Shallow remineralization in the Sargasso Sea estimated from seasonal variations in oxygen, dissolved inorganic carbon and nitrate

A diagnostic model of the mean annual cycles of oxygen, dissolved inorganic carbon (DIC) and nitrate below the mixed layer at the Bermuda Atlantic Tim e-Series Study (BATS) site is presented and used to estimate organic matter remineralization in the seasonal thermocline. The model includes lateral a nd vertical advection as well as vertical diffusion, which are found to be significant components of the seasonal budgets of oxygen, DIC and nitrate. The vertical and seasonal variation of the remineralization rates deduced f rom the oxygen and DIC distributions are very similar. Both locate the spri ng-summer community compensation depth at similar to 85 m and the remineral ization rate maximum at similar to 120 m; nitrate-based estimates of these depths are about 40m greater. Remineralization rates based on oxygen, DIC a nd nitrate all show the seasonal maximum to occur in the late spring, presu mably reflecting the decomposition of organic matter formed during the spri ng bloom. The remineralization rate integrated between 100 and 250 m and be tween mid-April and mid-December is estimated to be 2.08 +/- 0.38 mol O-2 m (-2), 1.53 +/- 0.35 mol Cm-2 and 0.080 +/- 0.046 mol N m(-2). These imply r emineralization ratios of O-2:C = 1.4 +/- 0.40 and C:N = 19 +/- 12. The for mer agrees well with the canonical Redfield ratio and the latter is signifi cantly larger. The analysis is consistent with the export and remineralizat ion of nitrogen-poor organic matter from surface waters. (C) 2001 Elsevier Science Ltd. All rights reserved.