Seston food quality and Daphnia production efficiencies in an oligo-mesotrophic Subalpine Lake

Because of major biochemical imbalances between plants and animals, ecological efficiency at this interface may have a major impact on overall energy flow in ecosystems. In order to study relationships between seston food quality and energy transfer between primary producers and herbivores, we conducted five microcosm experiments in Castle Lake, California, USA during the summer of 1996. We simultaneously performed life table experiments to determine the effects of highly unsaturated fatty acids (HUFA) on Daphnia rosea growth, reproduction and survival. The results of these experiments suggest strong energy limitation of D. rosea growth in Castle lake during the study. D. rosea production was coupled with primary production in Castle Lake and in the microcosm experiments. D. rosea production efficiencies, i.e., the ratios of D. rosea productivity to primary productivity, decreased towards the end of the summer. A food quality index based on phytoplankton species composition and seston carbon to phosphorus ( C: P) ratio were good predictors of D. rosea production efficiencies. The predicted D. rosea production pattern based on phytoplankton composition and primary productivity matched the zooplankton biomass dynamics in Castle Lake during 1991. Life table experiments showed HUFA effects on D. rosea population growth rates, reproduction and survival in support of the HUFA limitation hypothesis.