Nutrient cycling by Antarctic marine microbial plankton
Three sites along a shelf to deep-sea transect near the island of South Georgia (55 degrees S) were sampled in the latter part of the austral summer for water column structure, inorganic nutrients, particulate material and microplankton. Median concentrations at the 3 sites were 17 to 24 mmol m(-3) nitrate and approximately 1.6 mmol m(-3) phosphate, whilst silicate concentrations remained at 11 mmol m(-3) nearshore but were reduced to approximately 1 mmol m(-3) offshore. Microplankton community composition appeared to be characteristic of post-bloom conditions. Heterotrophs, including large protists, dominated microbial biomass at the offshore site. Pigment characterisation was consistent with the domination of the microplankton by diatoms at all sites. Changes in the carbon and nitrogen content of particulate material down the water column were consistent with significant remineralisation of nitrogen in the mixed layer, with molar C:N ratios of 6.5 to 7.7 in surface waters and up to 11.9 immediately below the mixed layer. In most cases, peak concentrations of ammonium were found to be associated with the pycnocline, suggesting that remineralisation of nitrogen was occurring here. However, no component of the microbial community could be associated with this region of ammonium production. Carbon fixation over the growing season predicted from published instantaneous nitrogen uptake rates, from biological nitrate deficit in the mixed layer and from the corresponding silicate deficit suggested that at least 25 g C m(-2), was produced by phytoplankton during the growth season. However, there is strong evidence that this is a serious underestimate. Even whilst nitrate concentrations remained high, ammonium and possibly other 'recycled nitrogen' appeared to be key inorganic nutrients. Particulate carbon and silicon appeared to be removed from the mixed layer whilst nitrogen was regenerated in situ. In such systems, nitrate removal does not indicate carbon export, and nitrogen recycling may be a crucial factor in determining productivity throughout the growing season.