Spring summer imbalance of dissolved inorganic carbon in the mixed layer of the northwestern Sargasso Sea

Marchal, Olivier, Monfrey, Patrick and Bates, Nicholas R. (1996) Spring summer imbalance of dissolved inorganic carbon in the mixed layer of the northwestern Sargasso Sea. Tellus A, 48 (1), 115-134. (doi:10.1034/j.1600-0889.1996.00011.x).

Abstract

The surface concentration of dissolved inorganic carbon (DIC) at the Bermuda Atlantic Time-series Study site (BATS) decreased gradually by ca. 30 µ mol kg^-1 from April to October in 1989. This decrease occurred almost in the absence of measurable nitrate in the mixed layer. Although ancillary data about the C system point to the possible importance of lateral transport, horizontal gradients in surface [DIC] and the mean flow in the area indicate that local effects should prevail in the seasonal drawdown of DIC. On the basis of an one-dimensional model, we hence estimate the mixed layer budget of DIC for this period, from surface [DIC] data, temperature profiles, and concomitant meteorological records. According to model uncertainties, the [DIC] drawdown should be mostly explained (71–93%) by a net community production (NCP) averaging 1.4–2.3 mg C m^-3 d^-1, and to a lesser extent, by outgassing of CO₂ to the atmosphere. These losses are partially compensated (< 30%) by mixing with DIC-rich waters of the thermocline. This NCP must be regarded as a lower estimate, since the mean flow from the northeast should bring waters with slightly higher [DIC] to the mixed layer at the BATS site. The model, which is sensitive to short-term variations in atmospheric forcing (< 1 day), indicates that this layer has never reached the nitrocline for spring–summer 1989, even as a hurricane passed through the region. Hence, the surface NCP should not have been supported by unsampled, pulse-like supplies of deep nutrients. Wet atmospheric deposition of nitrogen measured concurrently on Bermuda could contribute to the biological N requirement (10–20%). According to historical estimates, N₂ fixation seems however, insufficient to meet the remaining demand. Comparison between NCP and primary production measured in situ suggests that most of photosynthetically fixed C (> 50%) is not respired in the mixed layer.

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