SEASONAL COUPLING OF ORGANIC-MATTER PRODUCTION AND PARTICLE-FLUX IN THE WESTERN BRANSFIELD STRAIT, ANTARCTICA

Simultaneous measurements of seasonal depletions in dissolved inorganic carbon (SIGMA-CO2), nitrogen [NO3- + NO2-] and phosphate (HPO42- and concentrations of C, N and P collected using sediment traps at five representative sites in a 25,000 km2 sector of the western Bransfield Strait over a 4-month period have allowed us to evaluate the coupling between primary production and particle flux. During the spring bloom period (December to January), we observed a substantial removal of SIGMA-CO2, [NO3- + NO2-] and HPO4(2-) corresponding to a net upper water column (0-50 m) seasonal production of 8410 mmol C m-2, 827 mmol N m-2 and 53.1 mmol P m-2. This equates to an average C:N:P ratio of 158:15.6:1. These C:N (and C:P) ratios are higher than the expected Redfield ratios and also exceed the C:N and C:P ratios directly measured for suspended and sinking particles. We hypothesize a coupled formation of carbon-rich dissolved organic matter as a sink for approximately 30-50% of the SIGMA-CO2 removed during net photosynthesis. The rapid production and accumulation of phytoplankton in the northern portion of Gerlache Strait during the spring bloom resulted in a substantial pCO2 undersaturation (pCO2 less-than-or-equal-to 100-mu-atm) compared to surface samples collected from lower productivity areas in Drake Passage (pCO2 greater-than-or-equal-to 325-mu-atm). Nevertheless, CO2 invasion from the atmosphere was minimal, in part due to the fact that there was little or no wind-driven mixing at this time of the year. Particulate C, N, P and ATP fluxes derived from free-floating sediment traps, revealed a substantial variability in time and space. The greatest seasonal variation occurred at the stations with the highest rates of primary production. For example, both the highest (31.1 mmol C m-2 day-1) and lowest (< 3.0 mmol C m-2 day-1) rates of POC flux were measured in the northern portion of Gerlache Strait; the other sites exhibited moderate (4.1-12.5 mmol C m-2 day-1) but more constant fluxes during the period of our observations. Nevertheless, the total seasonally integrated C and N fluxes were nearly identical (1.3-1.5 mol C m-2 and 0.16 mol N m-2). For the northern Gerlache Strait station, new production was 26% of total production during the December to January spring bloom period.