Phyto- and zooplankton community structure and production around South Georgia (Southern Ocean) during Summer 2001/02

During Austral summer 2001/02 the spatial distribution of phytoplankton and zooplankton communities and associated production processes were investigated in waters to the north of the island of South Georgia. Nearest neighbour cluster analysis of phytoplankton and zooplankton data sets indicated the presence of 3 major station groupings within each analysis that collectively had great geographic integrity. Thus a shelf station group was characterised by low phytoplankton biomass and primary production rates (median values 69 mg chl a m−2; 418 mg C m−2 d−1, respectively) and a dominance of nano- and pico-phytoplankton, on the one hand, and low overall mesozooplankton levels (median 50,135 ind. m−2), which was also coincident with elevated krill biomass, on the other. At the western end of the island high mesozooplankton abundance (median >250,000 ind. m−2) in oceanic waters was co-incident with a phytoplankton bloom that originated over the North West Georgia Rise (NWGR). These ‘bloom’ stations were characterised by high phytoplankton biomass and primary production rates (median 218 mg m−2 and 916 mg C m−2 d−1, respectively) and were dominated by microphytoplankton. This elevated production was associated with an anticyclonic eddy which was considered to have been initiated where the Southern Antarctic Circumpolar Current Front (SACCF) interacted with the bathymetry of the NWGR. SeaWiFS and drifter buoy data indicated that this production was subsequently entrained and transported cyclonically around the Georgia Basin downstream from the island. In contrast, at the eastern end of the island, in waters south of the SACCF, phytoplankton biomass and primary production rates were lower (median 40 mg m−2 and 214 mg C m −2 d−1, respectively) although median zooplankton abundance was intermediate (not, vert, similar157,000 ind. m−2). Of the environmental variables measured in this study the proportion of microphytoplankton within samples, rather than for example, phytoplankton biomass, explained the greatest proportion of the variance associated with the carbon mass of individual copepod species stages, egg production rates and total zooplankton abundance. We conclude that stations in the western and eastern oceanic regions were dominated by bottom up controls, with physical forcing and adequate nutrient availability leading to bloom conditions of large diatoms in the west, whereas in the east, phytoplankton growth was lower (possibly due to micronutrient limitation) and zooplankton abundance was lower. Over the shelf we suggest that a higher average biomass of krill (not, vert, similar80 g wet mass m−2) compared to offshelf (43 g wet mass m−2) may have exerted top down control and grazed out chl a and thus have been responsible for low mesozooplankton abundance.

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