Trophic dynamics in turbid and clearwater lakes with special emphasis on the role of zooplankton for water clarity

Within a certain nutrient level shallow lakes may alternate between two sta tes, a clearwater and a turbid state. To obtain more information on the cha racteristics of these two states, we compared seasonal variations in trophi c structure and physico-chemical variables of two clear and two turbid lake s studied during seven or eight years. The clearwater lakes were characteri sed by a high abundance of submerged macrophytes, high piscivorous:planktiv orous fish ratios, high zooplankton:phytoplankton ratios and low chlorophyl l a during summer. Submerged macrophytes were almost absent from the turbid lakes, planktivorous fish dominated, the zooplankton:phytoplankton ratio w as low and summer chlorophyll a was high. While total phosphorus (TP) was a lmost constant throughout the season in the clearwater lakes, TP was substa ntially higher during summer in the turbid lakes reflecting high internal l oading. In the clearwater lakes, mean summer chlorophyll a was only 45-51% of winter values, while summer chlorophyll a was 118-259% of winter values in the turbid lakes. Our data suggests that zooplankton, by grazing on phyt oplankton, play a major role in maintaining clearwater conditions in eutrop hic macrophyte-rich lakes, in particular during summer. In addition, result s from a multiple regression on data from 37 lakes and the analyses of the seasonal dynamics in suspended solids provide some evidence that zooplankto n grazing diminishes concentrations of detritus and inorganic suspended sol ids either directly by grazing or more indirectly. Using information also f rom literature, we argue that the role of zooplankton grazing for water cla rity in macrophyte-rich lakes may increase from mesotrophic to eutrophic la kes.