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 c haracterized by low biodiversity and absence of vertebrate predators. We conducted manipulations of the planktonic food web of two such lake s at South Georgia, where herbivorous copepods constitute the highest trophic level of quantitative importance for food web interactions. Ab undances of algae (as chlorophyll a), bacterivorous flagellates, and h eterotrophic bacteria were monitored in 20-I microcosms and in 0.5-m(3 ) mesocosms with manipulated copepod density. Microcosm experiments wh ere 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 p ositively to the addition of copepods at rates of 0.08-0.16 I (mg cope pod) d(-1) due to the release from grazing by bacterivorous flagellate s. In subsequent experiments, the two herbivorous copepods present in the lakes were studied separately. The grazing mortality of flagellate s, as well as the positive response of bacteria, could be attributed t o Boeckella michaelseni. The larger Pseudoboeckella poppei had no impa ct on any of the microbial components, even at high densities. This sp ecies was specialized on larger food particles (algae). Mesocosms with reduced copepod abundance contained more heterotrophic flagellates bu t less bacteria than mesocosms with abundant copepods. Thus, we demons trate strong trophic interactions of copepods, bacterivorous flagellat es, and bacteria. In these sub-Antarctic lakes, copepods play a crucia l role in the regulation of microbial food webs.