Cycling of high-molecular-weight dissolved organic matter in the middle Atlantic bight as revealed by carbon isotopic (C-13 and C-14) signatures

Carbon isotopes (C-13 and C-14) 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 Eight (MAE). COM(1) (1 kDa-0.2 mu 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 mu 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. Delta(14)C values of COM(1) decreased from nearshore (-21 to +12 parts per thousand) to offshore and from surface (-166 to -85 parts per thousand) to bottom waters (-400 to -304 parts per thousand). Although Delta(14)C values of surface-water HMW COM(10) were generally high (-2 to -7 parts per thousand), values for bottom-water COM(10) were much lower (-129 to -709 parts per thousand). The high Delta(14)C values in the surface water suggest a particulate origin of pelagic COM, consistent with the contemporary Delta(14)C values of particulate organic matter (POM). The very low Delta(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 Delta(14)C values, high C:N ratios, and lower delta(13)C values; offshore surface water colloids with intermediate Delta(14)C values, lower C:N ratios, and higher delta(13)C values; and offshore deep-water colloids with low Delta(14)C values, intermediate C:N ratios, and variable delta(13)C values.