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 usuall y high all year round, it was hypothesized in the past that food limit ation is of major importance for the seasonal dynamics of herbivorous zooplankton in such lakes. Since direct measurements of food are hampe red by the lack of knowledge on the exact nature of the food in eutrop hic conditions, indirect measurements are necessary to estimate the de gree of food limitation in these circumstances. Hence, we used laborat ory- and field-derived relationships between the body length and body carbon content of different species of Daphnia, which were collected f rom the highly eutrophic Tjeukemeer, the Netherlands. From a seasonal survey of the carbon content of the daphnids, we concluded that in Tje ukemeer D.galeata, D.galeata x cucullata and D.cucullata are food limi ted during the largest part of the year. Since the condition of the hy brids was relatively high as compared with the parental species when t he food concentration was high. D.galeata x cucullata is expected to b e the more successful taxon during periods of high food availability.