Phytoplankton biomass reduction after planktivorous fish reduction in a shallow, eutrophic lake: a combined effect of reduced internal P-loading and increased zooplankton grazing

Abstract

No recovery was recorded in the shallow and eutrophic Lake Væng, Denmark, after a sewage diversion in 1981, due to an internal phosphorus loading and a dominance of planktivorous fish. In order to improve lake water quality by means of biomanipulation, a total of 2.5 tons of bream (Abramis brama)and roach (Rutilus rutilus)was removed during 1986 and the spring of 1987. The planktivorous/benthivorous fish biomass was thereby reduced by approximately 50%, from 30 to 15 g WW m⁻². After the reduction, the biological structure of the lake changed markedly. The zooplankton community changed from a dominance of rotifers before the fish reduction to larger cladocerans afterwards. Zooplankton mean summer biomass increased from 0.4 mg DW 1⁻¹ in 1986 to 2.7 mg DW 1⁻¹ in 1987 and to 1.3 mg DW 1⁻¹ in 1988. Phytoplankton biomass decreased from a mean summer level of 25 mm³ 1⁻¹ in 1986 to 12 in 1987 and to 7 mm³ 1⁻¹ in 1988. Qualitatively, the phytoplankton changed from a dominance of cyanobacteria and small diatoms to a dominance of larger diatoms, larger greens and cryptophytes. Secchi depth increased from a mean summer level of 0.6 m in 1986 to 1.0 m in 1987 and 1.3 m in 1988. After the fish removal, the internal phosphorus loading was reduced markedly. This reduction is considered primarily to be caused by the improved redox conditions in the sediment due to reduced sedimentation, and by the increased micro-benthic primary production and phosphorus uptake. Submerged macrophytes, mainly Potamogeton crispus and Elodea canadensis, increased in abundance due to improved light climate at the lake bottom.