Essential fatty acid content and the phosphorus to carbon ratio in cultured algae as indicators of food quality for Daphnia

Abstract

1. A series of experiments examined growth of Daphnia magna on three algal diets (Rhodomonas minuta, Scenedesmus acutus and Synechococcus sp.) at varying physiological states [nitrogen and phosphorus (P) limitations] to test whether variation in algal fatty acid and/or elemental composition can predict Daphnia growth.2. These algae differed widely in their essential fatty acid (EFA) composition while phosphorus (P) or nitrogen limitation had only a small influence on their ω3‐polyunsaturated fatty acid (PUFA) content.3. Individually, algal ω3‐PUFA content explained 69% of the variation in the normalised growth of D. magna, while algal phosphorus content explained 11% of the variation. Quantitative models for D. magna growth used both algal ω3‐PUFA content and algal C : P ratio as food quality indices. Together, algal ω3‐PUFA content and C : P ratio explained 70% of the variation in the normalised growth rate of D. magna.4. Our results indicate that EFA influenced algal food quality much more strongly than P. The EFA and mineral P impacts appear to be independent.