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This study relates measured sestonic food parameters to growth rates of Daphnia galeata. Daphnia growth rates were obtained from standardized laboratory experiments feeding the animal natural lake seston. The experimental period spanned one season. Elemental (carbon, nitrogen, phosphate) and biochemical (fatty acids) food parameters of this natural food source were compared. The correlation between growth rates and sestonic phosphorus was very weak (r-2 = 0.08). The regressions with the nitrogen (r-2 = 0.42) and carbon (r-2 = 0.62) content were significantly better but still exhibited the usual considerable scatter around the regression line. In contrast, a very tight correlation (r-2 = 0.93) was found between Daphnia growth rates and the sestonic content of eicosapentaenoic acid (2Q:5-omega-3), an essential, highly unsaturated fatty acid. This relationship exhibited a normal asymptotic curve with a distinct plateau above concentrations of 0.8 mu-g 20:5-omega-3/L. In contrast, a plateau was not observed for particulate organic carbon, nitrogen or phosphate. Carbon, nitrogen and other essential fatty acids were not only moderately correlated to Daphnia's growth rates but also to 20:5-omega-3. Thus, the modest suitability of carbon and nitrogen as food parameters seems to be due to co-occurrence with 20:5-omega-3. This shows, that the sestonic 20:5-omega-3 content is a parameter for natural food quantity including its food quality. High 20:5-omega-3 concentrations within the plateau region occurred mostly during spring. These results suggest that Daphnia was limited by the 20:5-omega-3 concentration during most of the summer
