Effects of nutrient enrichment on the trophic structure and species composition of freshwater nematodes-a microcosm study

We explored changes in trophic structure and species composition of freshwater nematodes in response to nutrient enrichment in microcosms for 16 mo. Succession of functional groups differed significantly among the 5 nutrient treatments, which ranged from oligotrophic to polytrophic conditions, in the 2(nd) half of the experiment. Differences among nutrient levels were most important for representatives of lower trophic levels, such as bacterial and algal feeders, whereas suction feeders and predators remained relatively unaffected. In contrast to our expectations, density and biomass of bacterial and algal feeders increased in the nutrient-poor treatments because of the presence of a high fraction of edible unicellular green algae and diatoms, whereas large standing stocks of inedible macrophytes developed in the highly nutrient-enriched treatments. Nematode assemblages in microcosms with intermediate or high nutrient levels were characterized by a large proportion of omnivores and predators, respectively, because of relatively low total densities. Our results also suggest a gradual change in species composition along the nutrient gradient because the degree of pairwise assemblage dissimilarity increased with corresponding differences in nutrient enrichment. Our study highlights the potential of bottom-up forces in shaping freshwater nematode assemblages and provides further evidence of pelagic-meiobenthic coupling.