Daphnia Growth on Varying Quality of Scenedesmus: Mineral Limitation of Zooplankton

Because the biochemical content of primary producers varied with their growth rate, herbivores feeding on foods of high growth rate have different diets than herbivores feeding on foods of low growth rate. Herbivores' food quality is thus related to their resource's growth rate. Here, the growth in body mass of individuals of the herbivore Daphnia obtusa was measured for a variety of foods, which consisted of the chlorophyte Scenedesmus acutus cultured under a suite of N— and P—limited growth rates. Food quantity (measured as carbon) was held constant at 0.5 mg/L. In general, low growth rate algae were poorer in quality than high growth rate algae. The logarithm of Daphnia mass was well predicted by the logarithm of algal growth rate. Algal growth rates at <50% of resource saturation demonstrated marked reductions in quality. Animals feeding on low growth rate foods, and which themselves tended to grow slowly, nevertheless accumulated lipid to a higher degree than faster growing animals. A biochemical analysis of elemental (C, H, N, and P) and biochemical (protein, carbohydrate, and ash) content (all as percentage of total dry mass) suggested the nutritional basis for these differences in food quality. Phosphorus content was especially well correlated with food quality. Daphnia mass was explained with an r 2 of 0.97 by a regression model with P, C, and H content. Algal foods of >1% P were high in quality, while foods lower than this exhibited reduced quality. These results are consistent with P limitation of Daphnia growth when feeding on foods of low P content.