Nutrient and carbon removal ratios and fluxes in the Ross Sea, Antarctica

Net community production (NCP) and nutrient deficits (Def(X)) were calculat ed using decreases in dissolved CO2 and nutrient concentrations due to biol ogical removal in the upper 200 m of the water column during four cruises i n the Ross Sea, Antarctica along 76 degrees 30'S in 1996 and 1997. A compar ison to excess dissolved and particulate organic carbon showed close agreem ent between surplus total organic carbon (TOC) and NCP during bloom initiat ion and productivity maximum; however, when TOC values had returned to low wintertime values NCP was still significantly above zero. This seasonal NCP , 3.9 +/- 1 mol C m(-2), must be equivalent to the particle export to depth s greater than 200 m over the whole productive season. We estimate that the annual export was 55 +/- 22% of the seasonal maximum in NCP. The fraction of the seasonal maximum NCP that is exported through 200 m is significantly higher than that measured by moored sediment traps at a depth of 206 m. Th e removal of carbon, nitrate and phosphate (based on nutrient disappearance since early spring) and their ratios showed significant differences betwee n regions dominated by diatoms and regions dominated by the haptophyte Phae ocystis antarctica. While the DeltaC/DeltaN removal ratio was similar (7.8 +/- 0.2 for diatoms and 7.2 +/- 0.1 for P. antarctica), the DeltaN/DeltaP a nd DeltaC/DeltaP removal ratios for diatoms (10.1 +/- 0.3 and 80.5 +/- 2.3) were significantly smaller than those of P. antarctica (18.6 +/- 0.4 and 1 34.0 +/- 4.7). The similarity in DeltaC/DeltaN removal ratios of the two as semblages suggests that preferential uptake of phosphate by diatoms caused the dramatic differences in DeltaC/DeltaP and DeltaN/DeltaP removal ratios. In contrast to low DeltaC/DeltaP and DeltaN/DeltaP removal ratio in diatom -dominated areas early in the growing season, deficit N/P and C/P ratios in late autumn indicate that the elemental stochiometry of exported organic m atter did not deviate significantly from traditional Redfield ratios. Chang es in biologically utilized nutrient and carbon ratios over the course of t he growing season indicated either a substantial remineralization of phosph ate or a decrease in phosphate removal relative to carbon and total inorgan ic nitrogen over the bloom period. The species dependence in C/P ratios, an d the relative constancy in. the C/N ratios, makes N a better proxy of biol ogical utilization of CO2. (C) 2000 Elsevier Science Ltd. All rights reserv ed.