Dynamics of pelagic rotifers subject to trophic fluctuations in Upper Lake Constance (1963-2012)

Abstract

This study evaluates a long-Term monitoring dataset recording the pelagic rotifers of Upper Lake Constance (ULC) over almost 50 years. The same period has seen profound changes in trophic conditions in the lake, with total phosphorus concentrations rising from 18 μg L â '1 in 1963 to 84 μg L â '1 in 1984, and then declining back to 6.3 μg L â '1 by 2012. A severe bias identified in calculations for density and abundance was successfully controlled using a density function for vertical rotifer distribution developed from a supplementary dataset. Six out of 21 genera were lost from the rotifer assemblage during eutrophication phase and failed to re-Appear when oligotrophic conditions were restored. From 2007 onwards, individuals of the remaining rare genera remained scarce, while densities of the dominant genera increased significantly when phosphorus levels declined. Unlike other water bodies, and in contrast with the situation prior to eutrophication, the diversity of ULC rotifers today correlates negatively with trophic state. Grazing pressure exerted by both rotifer and crustacean communities decreased with increasing P concentrations indicating increasing top down control. Surprisingly, however, since 2008, the influence of dominant rotifers has even exceeded that of crustacean grazers with likely implications for the entire pelagic food web.