Seasonal patterns of food limitation in Daphnia galeata: Separating food quantity and food quality effects

Abstract

Food limitation for the cladoceran Daphnia galeata was measured growing them with natural lake seston for one growing season under standardized conditions in the laboratory. Growth rates were related to several measures of food quantity. Particulate organic carbon (POC) <30 μm was the best predictor of total food availability measured. It was better than chlorophyll-a (Chl-a) of the same size fraction, algal volume calculated from cell counts or total particle volume from Coulter Counter measurements. Daphnids were also grown with Scenedesmus acutus under the same conditions and their growth rates were compared. Sestonic carbon concentrations were related to Scenedesmus carbon concentrations, that provided equal growth rates. The ratio of the carbon concentrations of both food types (seston, Scenedesmus) represented the food quality of seston. Thus, the nutritional quality of natural lake seston relative to a standard alga was determined over one growing season. Growth rates of seston fed animals revealed that they were food limited during long periods of the year. This was also ascertained by standard clutch sizes of field animals. However, the causes of food limitation varied within the duration of the studied period. Whereas during the clear-water phase the food quantity was limiting (while the quality was high; 100% Scenedesmus), shortly after on 10 June and 17 June food conditions were close to threshold concentrations due to low food quality. For the remainder of the summer, a POC of 0.3-0.6 mg C l-1 with a quality of ~50% Scenedesmus, resulted in moderate food limitation of Daphnia.