Carbon-13 constraints on the seasonal inorganic carbon budget at the BATS site in the northwestern Sargasso Sea

The seasonal budget of dissolved inorganic carbon (C) in the mixed layer at the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site is assessed on the basis of a 4-yr time series (1991–1994) of high-precision C, alkalinity and the 13C/12C ratio of C. Compared to previous studies, our budget is constrained by observed changes in 13C/12C, which permit calculation of the net community production. We are thus able to quantitatively separate all processes that contribute to the observed seasonal cycle in the mixed layer. Uncertainties in the C budget are determined using a Monte Carlo method. We found that net community production is mainly responsible in generating the observed C drawdown of about 26 μmol kg-1 between April and October by removing on the average −40±4 μmol kg-1 from the mixed layer. This net community production occurs in the absence of measurable nitrate and phosphate concentrations. We hypothezise that N2 fixation in combination with vertical migration may be large enough to meet the biological nutrient demand in the mixed layer during this summer/fall period. Physical processes do not explain the observed summer/fall drawdown except when very improbably large horizontal advective velocities are assumed. Annual net community production of 2.3±0.9 mol m-2 is not well constrained by 13C/12C, but if tentatively extrapolated to the whole euphotic layer, it is well within the range of estimates based on a wide variety of methods. The region around BATS is computed to be a moderate sink for atmospheric CO2 with an annual uptake of 1.8±0.5 mol m-2, with about 25% of this influx associated with the uptake of anthropogenic CO2.