Oligochaete distribution patterns in two German hardwater lakes of different trophic state

We investigated the effect of contrasting trophic conditions on the distribution and dominance patterns of Oligochaeta species assemblages in lake sediments and their relation to the environmental variables depth, season, and substrate. The study was performed on the highly eutrophic Lake Haussee and the oligotrophic Lake Stechlin, both hardwater lakes in the Baltic Lake District of Northern Germany. Quantitative monthly and seasonal sampling took place over one year at 14 representative sites, covering littoral and profundal sediments of both lakes. Between-lake differences in the profundal were clearcut with an absence of any zoobenthos in Lake Haussee and a peculiar meiobenthic species assemblage in Lake Stechlin (Collado et al. 1999). Between-lake differences in the littoral, however, were small and mainly attributable to a small number of species exclusive to Lake Stechlin and an overall higher abundance of oligochaetes, especially naidids, in Lake Haussee. Species-richest family in both lakes were Naididae; Tubificidae were dominant in Lake Stechlin; in Lake Haussee Tubificidae and Naididae were equally abundant. Most striking were diversity of habitat types and species distribution patterns in the littoral. Multivariate analysis (CA, CCA) showed that the distribution patterns of oligochaete species assemblages are significantly correlated with depth, season and substrate. Depth is the major factor when the whole water body is considered. When confining to the littoral, species abundance and distribution are strongly related to seasonality and substrate type. Naididae show, in general, maximal abundances in autumn and summer and a preference for plants, plant debris and soft sediments; Tubificidae are more abundant in spring and prefer mineral substrate. The relation between seasonality, substrate and food availability is discussed.