Variability of phytoplankton distribution and primary production around Elephant Island, Antarctica, during 1990?1993

Abstract

The distribution and abundance of phytoplankton within a sampling grid of 50×103 km2 around Elephant Island were determined from early January to mid-March of 4 successive years, 1990–1993. The number of stations where physical-optical-biological data were obtained from the surface to a maximum of 750 m ranged from 74 in 1990 to 206 in 1993. Contour maps of chlorophyll-a (chl-a) concentrations showed marked mesoscale patchiness that varied from month to month and also interannually. The distribution patterns for chl-a were similar when plotting surface concentrations or integrated values to 100 m. Three major zones could be distinguished that differed in both physical and biological characteristics. Stations in the northwest portion of the grid (Drake Passage waters) and in the southeast portion of the grid (Bransfield Strait waters) showed the most pronounced interannual variations, with phytoplankton biomass and rates of primary production being considerably higher in 1990–91 than in 1992–93. The central portion of the sampling grid, which included the major frontal system north of Elephant Island, showed the smallest interannual variations in both biological and physical parameters and the highest rates of primary production. Phytoplankton biomass and rates of primary production were correlated with depth of the upper mixed layer (UML), which in turn was correlated with the measured wind stress. The mean depth of the UML was 50 m, while the mean depth of the euphotic zone was 90 m. Using the measured mean surface solar irradiance (550 μEinsteins m−2 s−1), the mean irradiance experienced by cells in the UML of 50 m would be around 105 μE m−2 s−1, which is similar to the measured Ik (light saturation) value for photosynthesis (101 μEm−2 s−1). The mean value from all cruises for chl-a in surface waters was 0.7 mg m−3, while the mean rate of primary production was 374 mg Cm−2 day−1.