SEASONAL DYNAMICS OF THE MYSID NEOMYSIS INTEGER AND ITS PREDATION ON THE COPEPOD EURYTEMORA-AFFINIS IN A SHALLOW HYPERTROPHIC BRACKISH LAKE

The study included field sampling, field experiments in enclosures and laboratory predation experiments. Neomysis integer, which underwent 2 generations during the study year (1992), occurred in high density, the maximum being 0.8 ind. l(-1) or 1250 ind. m(-2). Estimated production was 2.2 g DW (dry wt) m(-2) yr(-1) and peaked in August at 25 mg DW m(-2) d(-1). The laboratory predation experiments revealed a high predation potential on Eurytemora affinis, the maximum predation rates on nauplii and copepodites + adults being 52 and 8 ind. mysid(-1) h(-1), respectively. Stepwise multiple regressions on the data obtained from enclosure experiments conducted at various densities of N. integer and three-spined sticklebacks Gasterosteus aculeatus revealed that the number of nauplii and the total number of E. affinis were significantly negatively related to the densities of both N. integer and fish, albeit positively the interaction between fish density and N. integer density. In addition, the density of the advanced stages was significantly negatively related to that of fish, while the chlorophyll a concentration was weakly negatively related to E. affinis biomass and highly positively related to that of N. integer. The results provide further evidence that N. integer enhances eutrophication in nutrient-rich brackish lakes, i.e. N. integer predation on zooplankton reduces grazing pressure on the phytoplankton. Moreover, the results indicate that mysids also stimulate phytoplankton growth more directly.