Growth and reproduction of Daphnia galeata in response to changes in fatty acids, phosphorus, and nitrogen in Chlamydomonas reinhardtii

Abstract

The importance of changes in elemental and fatty acid composition of the algal food for Daphnia galeata was investigated. The green alga Chlamydomonas reinhardtii was grown under nitrogen or phosphorus limitation to modify its elemental and biochemical composition. Both N‐ and P‐limited algae exhibited similar fatty acid compositions but differed from algae grown under N and P saturation. Nutrient limitation of algae caused the amounts of saturated fatty acids, monounsaturated, and diunsaturated fatty acids to increase, but those of polyunsaturated fatty acids to decrease markedly. Life‐history experiments with D. galeata, carried out to examine the effects of the varying N and P regimes to the food quality of Chlamydomonas, revealed that N‐limited and N+P‐saturated algae were of a comparable quality. In contrast, P‐limited alga was a very poor food; that is, both population growth and somatic growth rate were much lower than with N‐ and N+P‐saturated algae. Differences of algal fatty acid composition did not explain the differences in algal food quality as N+P‐saturated and N‐limited Chlamydomonas were both similar in quality despite differences in fatty acid composition. P limitation of daphnid growth is more consistent with the observed differences in growth and reproduction. The low growth rates of the daphnids when fed P‐limited alga, however, may also be a result of indirect effects because P limitation may induce changes in algal morphology or biochemical compounds other than fatty acids.