IMPACTS OF NUTRIENTS AND ZOOPLANKTON ON THE MICROBIAL FOOD-WEB OF AN ULTRA-OLIGOTROPHIC LAKE

In ultra-oligotrophic lakes and the sea, calanoid copepods are the dom inant mesozooplankton and cladocerans are generally sparse or absent. To determine the effects of predation and nutrient enrichment on the p elagic microbial food web of an ultra-oligotrophic lake, we added cope pods and cladocerans at low biomasses (<60 mu g l(-1)) to in situ encl osures in Lake Wakatipu, New Zealand, in the presence and absence of a dded nutrients (nitrogen and phosphorus). In response to nutrient fert ilization, the concentrations of phototrophs >3 mu m and heterotrophic bacteria increased by 50 and 15%, respectively, over 4 days, but thos e of cyanobacterial picoplankton decreased by 68%. The presence of cal anoid copepods (Boeckella dilarata) at ambient densities (1 and 4 l(-1 )) rapidly and severely suppressed ciliate population growth over 4 da ys and also lowered that of flagellates >3 mu m, even when microbial g rowth was enhanced by added nutrients. The presence of a small cladoce ran, Ceriodaphnia dubia, at double the densities, but similar biomasse s, to those of copepods, depressed the net growth rates of ciliates an d flagellates to a lesser degree. The net growth rate of heterotrophic bacteria after 4 days declined with flagellate abundance, consistent with the possibility of regulation by flagellates. Although bacteria a nd algae increased in response to nutrient fertilization (bottom-up co ntrol), predation (top-down control) appeared to play an important rol e in structuring the microbial food web of this ultra-oligotrophic lak e in summer.