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

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 (1991Ð1994) 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 lmolkg~1 between April and October by removing on the average!40$4 lmolkg~1 from the mixed layer. This net community production occurs in the absence of measurable nitrate and phosphate concentra-tions. We hypothezise that N 2 Þxation in combination with vertical migration may be large enough to meet the biological nutrient demand in the mixed layer during this summer/fal