LIMNOLOGICAL RESPONSES TO THE COLLAPSE OF THE PERCH POPULATION IN A SMALL LAKE

Several limnological responses were recorded after an almost total col lapse of the population of perch, Perca fluviatilis, in the control ba sin of the limed Lake Iso Valkjarvi in autumn 1992. Increases in hypol imnetic concentrations of dissolved organic carbon and ammonium-nitrog en were the most striking changes in water chemistry. After a few mont hs, high densities of ciliates were recorded in the hypolimnion, sugge sting an increased microbial activity in the decomposition of fish car casses. Correspondingly, hypolimnetic bacterial biomasses in summer 19 93 were higher than earlier. In crustacean zooplankton: the densities of planktonic cladocerans were lower in the fishless side of the lake in summer 1993 than before the fish kill in the previous summer. The d ecline in cladocerans was probably due to predation by the rapidly inc reased numbers of Chaoborus larvae and corixids after the disappearanc e of fish. The mean biomass of zoobenthos also increased following the fish kill. The remaining perch in the control basin started to grow v ery rapidly. Instead of the normal length increase of 1-2 cm per year, they grew 5-7 cm in the summer of 1993 and their weight increased ten times more than that of perch of the limed basin of the lake. The mer cury concentrations and radioactivity of the perch showed a 50% decrea se, probably due to the growth dilution. A behavioural change in the g olden-eye, Bucephala clangula, was also recorded. The breeding individ uals continued to use both basins of L. Iso Valkjarvi, but the brood c learly preferred the control basin after the collapse of the perch pop ulation.