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

Ž . 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 Ž X X . in the nearshore Palmer Station grid 64846.77 S, 64804.36 W and spatial observations in a larger regional grid 200 km onroff-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 1991r1992 and 1994r1995 Ž seasons developed higher overall chl-a concentrations average maximum water column values reaching 8 and 16 mg chl-a y3 . Ž m , respectively than the 1992r1993 and 1993r1994 seasons average maximum water column values of less than 3 mg y3. chl-a m . The 1994r1995 season of extremely high chl-a concentrations also showed a prolonged bloom period into February, while the 1991r1992 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 for January of 1993, 1994 and 1995, respectively. The regional grid contains an onroff-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 Ž y3 . biomass than off-shore stations 2.18 vs. 0.59 mg chl-a m , respectively . There is evidence that this onroff-shore gradient is modulated alongshore by latitudinal variability which follows the annual advance and retreat of sea ice.