Limnetica 35

Due to their origin and their morphometric and hydrodynamic parameters, oxbow lakes represent a distinct type of aquatic ecosystem, especially compared with post-glacial lakes. The functioning of oxbows is directly and indirectly connected with temporal fluctuations of the water level in rivers. Periodic inundation of floodplain habitats provides primary producers with access to mineral and organic compounds, creates refuge conditions for aquatic organisms against disturbances in the river channel, and contributes to the increase of biodiversity in the whole river system. To determine the best predictors of zooplankton community indices, as well as those of rotifer and crustacean species, we examined them with respect to the following environmental parameters: the type of habitat, the type of land uses, the shading of the lake’s surface and the presence of fish. The study was performed on oxbows during limnophase. Despite the fact that the biotic structure in oxbows may be simplified, it was found that a high level of habitat complexity positively influences zooplankton species richness and the Shannon-Weaver index of diversity. This would support the fact that during limnophase, oxbows are in an ecotonal phase – between the flooding and the stabilization period. In contrast to other types of small water bodies, the type of land uses (forest vs. field) that affected the quality of the supplies of organic and mineral compounds, along with the presence of fish, did not have a pronounced effect on either zooplankton diversity or their community abundance. Particular zooplankton species also exhibited habitat segregation in accordance with their ecological group (pelagic vs. littoral species) or taxonomic affiliation (rotifers vs. crustaceans). Littoral species, attributed to elodeids, preferred oxbows with a high degree of overshading. Among the pelagic zooplankton, which preferred to remain in the open water zone, a large representation of eutrophic species was recorded due to the overload of the water by mineral nutrients and organic matter. The high zooplankton diversity of the studied aquatic ecosystems and the abiotic features, which revealed an unstable and disturbed status, confirmed the fact that oxbows during the limnophase reflect an ecotonal phase. This may be a complementation to the river flood pulse concept.