Laboratory for Oceanographic and Environmental Research

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Cycling of Organic Carbon in Marine Environments

Dissolved organic carbon (DOC) is the largest organic carbon reservoir in seawater and plays an important role in the marine carbon cycle and other biogeochemical processes in the ocean. The cycling of dissolved organic matter in the ocean is more complex than previously thought. Recently, fine-size colloidal organic carbon (COC) have been found to be abundant in natural waters. Due to the high specific surface area and complexation capacity, colloids may strongly control the fate and transport of many trace elements and thus their bioavailability and toxicity in estuarine and marine environments. The fact that the bulk DOC pool is heterogeneous in terms of sizes or molecular weights, chemical reactivities, and turnover rates presents further challenges.

Major Research Interests Concerning OC Cycling

• Accurate and precise determination of the DOC concentration in seawater
• Evaluation of cross-flow ultrafiltration techniques for sampling aquatic colloids
• Variations of dissolved organic matter in estuarine and marine environments
• Isotopic and chemical characterization of colloidal organic matter

A vertical profile of dissolved organic carbon in an open Atlantic station (Guo et al., 1995). Results of concentrations and distributions of DOC in the Gulf of Mexico and North Atlantic are oceanographically consistent. A steep DOC concentration gradient in the mixed layer was followed by a decline in the subsurface layer and a consistent and subtle increase towards the bottom. Conservative mixing behavior of DOC in slope waters indicates that water mixing processes are important factors in controlling the distribution of DOC in the ocean.

An Average Molecular Weight Distribution of Dissolved Organic Carbon in Seawater of the Gulf of Mexico and the Middle Atlantic Bight (Guo et al., L&O, 1995). A considerable fraction of the traditionally defined DOC was observed in a colloidal form 1 kDa. COC1, 1 kDa-0.2 µm, comprised ~60% of the bulk DOC in estuarine waters, decreasing to ~30-40% in oceanic waters. High molecular weight (HMW) COC10, 10 kDa-0.2 µm, made up 3-15% of the DOC pool, with highest abundances in estuarine waters and lowest ones in deep oceanic waters).

Distribution of C/N ratio between suspended particulate organic matter (POM), COM10, and COM1 (Guo & Santschi, Mar. Chem., in press). The increase of C/N ratios from particulate to HMW COM to medium MW COM suggests that the direction of organic matter degradation is largely from particulate to HMW and to LMW organic fractions in estuarine environments.

Three types of COM with distinctly different 14C values and organic C:N ratios can be distinguished: estuarine colloids, offshore surface water colloids, and deep water colloids. As marine DOM components increased from estuarine to coastal waters, both C/N ratios and Æ14C values of COM1 decreased from near-shore to off-shore. Moreover, during COM transport from the upper water column to the bottom, C/N ratios of COM1 increased again from surface to bottom waters due to COM degradation, whereas Æ14C values decreased monotonically from surface to deep waters (Guo et al., L&O, 1996).