Sea ice microbial dynamics over an annual ice cycle in Prydz Bay, Antarctica

Microbial community dynamics within the fast sea ice of Prydz Bay (68°S 78°E) were investigated over an annual cycle at two sites (1 and 3 km offshore) between April and November 2008. There are few long-term sea ice studies, and few that cover the phase of winter darkness when autotrophic processes are curtailed. Mean chlorophyll a concentrations in the ice column ranged between 0.76 and 44.8 μg L⁻¹ at the 1-km site (Site 1) and 3.11–144.6 μg L⁻¹ at the 3-km site (Site 2). Highest chlorophyll a usually occurred at the base of the ice. Bacterial concentrations ranged between 0.30 and 2.08 × 10⁸ cells L⁻¹, heterotrophic nanoflagellates (HNAN) between 0.21 × 10⁵ and 2.98 × 10⁵ cells L⁻¹ and phototrophic nanoflagellates (PNAN) 0–1.06 × 10⁵ cells L⁻¹. While HNAN occurred throughout the year, PNAN were largely absent in winter. Dinoflagellates were a conspicuous and occasionally an abundant element of the community (maximum 17,460 cells L⁻¹), while ciliates were sparse. The bacterial community showed considerable morphological diversity with a dominance of filamentous forms. Bacterial production continued throughout the year ranging between 0 and 22.92 μg C L⁻¹ day⁻¹ throughout the ice column. Lowest rates occurred between late June and early August. The sea ice sustained an active and diverse microbial community through its annual extent. The data suggest that during winter darkness the microbial community is dominated by heterotrophic processes, sustained by a pool of dissolved organic carbon.