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

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Deep-Sea Research Part Ii-Topical Studies in Oceanography
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Net community production (NCP) and nutrient deficits (Def(X)) were calculated using decreases in dissolved CO2 and nutrient concentrations due to biological removal in the upper 200 m of the water column during four cruises in the Ross Sea, Antarctica along 76 degrees 30'S in 1996 and 1997. A comparison to excess dissolved and particulate organic carbon showed close agreement between surplus total organic carbon (TOC) and NCP during bloom initiation 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 depths 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. The removal of carbon, nitrate and phosphate (based on nutrient disappearance since early spring) and their ratios showed significant differences between regions dominated by diatoms and regions dominated by the haptophyte Phaeocystis 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 and 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 134.0 +/- 4.7). The similarity in DeltaC/DeltaN removal ratios of the two assemblages 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 matter did not deviate significantly from traditional Redfield ratios. Changes in biologically utilized nutrient and carbon ratios over the course of the growing season indicated either a substantial remineralization of phosphate or a decrease in phosphate removal relative to carbon and total inorganic nitrogen over the bloom period. The species dependence in C/P ratios, and the relative constancy in. the C/N ratios, makes N a better proxy of biological utilization of CO2. (C) 2000 Elsevier Science Ltd. All rights reserved.


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