NUTRIENT CYCLING BY ANTARCTIC MARINE MICROBIAL PLANKTON

Three sites along a shelf to deep-sea transect near the island of Sout h Georgia (55 degrees S) were sampled in the latter part of the austra l 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 b ut were reduced to approximately 1 mmol m(-3) offshore. Microplankton community composition appeared to be characteristic of post-bloom cond itions. Heterotrophs, including large protists, dominated microbial bi omass at the offshore site. Pigment characterisation was consistent wi th the domination of the microplankton by diatoms at all sites. Change s in the carbon and nitrogen content of particulate material down the water column were consistent with significant remineralisation of nitr ogen in the mixed layer, with molar C:N ratios of 6.5 to 7.7 in surfac e waters and up to 11.9 immediately below the mixed layer. In most cas es, peak concentrations of ammonium were found to be associated with t he pycnocline, suggesting that remineralisation of nitrogen was occurr ing here. However, no component of the microbial community could be as sociated with this region of ammonium production. Carbon fixation over the growing season predicted from published instantaneous nitrogen up take rates, from biological nitrate deficit in the mixed layer and fro m 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 wh ilst nitrate concentrations remained high, ammonium and possibly other 'recycled nitrogen' appeared to be key inorganic nutrients. Particula te carbon and silicon appeared to be removed from the mixed layer whil st nitrogen was regenerated in situ. In such systems, nitrate removal does not indicate carbon export, and nitrogen recycling may be a cruci al factor in determining productivity throughout the growing season.