Microbial dynamics in coastal waters of East Antarctica: Bacterial production and nanoflagellate bacterivory

Bacterial production and heterotrophic nanoflagellate (HNAN) bacterivory were determined concurrently with measurements of abundance and biomass at weekly intervals between 30 December 1993 and 11 February 1994 at a shallow, coastal location in Prydz Bay, eastern Antarctica. Bacterial production was measured by [3H]thymidine incorporation and HNAN bacterivory by the uptake of fluorescently labelled bacteria. Bacterial abundance, biomass and production ranged from 2 to 8 x 108 l-1, 13 to 64 µg C l-1 and 8 to 14 µg C l-1 d-1, respectively, with maximum values recorded in mid January. The HNAN community comprised choanoflagellate, non-collared and colonial taxa, with non-collared forms dominating abundance and biomass in late January and early February. Total HNAN abundance and biomass ranged from 1.6 to 4.2 x 106 l-1 and 8 to 16 µg C l-1, respectively. HNAN cellular ingestion and clearance rates differed between taxa with maximum rates of 8.28 particles cell-1 h-1 and 9.32 nl cell-1 h-1 recorded for large non-collared forms. During the study period the HNAN community grazed 0.9 to 4.7 µg bacterial C l-1 d-1, equivalent to 3 and 12% of bacterial biomass, and 10 and 36% of daily bacterial production; however, these values are likely to be minimal estimates and grazing impact may have been higher on occasion. Choanoflagellates were responsible for much of the grazing impact at the beginning of the study period, while non-collared HNAN were the dominant grazers in late January and early February. The HNAN community therefore appears to graze substantial bacterial production in Antarctic coastal waters during the austral summer, although alternative sources of bacterial mortality are likely to be of importance.