Changes in nitrogen retention in shallow eutrophic lakes following a decline in density of cyprinids

To study how changes in biomass of cyprinids (mainly roach, Rutilus rutilus L., and bream, Abramis brama L.) affect nitrogen retention in shallow lakes, we conducted mass balances of total nitrogen for 6-11 years in four eutrophic lakes in which the fish biomass changed markedly, either from natural causes or due to manipulation. The decline in cyprinids led to a shift from a turbid to a clearwater state in three of the four lakes. In these lakes total nitrogen (N) concentrations decreased and the percentage of N retained in the sediment, or lost by denitrification (N ret% ) increased substantially. In Lake Vmng, summer N ret% increased from 22-39 % before to 60-72 % after the biomass of cyprinids had been reduced by 50 %. N ret% temporarily decreased to 42 % during a short-term return to the turbid state. In Lake Engelsholm, a 90 % reduction in cyprinids resulted in an increase in summer mean N ret% from 13-50 % to 58-60 %, and in Lake Arreskov the annual mean N ret% increased from -4-34 % before a major fish kill to 54-59 % after. A comparison with data from 16 non-manipulated lakes revealed that these changes could not be ascribed to natural interannual variations. No significant changes in N concentrations or N ret% were found in Lake Sobygard, which remained turbid and maintained a relatively high biomass of cyprinids. In the three lakes that shifted to a clearwater state, N ret% was significantly inversely related to chlorophyll-a, but independent of the abundance of submerged macrophytes and biomass of N-fixing cyanobacteria. The increase in N ret% might have resulted from 1) a decrease in organic N in the lake and the outlet due to the decrease in phytoplankton biomass and thus phytoplankton-N, which was not compensated by an increase in inorganic N, 2) reduced resuspension, probably reflecting both a decrease in the number of fish foraging in the sediment and a suggested increase in benthic algal growth, 3) higher denitrification in the sediment, reflecting less competition between denitrifiers and phytoplankton for nitrate, enhanced N retention by phyto- and zoobenthos and enhanced sediment nitrification due to higher oxygen concentrations, the latter reflecting lower sedimentation, higher density of zoobenthos and higher oxygen production by benthic algae. More research is needed to elucidate the relative importance of these mechanisms. It may, however, be concluded that fish manipulation or phosphorus-loading reduction leading to a shift from a turbid to a clearwater state in eutrophic lakes may markedly enhance lake N ret% and consequently reduce the transfer of nitrogen to coastal waters.