way-ANOVA, p = 0.03). Total abundance of benthic organisms was ca. 1.5 times higher @ = 0.09) in the macrofauna treatment than in the meiofauna treatment mainly due to the increased gastropod density. Abundances in the pipe exclosures were much lower than in meio and macro treatments, with the exception of annelids and rotifers. Outside the exclosures, total abundance was ca. three times higher than in the meiofauna (one-way-ANOVA, p = 0.01) and two times higher than in the macrofauna treatments (one-way-ANOVA, p = 0.03). Inside both of the cages, ambient densities were reached only by copepods (both treatments), gastropods (in meiofauna treatments) and acari (one-way-ANOVA, p>0.05). Ostracods (in both treatments) and gastropods in the macrofaunal cages clearly exceeded the abundances in the surrounding sediment (one-way-ANOVA, p<0.05).

Chlorophyll a content Treatment significantly influenced the chlorophyll a content both on tiles and in the

sediment (Fig. 2). Compared to macrofaunal cages, in meiofaunal cages there was more chlorophyll a on tiles and in the sediment (one-way-ANOVA, p = 0.04 for tiles and

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p = 0.1 for sediment). The chlorophyll a content of the sediment surrounding the exclosures was significantly lower than the chlorophyll a content in the meiofaunal and macrofaunal treatments (one-way-ANOVA, p<0.05). Also in the pipe exclosures, chlorophyll a contents both on tiles and in the sediment were significantly lower than in meiofauna (one-way-ANOVA, p = 0.001 for sediment, p = 0.04 for tiles) and in macrofauna treatments (one-way-ANOVA, p = 0.02 for sediment, p = 0.02 for tiles, Fig. 2). Cage location had no effect on chlorophyll a (two-way-ANOVA, p>0.1). Mean grain size of the sediments was 226 pm (middle sand) in the macrofaunal cages, 184 pm (fine sand) in the meiofaunal cages, and 128 pm (fine sand) in pipe exclosures.

pipe meio macro

outs ide

tiles sediment Figure 2. Chlorophyll a content (pg/cm2) in the pipe, meio- and macrofaunal treatments -

measured on tiles and in the sediment - and in the outside sediment (mean * SE, n = 5) .

Nematodes - species and feeding types Deposit feeders were the dominant feeding type in meio as well as in macro

treatments (Fig. 3). They were more numerous in meio than in macro treatments, but the difference was not statistically significant (one-way-ANOVA, p = 0.1 8). Epistrate feeders were the least abundant feeding type in both treatments. The abundances of suction feeders, chewers and epistrate feeders were similar in macrofaunal and meiofaunal treatments (Fig. 3, one-way-ANOVA, p>0.1). In the surrounding sediment outside the

18

16

14 1 12 W meio H macro 5 10 outside

8 0

5 6 9 4

2

0 deposlt feeders epistrate feedem chewers suction feeders

feeding types

Figure 3. AbundancetlO cm2 of the four different feeding types of nematodes in meio- and macrofaunal treatments and in the outside sediment (mean * SE; n = 5; p- values of a one-way-ANOVA of the effects of treatment are p>0.1 for all feeding types).

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cages, a similar distribution of the feeding type abundances was found, yet with a higher total number of individuals (Fig. 3); however, only epistrate feeders were significantly more abundant outside than inside the exclosures (one-way-ANOVA, p<0.05).

The total number of nematode species (24) did not differ between the macrofaunal and meiofaunal treatments, whereas in the surrounding sediment 3 1 nematode species were identified (Table 2).

Species of the genus Eumonhystera (mainly Eumonhystera simplex and Eumonhystera vulgaris) and Plectus spec., all deposit feeders, were numerically dominant (>50% of all individuals) in meio exclosures, while Rhabdolaimus terrestris, also a deposit-feeding species, and the suction feeders Dorylaimus stagnalis and Mesodorylaimus spec. appeared to be more abundant in macro exclosures. Outside the exclosures, the Eumonhystera species (E. simplex and E. vulgaris) and Prismatolaimus intermedius, an epistrate feeder, were numerically dominant.

Table 2. List of nematode species and classification into feeding types (D = deposit feeder, E = epistrate feeder, C = chewer, S = suction feeder) and their contributions to total nematode numbers (%) in the meio- and macrofaunal treatments and the outside sediment (n = 64 for meio, n = 50 for macro and n = 97 for outside).

