AME 64:253-266 (2011)  -  DOI: https://doi.org/10.3354/ame01525

ABSTRACT: Marine microbial communities have been little studied in Arctic waters, especially ­during the winter–spring transition before the development of extensive phytoplankton blooms. This study investigated microbial plankton in the ice-covered polar surface waters of the northwestern Fram Strait (75 to 80°N) at the onset of the 24 h light period in spring (April to May). The system we encountered was characterised by low concentrations of chlorophyll a (<0.2 µg l⁻¹) and a low abundance of both bacteria (1.4 to 2.5 × 10⁸ cells l⁻¹) and protists (1 to 1.7 × 10⁵ cells l⁻¹). Bacterial production was very low (≤0.63 µg C l⁻¹ d⁻¹), despite the dominance of nucleic-acid-rich bacteria (58 ± 6% of total bacterial abundance). Small (2 to 5 µm) phototrophs dominated the eukaryotic assemblage in the surface and most probably had profound effects on the composition and metabolic balance of the microbial community as a whole. Most stations appeared to have been net-autotrophic, and calculations of phagotrophy indicated a balanced carbon budget for the microbial community. Mixotrophy was seen in a large part of the ciliate assemblage and may have contributed to the productivity and stability of the pre-bloom system that we encountered.

KEY WORDS: Flow cytometry · Bacteria · Microzooplankton · Mixotrophy · Planktonic metabolism