RELATIVE IMPACTS OF COPEPODS, CLADOCERANS AND NUTRIENTS ON THE MICROBIAL FOOD-WEB OF A MESOTROPHIC LAKE

Calanoid copepods. rather than cladocerans, frequently dominate the zo oplankton of lakes in New Zealand. The potential consequences of this domination for the microbial community of mesetrophic Lake Mahinerangi , New Zealand, were determined by field experiments in which Boeckella and Daphnia were added to in situ enclosures in the presence and abse nce of added nutrients. Boeckella hamata at ambient densities (2 and 8 1(-1)) rapidly and severely suppressed ciliate population growth over 4 days, even when microbial growth was enhanced by added nutrients, bu t effects of copepods on other components of the microbial community ( bacteria, photosynthetic picoplankton, heterotrophic nanoflagellates, algae) were slight. In contrast, Daphnia carinata at the same densitie s (but 5-fold higher biomasses per litre) had a relatively weak effect on ciliates, suppressing ciliate abundance only after 4 days at 8 Dap hnia l(-1) (330 mu g l(-1)); this daphniid density also depressed abun dances of large bacterial rods, some photosynthetic picoplankton and t he dominant alga, Cyclotella. These results highlight the relative imp ortance of specific trophic linkages in a microbial food web; they als o suggest that the dominance of Boeckella in many southern hemisphere lakes may account for relatively low ciliate abundances in these lakes .