Species Feeding % % YO type meio macro outside

Achromadora micoletzkyi de Man E 3.2 4.0 0 Achrornadora spec. Dorylaimus stagnalis Dujardin Eumonhystera barbata Andrhsy Eumonhystera dispar Bastian Eumonhysterafiliformis Bastian Eumonhystera gerlachi Andrhsy Eumonhystera longicaudatula Gerlach and Riemann Eumonhystera simplex de Man Eumonhystera vulgaris de Man Hemicycliophora spec. Ironus ignavus de Man Ironus tenuicaudatus de Man Mesodorylaimus bastiani Andrhssy Mesodorylaimus spec. Monhystera spec. Paramphidelus spec. Plectus spec. Prismatolaimus intermedius Biitschli Prodesmodora cf: circulata Micoletzky Rhabdulaimus spec. Rhabdolaimus terrestris de Man Tobrilus spec. Undetermined species Other species TOTAL Number of species

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DISCUSSION

Colonization We found two pronounced effects of our treatments. Gastropod grazers were largely

excluded from meio cages and, correspondingly, chlorophyll a levels were higher in meio than in macro cages. An even (significantly) lower chlorophyll a content was measured in the surrounding untreated sediment (control), where grazing was open to organisms of all sizes. This strongly supports our second hypothesis, an impact of macrofauna on algal biomass, but the possibility of a caging effect during the experiment cannot be ruled out. In contrast, our first hypothesis, an impact of macrofauna on meiofaunal density, was not supported by the data. All meiobenthic organisms colonized the meio- and macrofaunal treatments in similar densities. Similar results were reported by Reise (1979) for marine environments. He found that macrobenthos had little or no effect on nematode, ostracod, and copepod densities; only larger organisms like crabs had some (relatively small) effects. Fitzhugh (1982) observed no responses of nematodes to the exclusion of macrobenthos, whereas copepods and annelids responded with a population increase.

Gastropods are the only macrobenthic organisms that were successfully excluded in the meiobenthic treatment. For other organisms, there were no differences in colonization of the meio- and macrofaunal treatments. Gastropods are not considered as predators of meiofauna, but there are no studies on indirect effects of gastropods on meiobenthos mediated through grazing. Interestingly, gastropods were only found in very low densities outside the cages. Conditions inside the (macro) cages seemed to have been particularly favorable for them, as was indicated by the six times higher chlorophyll a levels inside the macrofaunal cages compared to the outside sediments. Also, cages may have provided shelter ftom larger predators. The dominance of ostracods in the cages, compared to the abundance outside, may also be due to the increased chlorophyll a levels since many ostracods feed on microalgae (Pennak 1989), or due to reduced flow.

Ambient densities were not reached by most taxonomic groups inside the exclosures. The pipe exclosures were colonized only in very low densities, the most abundant taxonomic group being rotifers, which can be explained by their small size, and annelids, which was due to a high abundance in only one of the cages, probably caused by a leak.

Chlorophyll a Algal biomass was clearly higher in meio- than in macrofaunal treatments, both for

epiIithic aufwuchs, which is primarily available as food to epibenthic organisms (gastropods or insect larvae; Feminella et al. 1989, McCormick and Stevenson 1989, Lamberti et al. 1987), as well as for algal biomass in the sediment, which is important for organisms living in the sediment (annelids or nematodes; Borchardt and Bott 1995, Cummins and Klug 1979).

In the sediment surrounding the cages, even less chlorophyll was measured. This may have been due to grazing activities of larger macrobenthos such as insect larvae or megafauna like crabs or fuh, which can strongly reduce algal biomass (Cooper 1973, Power et al. 1988). However, the high chlorophyll a levels inside the exclosures could also be due to caging effects. Inside the cages one has to expect reduced flow velocities (Peckarsky and Penton 1990) that may have led to reduced losses of algal biomass. Due to methodological difficulties this effect has not been measured in any in situ caging experiments in running waters (Peckarsky and Penton 1990). Nevertheless, caging effects may explain many of our obtained results, including the difference of chlorophyll contents in meio- and macrofaunal treatments.

In the pipe treatments, only very low chlorophyll a levels were measured which was

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unexpected. Obviously, the extremely fine mesh (35 pm) was rapidly clogged by fine particles and therefore light was not able to enter these cages which consequently inhibited growth of algae. Thus, the design of a control treatment for algal growth undisturbed by meiobenthic grazing activities could not be achieved and remains an item to be studied, as no evidence was found in the literature.

Meiobenthic community structure Why did the presumably more favorable food situation in the meiofaunal cages not

result in higher densities of meiobenthos there? The following are possible explanations. (1) Benthic algae are not the main food resource for meiobenthos, and treatment effects were therefore comparatively small and hard to detect. Algae have a higher nutritional value (C:N ratio) than detritus, but they are a less valuable food than bacteria (Cummins and KIug 1979). This idea is supported by the results of the composition of the nematode community in the exclosures based on feeding types. Bacterivorous nematodes were by far more abundant than algal feeders, so bacteria seemed to be the main food resource. (2) The exposure period of 21 days enabled colonization by organisms (by immigration) but was too short for the development of a complete spectrum of species (by local reproduction). Townsend and Hildrew (1976) point this out as well. Certain (algivorous) organisms would have been underrepresented in such a scenario. For nematodes this is likely. Typical diatom feeders, such as Ethmolaimuspratensis, have a life cycle of three to four months (Bretschko 1984, Traunspurger 1998) and could therefore only benefit from increased algal supply in substantially longer experiments (six to nine months). It is questionable whether it is meaninghl to expose exclosures in streams for such a long time. The pushing force of stones, sedimentation, and floods are factors that may discount the idea of very long exposure times in the study area. Hence, these short-term experiments mainly study pioneer species.

