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Carbon isotopes ( 13 C and 14 C) and elemental composition (C and N) in two fractions of colloidal organic matter (COM) were measured to study the origin and cycling of dissolved organic matter (DOM) in the Middle Atlantic Bight (MAB). COM 1 (1 kDa–0.2 µ m) was 59% of the bulk DOM in surface Chesapeake Bay waters and decreased to 30–35% in waters of the MAB. COM 10 (10 kDa–0.2 µ m), which was the high‐molecular‐weight (HMW) component of COM 1 , comprised 3–12% of the bulk DOM, with highest concentrations in Chesapeake Bay waters and the lowest in deep waters in the MAB. Δ 14 C values of COM 1 decreased from nearshore (‒21 to +12‰) to offshore and from surface (‒166 to ‒85‰) to bottom waters (‒400 to ‒304‰). Although Δ 14 C values of surface‐water HMW COM 10 were generally high (‒2 to ‒7‰), values for bottom‐water COM 10 were much lower (‒129 to ‒709‰). The high Δ 14 C values in the surface water suggest a particulate origin of pelagic COM, consistent with the contemporary Δ 14 C values of particulate organic matter (POM). The very low Δ 14 C values of bottom‐water COM 10 imply that in addition to the pelagic origin, sedimentary organic C may serve as an important source for the benthic colloids in the bottom nepheloid layer. The general flow direction of organic carbon is from POM to HMW and to LMW DOM. Three colloidal end‐members were identified in the MAB as well as in the Gulf of Mexico: estuarine colloids with high Δ 14 C values, high C : N ratios, and lower δ 13 C values; offshore surface water colloids with intermediate Δ 14 C values, lower C : N ratios, and higher δ 13 C values; and offshore deep‐water colloids with low Δ 14 C values, intermediate C : N ratios, and variable δ 13 C values.
Guo et al. (Sun,) studied this question.