Predator regulation of aquatic microbial abundance in simple food webs of sub-Antarctic lakes

Lakes along the coast of Antarctica and on sub-Antarctic islands are characterized by low biodiversity and absence of vertebrate predators. We conducted manipulations of the planktonic food web of two such lakes at South Georgia, where herbivorous copepods constitute the highest trophic level of quantitative importance for food web interactions. Abundances of algae (as chlorophyll a), bacterivorous flagellates, and heterotrophic bacteria were monitored in 20-I microcosms and in 0.5-m(3) mesocosms with manipulated copepod density. Microcosm experiments where copepod abundance was adjusted to 0.1 to IO times in situ density in the lakes, yielded clearance rates for bacterivorous flagellates of 0.10-0.25 l (mg copepod) d(-1). On the contrary, bacteria responded positively to the addition of copepods at rates of 0.08-0.16 I (mg copepod) d(-1) due to the release from grazing by bacterivorous flagellates. In subsequent experiments, the two herbivorous copepods present in the lakes were studied separately. The grazing mortality of flagellates, as well as the positive response of bacteria, could be attributed to Boeckella michaelseni. The larger Pseudoboeckella poppei had no impact on any of the microbial components, even at high densities. This species was specialized on larger food particles (algae). Mesocosms with reduced copepod abundance contained more heterotrophic flagellates but less bacteria than mesocosms with abundant copepods. Thus, we demonstrate strong trophic interactions of copepods, bacterivorous flagellates, and bacteria. In these sub-Antarctic lakes, copepods play a crucial role in the regulation of microbial food webs.