Implications of phytoplankton chemical composition for zooplankton production: experimental evidence.

Growth and reproduction of twoDaphnia species (Cladocera) from Lake Constance were measured experimentally in a through-flow laboratory system under different food conditions, in order to investigate the effect of food quality on zooplankton populations. The cryptomonadRhodomonas sp. and the diatomStephanodiscus hantzschii were used as food. BothDaphnia species grew well at concentrations of 0.2-2.0 mgC/l ofRhodomonas sp., and 0.4-2.0 mgC/l ofS. hantzschii, whileStephanodiscus concentrations of 0.2 mgC/l did not supportDaphnia growth. Nitrogen content, which was very different in the two algae (C/N ratio = 4.9 forRhodomonas and 10.9 forS. hantzschii), was presumed to be the limiting factor. To test this hypothesis, nitrogen-limitedRhodomonas was offered to the cladocerans and their growth observed: the reduction in algal nitrogen content hindered animals' growth and reproduction. Results suggested that zooplankton production may also be affected by the chemical composition of the food algae, and that nitrogen potentially limits production, even in freshwater environments that are generally not N-limited.