INTERRELATIONSHIPS BETWEEN CRUSTACEAN ZOOPLANKTON AND PHYTOPLANKTON -RESULTS FROM 15 YEARS OF FIELD OBSERVATIONS AT THE MESOTROPHIC SAIDENBACH RESERVOIR (GERMANY)

Fifteen years of data (1975-1990) on the phyto- and crustacean plankto n in the meso/eutrophic Saidenbach Reservoir were analysed to reveal c orrelations between these groups or members of them. The weekly or for tnightly samples were collected from different depths and times, and w ere integrated to form seasonal averages. For the dominant organisms, summer means of abundance were plotted against one another. Among the Crustacea, Daphnia galeata exerts a strong influence on nanoplankton r esulting in an increased Secchi depth in years with a high standing st ock of Daphnia. No such correlations were found for Eudiaptomus and th e Cyclopoida, which are not able to remove such small particles. The a bundance of Daphnia showed no obvious effects on Cyanophyta, colony fo rming Chlorophyta and Asterionella formosa. This implies that in the S aidenbach Reservoir, factors other than crustacean grazing are of cruc ial importance for the population dynamics of the latter algal groups. It is shown, that the influence of the Crustacea on the phytoplankton is evident not only for short periods, i.e. in clear-water stages, or in biomanipulated lakes, but also can be observed under 'non-manipula ted' conditions for longer time periods.