Biogeochemistry of total organic carbon and nitrogen in the Sargasso Sea: control by convective overturn

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The contributions of total organic carbon and nitrogen to elemental cycling in the surface layer of the Sargasso Sea are evaluated using a 5-yr time-series data set (1994–1998). Surface-layer total organic carbon (TOC) and total organic nitrogen (TON) concentrations ranged from 60 to 70 μM C and 4 to 5.5 μM N seasonally, resulting in a mean C : N molar ratio of 14.4±2.2. The highest surface concentrations varied little during individual summer periods, indicating that net TOC production ceased during the highly oligotrophic summer season. Winter overturn and mixing of the water column were both the cause of concentration reductions and the trigger for net TOC production each year following nutrient entrainment and subsequent new production. The net production of TOC varied with the maximum in the winter mixed-layer depth (MLD), with greater mixing supporting the greatest net production of TOC. In winter 1995, the TOC stock increased by 1.4 mol C m−2 in response to maximum mixing depths of 260 m. In subsequent years experiencing shallower maxima in MLD (<220 m), TOC stocks increased <0.7 mol C m−2. Overturn of the water column served to export TOC to depth (>100 m), with the amount exported dependent on the depth of mixing (total export ranged from 0.4 to 1.4 mol C m−2 yr−1). The exported TOC was comprised both of material resident in the surface layer during late summer (resident TOC) and material newly produced during the spring bloom period (fresh TOC). Export of resident TOC ranged from 0.5 to 0.8 mol C m−2 yr−1, covarying with the maximum winter MLD. Export of fresh TOC varied from nil to 0.8 mol C m−2 yr−1. Fresh TOC was exported only after a threshold maximum winter MLD of ≈200 m was reached. In years with shallower mixing, fresh TOC export and net TOC production in the surface layer were greatly reduced. The decay rates of the exported TOC also covaried with maximum MLD. The year with deepest mixing resulted in the highest export and the highest decay rate (0.003 d−1) while shallow and low export resulted in low decay rates (0.0002 d−1), likely a consequence of the quality of material exported. The exported TOC supported oxygen utilization at ΔC : ΔO2 molar ratios ranging from 0.17 when TOC export was low to 0.47 when it was high. We estimate that exported TOC drove 15–41% of the annual oxygen utilization rates in the 100–400 m depth range. Finally, there was a lack of variability in the surface-layer TON signal during summer. The lack of a summer signal for net TON production suggests a small role for N2 fixation at the site. We hypothesize that if N2 fixation is responsible for elevated N : P ratios in the main thermocline of the Sargasso Sea, then the process must take place south of Bermuda and the signal transported north with the Gulf Stream system.