Inference of past changes in zooplankton community structure and planktivorous fish abundance from sedimentary subfossils - a study of a coastal lake subjected to major fish kill incidents during the past century

Abstract

To describe past changes in trophic structure, analyses were conducted of zooplankton (Cladocera, Rotifera) assemblages recovered in a 210 Pb dated sediment core from a shallow brackish lake exposed to several fish kill episodes during the 20 th century. A weighted-averaging zooplankton-based transfer function was applied to infer planktivorous fish densities (PL-CPUE) and to elucidate the potential of identify- ing fish kill incidents using sedimentary subfossils. The stratigraphical record showed a shift during the 20 th century. The cladoceran community structure changed from high abundance of macrophyte-associated taxa and chydorids, characteristic of clear water conditions, towards a community composed mainly by sediment-associated taxa, small-sized pelagic taxa as well as rotifers and chydorids, characteristic of eutrophic turbid conditions. Coinciding with these changes, the mean size of Daphnia ephippia decreased while inferred PL-CPUE increased. The results point towards progressive eutrophication with additional impact of salinity. Reduced temporal resolution of the sediment record made identification of fish kill incidents difficult. The indications of high fish planktivory and a decline in macrophyte abundance are consistent with anec- dotal observations of past fish investigations and contemporary data. The study dem- onstrates the usefulness of applying zooplankton subfossils to detect long-term changes in the trophic structure of brackish lakes in the past. In addition, it draws at- tention to the difficulties of using sediment records to highlight short-term events, such as fish kills, in shallow wind-impacted lakes.