Microbial Ecology of the WAP Continental Shelf Region

We present results from a large data set (N=5427 individual measurements at 449 stations, 2003-2012) on bacterial abundance and production and related hydrographic properties collected on ten, regional-scale (700 X 200 km) annual summer cruises along the western Antarctic Peninsula. This region is strongly influenced by the duration and extent of sea ice cover. The region is also experiencing rapid climate warming causing reduction in sea ice cover. Our study area transitions from a prolonged ice-free season in the north to persistent summer sea ice in the south, and from a nearshore region influenced by summer runoff from glaciers to an offshore, open ocean region dominated by the Antarctic Circumpolar Current. These gradients provide a powerful opportunity to study regulation of bacterial production by physical-biological couplings such as climate-driven sea ice control of phytoplankton blooms. The summer bacterial assemblage is the product of the seasonal warming and freshening of cold, salty Winter Water and the plankton succession occurring in that evolving water mass. Bacterial production (BP) rates averaged ~20 mgC m-2 d-1, over all stations and years, and was a low (5%) fraction of the simultaneous primary production rates. There was significant variation in BP between regions and years, reflecting the variability in phytoplankton standing stocks and primary production rates. Leucine incorporation rates were significantly correlated with both chlorophyll and primary production rates across depths, regions and years. More tellingly, leucine incorporation rates were significantly correlated with chlorophyll in the Shelf region in all years, indicating the dependence of BP on biomass accumulation and strong phytoplankton-bacteria coupling. Relationships with temperature were variable, with no relationship in 3 years, positive relationships (i.e., cold inhibition) in 4 and negative relationships (i.e.,  cold stimulation) in 3 years. There was no evidence of cold suppression of BP at low substrate availability (low Chl).