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 dominant mesozoo-plankton and cladocerans are generally sparse or absent. To determine the effects of predation and nutrient enrichment on the pelagic microbial food web of an ultra-oligotrophic lake, we added copepods and cladocerans at low biomasses (<60 μg l −1 to in situ enclosures in Lake Wakatipu, New Zealand, in the presence and absence of added nutrients (nitrogen and phosphorus). In response to nutrient fertilization, the concentrations of phototrophs >3 μm and heterotrophic bacteria increased by 50 and 15%, respectively, over 4 days, but those of cyanobacterial picoplankton decreased by 68%. The presence of calanoid copepods ( Boeckella dilatata ) at ambient densities (1 and 4 l −1 ) rapidly and severely suppressed ciliate population growth over 4 days and also lowered that of flagellates >3 μm, even when microbial growth was enhanced by added nutrients. The presence of a small cladoceran, Ceriodaphnia dubia , at double the densities, but similar biomasses, to those of copepods, depressed the net growth rates of ciliates and 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 and algae increased in response to nutrient fertilization (bottom-up control), predation (top-down control) appeared to play an important role in structuring the microbial food web of this ultra-oligotrophic lake in summer.