Biochemical versus mineral limitation in Daphnia

Laboratory growth experiments were used to test whether observed phosphorus limitation in Daphnia is a direct effect of P availability. Daphnia was fed the algae Cyclotella and Scenedesmus , each P saturated or P limited. The algal species were chosen because of their distinct fatty acid composition, particularly with respect to their eicosapentaenoic acid (20:5 ω 3) content. Although both algal species had reduced 20:5 ω 3 content under P limitation, P‐limited Cyclotella had a substantially higher 20:5 ω 3 content than did P‐saturated Scenedesmus. In keeping with its P and 20:5 ω 3 content, P‐limited Scenedesmus was the lowest quality of food. Inconsistent with mineral P limitation in Daphnia but in agreement with the 20:5 ω 3 content, P‐starved Cyclotella was a better quality of food than was P‐sufficient Scenedesmus. Thus P limitation seems to be a rather indirect effect of changes in algal biochemical composition in response to nutrient stress.