DOES THE IMPACT OF NUTRIENTS ON THE BIOLOGICAL STRUCTURE AND FUNCTIONOF BRACKISH AND FRESH-WATER LAKES DIFFER

The effects of nutrients on the biological structure of brackish and f reshwater lakes were compared. Quantitative analysis of late summer fi sh, zooplankton, mysid and macrophyte populations was undertaken in 20 -36 shallow brackish lakes of various trophic states and the findings compared with a similar analysis of shallow freshwater lakes based on either sampling (fish) or existing data (zooplankton, mysids and macro phytes). Special emphasis was placed on differences in pelagic top-dow n control. Whereas the fish biomass (CPUE, multiple mesh-size gill net s) rose with increasing P-concentration in freshwater lakes, that of b rackish lakes was markedly reduced at P-concentrations above ca. 0.4 m g P l-1 and there was a concomitant shift to exclusive dominance by th e small sticklebacks (Gasterosteus aculeatus and Pungitius pungitius); as a result, fish density remained relatively high. Mysids (Neomysis integer) were found at a salinity greater than 0.5 parts per thousand and increased substantially with increasing P-concentration, reaching levels as high as 13 ind. l-1. This is in contrast to the carnivorous zooplankton of freshwater lakes, which are most abundant at intermedia te P levels. The efficient algal controller, Daphnia was only found at a salinity below 2 parts per thousand and N. integer in lakes with a salinity above 0.5 parts per thousand. Above 2 parts per thousand the filter-feeding zooplankton were usually dominated by the less efficien t algal controllers Eurytemora and Acartia. In contrast to freshwater lakes, no shift to a clearwater state was found in eutrophic brackish lakes when submerged macrophytes became abundant. We conclude that pre dation pressure on zooplankton is higher and algal grazing capacity lo wer in brackish eutrophic-hypertrophic lakes than in comparable freshw ater lakes, and that the differences in trophic structure of brackish and freshwater lakes have major implications for the measures availabl e to reduce the recovery period following a reduction in nutrient load ing. From the point of view of top-down control, the salinity threshol d dividing freshwater and brackish lakes is much lower than the conven tionally defined 5 parts per thousand.