Relative impacts of Daphnia grazing and direct stimulation by fish on phytoplankton abundance in mesocosm communities

1. Planktivorous fish were hypothesised to influence the abundance of algal biomass in lakes by changing zooplankton grazing, affecting zooplankton nu trient recycling and by direct recycling of nutrients to phytoplankton. The relative roles of direct fish effects vs. zooplankton grazing were tested in mesocosm experiments by adding to natural communities large grazing zoop lankton (Daphnia carinata) and small planktivorous fish (mosquitofish or ju veniles of Australian golden perch). 2. The addition of Daphnia to natural communities reduced the numbers of al l phytoplankton less than 30 mu m in size, but did not affect total biomass of phytoplankton as large Volvox colonies predominated. 3. The addition of Daphnia also reduced the abundance of some small (Moina, Bosmina, Keratella) and large (adult Boeckella) zooplankton, suggesting co mpetitive interactions within zooplankton. 4. The addition of mosquitofish to communities containing Daphnia further r educed the abundance of some small zooplankton (Moina, Keratella), but incr eased the numbers of Daphnia and adult Boeckella. In spite of the likely in crease in grazing due to Daphnia, the abundance of total phytoplankton and dominant alga Volvox did not decline in the presence of mosquitofish but wa s maintained at a significantly higher level than in control. 5. The addition of juveniles of golden perch to communities containing Daph nia reduced the abundance of small zooplankton (Moina), increased the abund ance of large zooplankton (adult Boeckella) but had no significant effect o n Daphnia and total phytoplankton abundance. 6. The results of the present study suggest that some planktivorous fish ca n promote the growth of phytoplankton in a direct way, probably by recyclin g nutrients, and even in the presence of large grazers. However, the manife station of the direct effect of fish can vary with fish species.