The nutritional value of several planktonic algae was tested by means of feeding trials with three cladoceran zooplankters. The algae were monocultures and included two blue-greens, four greens and four flagellates with a size range of 5-48 u,m. The specific growth rates of the zooplankters were chosen as the measure of the nutritional value of the algae. The three cladocerans showed large differences in growth rate in the different algae, but the two cryptomonads were without doubt best suited as food for all. The fatty acid composition for the cryptomonads were different from the other algae. They contained high percentages of the polyunsaturated fatty acids 20:5u)3 (EPA) and 22:6w3 (DHA), which also are common in fish. It is suggested that the lipid composition is a probable factor determining the nutritional quality of the algae.

Population growth rates for Daphnia longispina, Bosmina longispina, and Chydorus sphaericus were determined in monocultures or mixtures of algae typical of the spring bloom (Stephanodiscus, Rhodomonas, and Chlamydomonas) and summer (Scenedesmus, Chlorella, and Microcystis). Algae from the spring bloom resulted in optimal population growth, whereas the green algae and cyanobacteria were of poorer quality. Daphnia showed the highest growth rates in all resources tested. Chydorus attained similar positive growth rates in most of the resources. In contrast, Bosmina achieved positive growth only in Stephanodiscus and Rhodomonas. For Bosmina, a combination of Stephanodiscus and Rhodomonas resulted in a positive synergisic effect on growth. The quality of resources depends greatly on the grazer of interest and cannot be generalized from results with Daphnia alone.