SEASONAL PATTERNS OF FOOD LIMITATION IN DAPHNIA-GALEATA - SEPARATING FOOD QUANTITY AND FOOD QUALITY EFFECTS

Food limitation for the cladoceran Daphnia galeata was measured growin g them with natural lake seston for one growing season under standardi zed conditions in the laboratory. Growth rates were related to several measures of food quantity. Particulate organic carbon (POC) <30 mu m was the best predictor of total food availability measured. It was bet ter than chlorophyll-a (Chi-a) of the same size fraction, algal volume calculated from cell counts or total particle volume from Coulter Cou nter measurements. Daphnids were also grown with Scenedesmus acutus un der the same conditions and their growth rates were compared. Sestonic carbon concentrations were related to Scenedesmus carbon concentratio ns, that provided equal growth rates. The ratio of the carbon concentr ations of both food types (seston, Scenedesmus) represented the food q uality of seston. Thus, the nutritional quality of natural lake seston relative to a standard alga was determined over one growing season. G rowth rates of seston fed animals revealed that they were food limited during long periods of the year. This was also ascertained by standar d clutch sizes of field animals. However, the causes of food limitatio n varied within the duration of the studied period. Whereas during the clear water phase the food quantity was limiting (while the quality w as high; 100% Scenedesmus). shortly after on 10 June and 17 June food conditions were close to threshold concentrations due to low food qual ity. For the remainder of the summer, a POC of 0.3-0.6 mg C l(-1) with a quality of similar to 50% Scenedesmus, resulted in moderate food li mitation of Daphnia.