Seasonal and interannual variability of phytoplankton biomass west of the Antarctic Peninsula

The spatial and temporal variability of phytoplankton biomass, estimated as chlorophyll-a (chl-a) concentration, is examined in the continental shelf-slope region west of the Antarctic Peninsula. Relationships between temporal observations in the nearshore Palmer Station grid (64°46.77′S, 64°04.36′W) and spatial observations in a larger regional grid 200 km on/off-shore and 900 km alongshore are presented. Average chl-a concentrations in the upper layers of the water column in the immediate vicinity of Palmer Station show strong seasonal and interannual variability. Biomass accumulation typically starts during mid-November, while strong blooms develop from December through January. The 1991/1992 and 1994/1995 seasons developed higher overall chl-a concentrations (average maximum water column values reaching 8 and 16 mg chl-a m−3, respectively) than the 1992/1993 and 1993/1994 seasons (average maximum water column values of less than 3 mg chl-a m−3). The 1994/1995 season of extremely high chl-a concentrations also showed a prolonged bloom period into February, while the 1991/1992 season did not. Similar interannual variability was observed in the regional grid. Average chl-a concentration in the top 30 m was 0.91, 1.24 and 1.66 mg chl-a m−3 for January of 1993, 1994 and 1995, respectively. The regional grid contains an on/off-shore gradient in bottom topography, measured physical and optical characteristics, as well as chl-a concentrations. Regional inshore grid stations in January had, on average, almost four times more chl-a biomass than off-shore stations (2.18 vs. 0.59 mg chl-a m−3, respectively). There is evidence that this on/off-shore gradient is modulated alongshore by latitudinal variability which follows the annual advance and retreat of sea ice.