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

Abstract

The net community production (NCP) of the Ross Sea, from the early austral spring (mid-October) to the austral summer (mid-February), has been estimated from the seasonal drawdown of CO 2 concentrations integrated over the top 100 m of the water column. The deficits in nutrients and CO 2 indicate three 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 growing in temperate regions. High NCP values (4.9–8.7 mol m −2) and low ratios 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 than the southwestern sector and consistent with the observed growth of the haptophyte Phaeocystis antarctica. An increase in Def(Si) and the Def(Si)/NCP ratio during re-occupations of selected stations indicate the presence and persistence of diatom growth late into the summer. The third regime (Region 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 distinguished this area also may be due to micro-nutrient deficiencies in addition to prolonged ice coverage. NCP over the continental shelf of the Ross Sea (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. By 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. During 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 regimes.