Differential effects of phosphorus and fatty acids on Daphnia magna growth and reproduction

We investigated the effects of various mineral and biochemical limitations on Daphnia magna. These daphniids have much lower saturation thresholds for growth for the polyunsaturated fatty acids (PUFA) eicosapentaenoic acid (EPA), and arachidonic acid (ARA) than has been previously described for other Daphnia species. Daphniids take up large amount of fatty acids from food, and different fatty acids are handled differently by D. magna. The saturated fatty acid (20:0; EPA) was not retained, and metabolized, the PUFAs were preferably stored. There were also differences among the PUFAs: EPA was found in higher concentrations in the eggs than ARA. In contrast, although there were some variations in D. magna phosphor-us levels with varying levels of phosphorus in the food, these differences were small compared with the changes in D. magna fatty-acid concentrations. Independent of these small changes, the P content of eggs was constant at 14 mg P (g dry wt)(-1). Storage of EPA, but not P, fully compensated D. magna growth during periods of bad food quality. Egg production was a major drain of fatty acids from female D. magna.