Factors influencing the distribution, biomass, and productivity of phytoplankton in the Scotia Sea and adjoining waters

During January and February 2000 four research vessels, from Russia, the UK, Japan, and the United States, conducted an oceanographic survey with 137 hydrographic stations within the Scotia Sea and adjoining waters as part of a survey sponsored by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) to estimate the biomass and distribution of Antarctic krill in the Scotia Sea. Chlorophyll-a (Chl-a) measurements showed great variability in phytoplankton biomass within the Scotia Sea, with some areas having among the lowest Chl-a concentrations found in Antarctic waters (<0.1 mg m−3 in surface waters) while other areas were among the richest with >10 mg m−3. This paper describes the distribution and concentration of Chl-a in the upper 100 m of the water column and relates the Chl-a profiles at individual stations to profiles of upper water-column stability, to the depth of the upper mixed layer, and to the mixing of different water masses. The 58 stations with the lowest Chl-a values in surface waters also had low values for integrated Chl-a (33.9±19.5 mg m−2) and a Chl-a maximum at depths of between 70 and 90 m, in contrast to all other stations where deep Chl-a maxima did not occur. The T/S diagrams at many of these stations were indicative of Antarctic Circumpolar Current (ACC) waters. The central Scotia Sea and areas to the west and north of South Georgia had significantly higher integrated Chl-a values (98.1±46.0 mg m−2, n = 57 ), in addition to five stations with very high Chl-a values (mean of 359±270 mg m−2). The mean rate of integrated primary production, which was estimated using the Chl-a data and the mean incident solar radiation measured from previous cruises as well as from satellite data, was estimated to be 994 mg carbon m−2 day−1. The temperature profiles at these stations suggested that considerable interleaving and mixing of water types had occurred, which was also evident in the T/S diagrams, which indicated mixing of ACC waters with coastal waters originating from Bransfield Strait or the Weddell Sea. There was no significant correlation between integrated Chl-a values and the profiles of upper water column stability or the depth of the upper mixed layer. The spatial variability in phytoplankton biomass within the Scotia Sea is discussed in relation to the hypothesis that low iron concentrations are the major factor controlling phytoplankton biomass in these pelagic Antarctic waters and that concentrations of iron available for phytoplankton uptake are strongly influenced by fronts and the mixing of different water masses.