Biogeochemical regimes, net community production and carbon export in the Ross Sea, Antarctica

The net community production (NCP) of the Ross Sea, from the early austral spring (mid-October) to the austral summer (mid-February), has been estimat ed from the seasonal drawdown of CO2 concentrations integrated over the top 100 m of the water column. The deficits in nutrients and CO2 indicate thre e distinct biogeochemical regimes. The regime in the southwestern Ross Sea (Region I) had relatively shallow mixed layers and was dominated by diatom growth, as evidenced by a silicate (Def(Si)) to NCP removal ratio (0.11 +/- 0.04) that was similar to the silicate-to-carbon ratio found in diatoms gr owing in temperate regions. High NCP values (4.9-8.7 mol m(-2)) and low rat ios of surplus total organic carbon (Surp(TOC)) to NCP (from 0.27 to 0.67) show high organic carbon export out of the upper 100 m of the water column. The second regime (Region II), located in the center of the southern Ross Sea polynya, also had high NCP's (4.4-10.8 mol m(-2)) but the mixed layers were deeper. The average Def(Si)/NCP ratio was 0.04 +/- 0.02, much lower th an the southwestern sector and consistent with the observed growth of the h aptophyte Phaeocystis antarctica. An increase in Def(Si) and the Def(Si)/NC P ratio during re-occupations of selected stations indicate the presence an d persistence of diatom growth late into the summer. The third regime (Regi on III), in the northeastern Ross Sea, had shallow mixed layers and a wider range of Def(Si)/NCP ratios (0.10-0.31), indicating variations in silicate -to-carbon uptake by diatoms. The low NCP's (1.2-4.2 mol C m(-2)) that dist inguished this area also may be due to micro-nutrient deficiencies in addit ion to prolonged ice coverage. NCP over the continental shelf of the Ross S ea (441,000 km(2), defined by the 1000-m isopleth) is estimated to be 25 +/ - 10 Tg of carbon per year, with a mean rate of 4.8 +/- 1.9 mol m(-2) yr. B y mid-February, the productivity peak of the 1997 growing season had passed , and 19 +/- 7% of NCP remained in the upper 100 m as DOC,which presumably would not be exported but remineralized prior to the next growing season. D uring the same time period, 16 +/- 25% of NCP had already been removed from the upper 100 m as sinking biogenic particles, leaving 65% present in the POC fraction to be exported later or remineralized. High export of organic carbon (> 50% of NCP) was shown in both diatom- and Phaeocystis-dominated r egimes. (C) 2000 Elsevier Science Ltd. Ail rights reserved.