Population response of pelagic fishes (ciscoes, Coregonus spp.) to rapidly accelerated eutrophication of an originally oligotrophic deep lake

Abstract

Not much is known about early responses of pelagic fish populations to rapidly increasing nutrient concentrations in originally oligotrophic lakes. Here we report on population parameters of a pair of pelagic ciscoes (Coregonus spp.) exposed to accelerating eutrophication of the deep (69.5 m) and stratified Lake Stechlin (NE Germany), in which total phosphorus (TP) concentration increased from about 15 mg m-3 to >60 mg m-3 within 10 years. With increasing TP concentrations, lower Secchi depths, declining oxygen concentrations in deeper water layers, higher phytoplankton and higher zooplankton biomass were observed. Correlation analyses supported strong bottom-up directed effects of higher TP concentrations on primary and secondary producers, and negative effects of TP on water quality parameters. Annual hydroacoustic records and midwater trawling showed an increase in average individual fish size. There was no temporal trend in fish biomass, but we observed cyclic annual biomass fluctuations. Diel vertical migration (DVM) amplitudes of ciscoes declined, primarily by occurrence of fish in shallower waters than in previous years during daytime. We propose that release from density-dependent food limitation, due to increased plankton food abundances, resulted in increased individual sizes for both the cisco species, which in turn increasingly blurs species discrimination because length distributions of both species are no longer substantially different. Our data show the value of long-term lake monitoring including fish population surveys, to understand the initially subtle changes in biotic communities, which may precede more drastic and potentially irreversible changes to anthropogenic pressures.