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

Abstract

A diagnostic model of the mean annual cycles of oxygen, dissolved inorganic carbon (DIC) and nitrate below the mixed layer at the Bermuda Atlantic Time-Series Study (BATS) site is presented and used to estimate organic matter remineralization in the seasonal thermocline. The model includes lateral and 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 from the oxygen and DIC distributions are very similar. Both locate the spring–summer community compensation depth at ?85 m and the remineralization rate maximum at ?120 m; nitrate-based estimates of these depths are about 40 m greater. Remineralization rates based on oxygen, DIC and nitrate all show the seasonal maximum to occur in the late spring, presumably reflecting the decomposition of organic matter formed during the spring bloom. The remineralization rate integrated between 100 and 250 m and between mid-April and mid-December is estimated to be 2.08±0.38 mol O2 m?2, 1.53±0.35 mol C m?2 and 0.080±0.046 mol N m?2. These imply remineralization ratios of O2 : C=1.4±0.40 and C : N=19±12. The former agrees well with the canonical Redfield ratio and the latter is significantly larger. The analysis is consistent with the export and remineralization of nitrogen-poor organic matter from surface waters.