The cross-flow ultrafiltration and multiple isotopic tracers (13C, 14C, 230Th, and 234Th) were used to study the dynamics of dissolved organic matter (DOM) in the Gulf of Mexico and the Middle Atlantic Bight. A considerable fraction of the traditionally defined DOM was actually in a colloidal form. Colloidal organic carbon (COC1, 1 kiloDalton (kDa)-0.2 micrometer) comprised ~60% of the bulk dissolved organic carbon (DOC) in estuarine waters, decreasing to ~30-40% in oceanic waters. High molecular weight (HMW) COC10 (10 kDa-0.2 microm) made up 3-15% of the DOC pool, with highest abundances in estuarine waters and lowest ones in deep oceanic waters. A conservative mixing behavior of DOC was found in slope waters with no difference between study areas and seasons, indicating that water mixing processes are important factors in controlling the distribution of DOC in the ocean and thus the overall residence times of oceanic DOC are on the similar time scales as water mixing. Delta (del) 14C values of both colloidal organic matter 1 (COM1) and COM10 in surface water were generally high, reflecting their contemporary inputs and shorter turnover times. Values of del 14C for bottom water COM10, in contrast, could be much lower depending on its source functions. Thus, apparent 14C ages of the bulk DOM are the weight-averaged ages of different DOM fractions with varying molecular weights and reactivities. A three-component del 14C model was used to further elucidate the heterogeneity of bulk DOM. Organic carbon/nitrogen (C/N) ratios of HMW DOM, which are significantly higher than those of bulk DOM pool, increased with decreasing sizes and decreased from nearshore to offshore. Three colloidal end-members were identified with distinct isotopic and chemical composition: estuarine colloids with high del 14C values, high C/N ratios, and lower del 13C values, offshore surface water (pelagic) colloids with intermediate del 14C values, lower C/N ratios, and higher del 13C values, and offshore deep water colloids with low del 14C values, intermediate C/N ratios, and variable del 13C values.

Turnover times of COM10 and COM1 derived from 234Th signatures were consistently short (1-60 days) regardless of apparent 14C ages, indicating that HMW colloids are turning over much faster than the bulk DOM pool. The partitioning of 234Th was broadly similar to that of organic carbon among dissolved, colloidal, and particulate phases, suggesting that Th isotopes can be used as a tracer to study the cycling of DOM in the ocean. The similarities in values of Th partitioning coefficients between dissolved, colloidal and particulate phases imply that the complexation between Th and organic matter is not solely controlled by the specific surface area or pool size of particle or colloid. Instead, the freshness and chemical composition of particulate organic matter (POM) and COM may strongly affect the scavenging of Th isotopes in seawater.