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Abstract The combined effects of grazing and nutrient regeneration by Daphnia and Eudiaptomus on the growth of Rhodomonas and heterotrophic bacteria was assessed experimentally. The responses of Rhodomonas and bacteria to the grazers were measured as net specific growth rate over the entire experimental periods, as well as production and specific production at the end of the experiments. Both zooplankton species had a negative effect on Rhodomonas net specific growth rate due to grazing and a positive effect on specific primary production due to nutrient regeneration. Daphnia had no effect on bacterial net specific growth rate, bacterial production or specific bacterial production in one of two experiments. In the other experiment, however, both bacterial growth rate and production decreased as a result of grazing. Furthermore, Daphnia had a negative effect on specific bacterial production, but Eudiaptomus had a positive effect on all bacterial parameters due to nutrient regeneration, probably of phosphorus. Positive effects of copepods on bacterial growth has previously been attributed to trophic cascades via protozoa. However, the present experiments show that regeneration of nutrients, especially phosphorus, may account for a large part of the stimulation of bacterial growth.

The effect of temperature and viscosity on the ingestion rate of Daphnia galeata and Chydorus sphaericus was investigated in short-term feeding experiments with 3H-labelled Scenedesmus quadricauda. Viscosity was manipulated independently from temperature by addition of inert polymers in two experiments. In the first experiment, the ingestion rate of D. galeata was found to be 50% lower at 10°C than at 25°C due to the combined effects of increased viscosity and lower temperature. Viscosity alone accounted for 61% of the decline in ingestion rate. In the second experiment, the ingestion rate of a different clone of D. galeata was 46–53% lower at 10°C than at 20°C and changes in viscosity alone accounted for most (>92%) of that decline. The ingestion rate of C. sphaericus was 80% lower at 10°C than at 20°C, and viscosity accounted for 65% of that decline. Thus, viscosity plays an important role in determining the ingestion rate of crustacean zooplankton. Adaptations to life in waters that undergo considerable variation in temperature and viscosity may therefore influence energy budgets and may eventually be of importance for zooplankton species succession, distribution and evolution.