'Olafsson and Moore (1990) investigated meiofaunal colonization of subtidal azoic sediment in cages and found that nematodes did not reach densities of the surrounding sediment even after one month. Also in caging experiments in estuarine sediment by Chandler and Fleeger (1983), nematodes did not reach densities of surrounding sediments after 29 days; by contrast, copepods did after only two days. On the other hand, results of Bell and Devlin (1983) showed that recolonization took place after seven hours to two days. Short life cycles of meiofauna and their ability to adapt rapidly to disturbances led Coull and Palmer (1984) to the conclusion that optimal time spans for such field experiments range from a few days to a few weeks. There is certainly a difference between colonization via water column by drift (for epibenthic organisms like copepods, but also for larvae or eggs) and infaunal colonization via sediment (for endobenthic organisms like annelids or nematodes; Fleeger and Decho 1987). In any case it must be considered that our study was a colonization experiment. Colonizers possibly do have an advantage of space and food inside the cages (Townsend and Hildrew 1976).

Nematodes: species, feeding types, and community structure Investigations of feeding behavior and the composition of nematode communities

with respect to feeding types and species have not yet been conducted in streams (Traunspurger 2000). In this study, the dominance of bacterivorous nematodes is obvious. This was observed in some lakes as well (Traunspurger 1991). In our experiment, neither the total abundance nor the number of identified species differed between meio and macro treatments. However, the species composition did differ. Samples taken from the undisturbed sediments of the pool yielded much higher densities and species numbers of nematodes than found later in the cages. Because nematode densities in rivers can fluctuate greatly during the course of the year (Traunspurger

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2000), we can, however, not necessarily conclude that nematode densities were generally lower inside the cages than outside. This supports the idea that representative population densities have not been reached inside the cages during the experiment.

In this study, we made the effort to distinguish nematodes at the species level and classified them into feeding types. On the level of feeding types, we found more bacterivorous nematodes in meio than in macro cages. Correspondingly, the percentages of algivorous nematodes, chewers, and suction feeders were higher in macro than in meio cages. These differences were not statistically significant which may be due to the low total number of nematodes and patchy distribution yielding rather high standard deviations. Except for the percentage of epistrate feeders, the distribution of the feeding types inside the cages resembled the one in the surrounding sediment, bacterivorous nematodes being the most abundant feeding type. Fine sediment was obviously more abundant in the fine-meshed meio cages than in the macro cages. An increased content of fine sediment and/or detritus would imply more food resources for bacteria and an enlarged surface for possible bacterial colonization (Virnsteiri 1978), both of which favor bacterivorous nematodes. Finally, bacterivorous nematodes may have benefited indirectly from the higher algal densities in the meio cages compared to the macro cages. Higher algal densities may have led to higher levels of algal detritus as a food source for bacteria.

Analysis of the nematode fauna on the species level also revealed that Eumonhystera (E. simplex and E. vulgaris) and Plectus spec. mainly colonized meiofaunal cages. As bacterivorous species they may have benefited from an increased content of detritus caused by the caging effect (Peckarsky and Penton 1990). Species of this genus are described as typical colonizers (Bongers 1990). Rhabdolaimus terrestris, by contrast, another bacterivorous nematode, and the suction feeders Dorylaimus stagnalis and Mesodorylaimus species mainly colonized the macrofaunal treatments. The sediment surrounding the exclosures was inhabited by a higher number of species and individuals, still mainly by Eumonhystera (E. simplex and E. vulgaris), but also the epistrate feeder Prismatolaimus intermedius. The reason for the differences in colonization patterns between these nematode genera remains unclear. Interactions among freshwater nematode species is a domain that remains to be studied (Traunspurger 2000), as is the autecology of nematode species.

ACKNOWLEDGEMENTS

We gratehlly acknowledge Sebastian Diehl and Lemart Weltje for helpful comments on the manuscript. We also thank Carlos Drews, Monika Springer, Astrid Michels and Esteban Estrada for assistance in Costa Rica. Co-operation by the Universidad de Costa Rica (Museo de Insectos), the Universidad Nacional de Heredia, the Centro de Investigaci6n de Ciencias del Mar y Limnologia, San Jod, Costa Rica, and the Area de Conservaci6n Guanacaste was most appreciated. Carlos Espinoza and Achim Weigert provided helpful lab and field assistance.

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Received: 18 April 2002 Accepted: 3 JU~V 2002

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