GROWTH AND REPRODUCTION OF DAPHNIA-GALEATA IN RESPONSE TO CHANGES IN FATTY-ACIDS, PHOSPHORUS, AND NITROGEN IN CHLAMYDOMONAS-REINHARDTII

The importance of changes in elemental and fatty acid composition of t he algal food for Daphnia galeata was investigated. The green alga Chl amydomonas reinhardtii was grown under nitrogen or phosphorus limitati on to modify its elemental and biochemical composition. Both N- and P- Limited algae exhibited similar fatty acid compositions but differed f rom algae grown under N and P saturation. Nutrient limitation of algae caused the amounts of saturated fatty acids, monounsaturated, and diu nsaturated 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-sat urated algae were of a comparable quality. In contrast, P-limited alga was a very poor food; that is, both population growth and somatic gro wth rate were much lower than with N- and N+P-saturated algae. Differe nces of algal fatty acid composition did not explain the differences i n 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 grow-th and reproduction. The low growth rates of the d aphnids when fed P-limited alga, however, may also be a result of indi rect effects because P limitation may induce changes in algal morpholo gy or biochemical compounds other than fatty acids.