Effects of fatty acid and phosphorus content of food on the growth, survival and reproduction of Daphnia

1. Mixtures of microencapsulated lipids and Scenedesmus quadricauda grown at different degrees of P limitation were used as food for Daphnia galeata in two growth experiments. Thereby, food quality in terms of omega 3-fatty acid (omega 3-FA) or phosphorus (P) content could be assessed without interference from other factors. 2. omega 3-highly unsaturated fatty acids (omega 3-HUFA), given to Daphnia as fish oil or eicosapentaenoic acid (EPA) together with non P-limited algae, decreased the time to first reproduction. When fed fish oil, somatic growth and survival were also enhanced. Linolenic acid also decreased the time to first reproduction but to a lesser extent than EPA. 3. Food quality depended to a large extent on the degree of P limitation of Scenedesmus, which is consistent with P limitation of Daphnia. The overall impact of P was always larger than the effect of omega 3-FA. Growth, survival and reproduction were elevated when Daphnia were fed non P-limited Scenedesmus compared to treatments with P-limited algae. 4. The relative importance of omega 3-HUFA and P content in the food changed over a C:P gradient, with stronger effects of omega 3-HUFA at low C:P ratios.