Cycling of high‐molecular‐weight dissolved organic matter in the Middle Atlantic Bight as revealed by carbon isotopic (13C and 14C) signatures

Abstract

Carbon isotopes (13C and 14C) 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). COM1 (1 kDa–0.2 µm) was 59% of the bulk DOM in surface Chesapeake Bay waters and decreased to 30–35% in waters of the MAB. COM10 (10 kDa–0.2 µm), which was the high‐molecular‐weight (HMW) component of COM1, comprised 3–12% of the bulk DOM, with highest concentrations in Chesapeake Bay waters and the lowest in deep waters in the MAB. Δ14C values of COM1 decreased from nearshore (‒21 to +12‰) to offshore and from surface (‒166 to ‒85‰) to bottom waters (‒400 to ‒304‰). Although Δ14C values of surface‐water HMW COM10 were generally high (‒2 to ‒7‰), values for bottom‐water COM10 were much lower (‒129 to ‒709‰). The high Δ14C values in the surface water suggest a particulate origin of pelagic COM, consistent with the contemporary Δ14C values of particulate organic matter (POM). The very low Δ14C values of bottom‐water COM10 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 Δ14C values, high C : N ratios, and lower δ13C values; offshore surface water colloids with intermediate Δ14C values, lower C : N ratios, and higher δ13C values; and offshore deep‐water colloids with low Δ14C values, intermediate C : N ratios, and variable δ13C values.