Microbial dynamics and flagellate grazing during transition to winter in Lakes Hoare and Bonney, Antarctica

The planktonic microbial communities of Lakes Hoare and Bonney were investigated during transition into winter. We hypothesized that the onset of darkness induces changes in the functional role of autotrophic and heterotrophic microplankton. Bacteria decreased in Lake Hoare during MarchApril, while in Lake Bonney bacterial abundances varied. Heterotrophic nanoflagellates (HNAN), phototrophic nanoflagellates (PNAN) and ciliates showed no marked decline with the onset of winter. PNAN outnumbered HNAN in both lakes. Grazing rates of HNAN in Lake Hoare ranged up to 30.8bacteriapercellday-1. The HNAN community grazed between 3.74 and 36.6ng of bacterialcarbonday-1. Mixotrophic PNAN had grazing rates up to 15.2bacteriapercellday-1, and their daily community grazing exceeded bacterial production. In Lake Bonney East, PNAN grazing rates ranged up to 12.48bacteriapercellday-1 and in Lake Bonney West up to 8.16bacteriapercellday-1. As in Lake Hoare, the mixotrophic PNAN grazing rates (up to 950ngCday-1) usually exceeded bacterial production. HNAN grazing rates were generally similar to those in Lake Hoare. As winter encroaches, these lakes move progressively towards heterotrophy and probably function during the winter, enabling populations to enter the short austral summer with actively growing populations.