IMPLICATIONS OF PHYTOPLANKTON CHEMICAL-COMPOSITION FOR ZOOPLANKTON PRODUCTION - EXPERIMENTAL-EVIDENCE

Growth and reproduction of two Daphnia 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 cryptomonad Rhodomonas sp. and the diatom Stephanodiscus hantzschii were used as food. Both Daphnia species grew well at concentrations of 0.2-2.0 mgC/l of Rhodomonas sp., and 0.4-2.0 mgC/l of S. hantzschii, while Stephanodiscus concentrations of 0.2 mgC/l did not support Daphnia growth. Nitrogen content, which was very different in the two algae (C/N ratio = 4.9 for Rhodomonas and 10.9 for S. hantzschii), was presumed to be the limiting factor. To test this hypothesis, nitrogen-limited Rhodomonas 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.