Dynamics of dissolved organic carbon (DOC) in oceanic environments

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Abstract
Cross‐flow ultrafiltration techniques and a high‐temperature combustion (HTC) method were used to investigate the distributions and fluxes of dissolved (DOC) and colloidal organic carbon (COC) in the Gulf of Mexico and in the Middle Atlantic Bight. Concentrations of DOC in both regions decreased from ≤80 uM in surface waters to ∼45 µM in deep waters and showed large vertical gradients in the subsurface layer. The vertical distributions of DOC were oceanographically consistent. A conservative mixing behavior of DOC was observed in slope waters, and water mixing processes were important factors in controlling the distribution of DOC in the ocean. Calculated downward fluxes of DOC are comparable to those measured for particulate organic C. Size fractionation results revealed that COC (1 kDa‒0.2 µm) comprised ∼40–50% of the total DOC in seawater off Cape Hatteras, while it comprised ∼30–40% in the Gulf of Mexico. High‐molecular‐weight COC10 (10 kDa‒0.2 µm) represented 4–10% of the total DOC in both study areas. Concentrations of COC1, COC3, and COC10 and their percentages in the total DOC decreased from nearshore to offshore and from surface to deep waters. The COC fractions seem to partition in a predictable way in seawater, with DOC concentration as a master variable. On average, 4–7% of the total DOC was in the COC10 fraction, 7–14% was in the 3–10‐kDa fraction, and ~24% was in the 1–3‐kDa fraction, leaving 55– 65% in the <1‐kDa fraction.

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