SEASONAL-VARIATIONS IN THE CONDITION OF 2 DAPHNIA SPECIES AND THEIR HYBRID IN A EUTROPHIC LAKE - EVIDENCE FOR FOOD LIMITATION

As seston and chlorophyll concentrations in eutrophic lakes are usually high all year round, it was hypothesized in the past that food limitation is of major importance for the seasonal dynamics of herbivorous zooplankton in such lakes. Since direct measurements of food are hampered by the lack of knowledge on the exact nature of the food in eutrophic conditions, indirect measurements are necessary to estimate the degree of food limitation in these circumstances. Hence, we used laboratory- and field-derived relationships between the body length and body carbon content of different species of Daphnia, which were collected from the highly eutrophic Tjeukemeer, the Netherlands. From a seasonal survey of the carbon content of the daphnids, we concluded that in Tjeukemeer D.galeata, D.galeata x cucullata and D.cucullata are food limited during the largest part of the year. Since the condition of the hybrids was relatively high as compared with the parental species when the food concentration was high. D.galeata x cucullata is expected to be the more successful taxon during periods of high food availability.