SEASONAL CARBON CYCLING IN THE SARGASSO SEA NEAR BERMUDA

Reference
Gruber, N., and C.D. Keeling, Seasonal carbon cycling in the Sargasso Sea near Bermuda, Scripps Institution of Oceanography, Bulletin, 30, Univ. Calif. Press, Berkeley, 98pp, 1999.

Abstract
By observing simultaneous changes in the concentration and 13C/12C ratio of dissolved inorganic carbon (DIC) in seawater over the annual carbon cycle it is possible to distinguish physical and biological processes which govern this cycle in near-surface waters. We report an analysis of approximately monthly data from 1983 through 1989 at Station `S' in the surface mixed-layer of the Sargasso Sea near Bermuda. The analysis also makes use of contemporary measurements of alkalinity, the calculated CO2 partial pressure in seawater, and measurements of atmospheric CO2 concentration and 13C/12C ratio from which the net flux of CO2 across the air-sea boundary was calculated as a function of wind-speed for each isotopic species. Limited additional chemical measurements in deeper water and physical oceanographic data afforded estimates of the seasonally varying upward flux of each isotopic species of DIC into the mixed-layer by vertical diffusion and entrainment. Net community production (the net transfer of carbon from inorganic to organic pools) was estimated for the mixed-layer from the observed change in the 13C/12C ratio of DIC after corrections were applied for air-sea exchange and vertical transport and using the fact of a strong fractionation of 13C during the photosynthetic uptake of DIC. As a second method, and to check our first approach, net community production was also calculated simply by difference from the observed change in DIC, after also taking into account air-sea exchange and vertical transport. Thus, both these estimates are obtained without reference to nutrient fluxes or concentrations of organic matter. We deduce that 11 gC m-2 yr-1 of DIC is lost on average as a result of a net community production of 19 gC m-2 during shoaling of the mixed-layer in spring and summer and a reversed net transfer from organic to inorganic of 8 gC m-2 during deepening. Air-sea exchange adds 21 gC m-2 yr-1 of CO2 to the mixed-layer and vertical transport a further 18 gC m-2 yr-1 of DIC. The sum of all fluxes is thus a net apparent gain of 28 gC m-2, which is nearly balanced by an apparent loss term associated with the variable mixed-layer depth. We have assessed the uncertainties in our calculations by sensitivity tests. Our estimate of the annual air-sea exchange lies between previously published estimates based on atmospheric modeling studies. When, on the basis of limited observational evidence we include approximately 7 gC m-2 yr-1 of net community production below the mixed-layer, the resulting tentative estimate of net community production in the entire euphotic layer is 18 gC m-2 yr-1. This estimate is comparable to those of net export of organic matter based on sediment traps and on water-column inventories, but considerably lower than estimates of euphotic new production based on measurements of oxygen and of noble gases and on other modeling studies for Station `S'.

see related article by Gruber et al. [1998]