The response of benthic macroinvertebrates to whole-lake biomanipulation

Abstract

The response of a benthic macroinvertebrate assemblage to whole-lake biomanipulation was studied in a small Finnish mesotrophic lake. From 1993 to 1997, over 200 kg ha −1 of fish, mainly roach (Rutilus rutilus (L.)) and bream (Abramis brama (L.)) were caught and the fish biomass was reduced by nearly 80%. The biomass and density of benthic invertebrates were investigated during the years of fish removal and for the following three years. The decrease in benthivorous fish stock led to a higher biomass and density of all major groups of benthic invertebrates during the early years of fish removal. Non-biting midges (Chironomidae), water mites (Hydrachnellae), mayfly nymphs (Ephemeroptera), sphaeriid clams (Bivalvia: Sphaeridae) and biting midges (Ceratopogonidae) seemed to respond most profoundly to changes in fish biomass. The biomass of most invertebrate groups correlated negatively with the catch-per-unit-effort (CPUE). The total biomass and density of invertebrates had strong negative correlations with the CPUE (r =− 0.85, p = 0.016, r =− 0.84, p = 0.019, respectively), but they did not correlate significantly with total phosphorus, chlorophyll a, or temperature. However, the variation in total biomass that was not explained by the CPUE, was significantly associated with total phosphorus. The fish stock recovered to almost its initial level within three years after fish removal had been discontinued. As an apparent response to increased predation pressure, the biomasses of many invertebrate groups decreased again in the years 1999–2000. The strong relationship between macrozoobenthos and fish populations in the studied lake is likely to be a consequence of the open and sparsely vegetated bottom, which offers minimal shelter to invertebrate prey. An additional factor behind the recent low biomass levels may be changes in primary production. Phosphorus and chlorophyll a concentrations started to decrease markedly after three years of fishing and they have remained at a low level.