The distribution of colloidal and dissolved organic carbon in the Gulf of Mexico

Abstract

Cross-flow ultrafiltration techniques have been used to extract colloidal organic carbon (COC) from seawater and to investigate different molecular weight fractions of dissolved organic carbon (DOC). Using a high-temperature catalytic oxidation (HTCO) method, DOC and COC of seawater in the Gulf of Mexico were measured during a R/V Gyre cruise in June 1992. DOC concentrations in surface water varied from 131 μM at a near-shore station (water depth ∼ 20 m) to 83 μM at an off-shore station (water depth ∼ 1550 m). DOC concentrations show statistically significant correlations with apparent oxygen utilization (AOU), as well as with temperature. However, as a upper limit, only 20–30% of the oxygen consumption could be due to dissolved organic carbon oxidation. Furthermore, a good correlation between DOC and AOU existed only in the upper water column across the pycnocline, which we ascribed to lateral exchange processes. Water mixing can be quite important in controlling the distribution of DOC and the relationship between DOC and AOU in the water column. Concentrations of COC > 1000 Dalton ranged from 20 to 69 μM, while COC > 10,000 Dalton ranged from 4 to 16 μM in the study area. On average, COC (> 1000 Dalton) comprised about 45% of the initial DOC, and the mass concentration of colloids was > 1 mg l −1. This was one order of magnitude higher than the concentration of suspended particulate matter, and indicates that COC may be an important component of the carbon cycle in the ocean. The relative abundance of COC (both > 1000 and > 10,000 Dalton) decreased from surface water to deep water, not only in terms of concentration but also relative to total DOC. The measurement of molecular weight distributions indicated that ∼ 35% of the initial DOC was in the 1000 –10,000 Dalton fraction, while only about 10% was in the > 10,000 Dalton fraction, leaving approximately 55% in the truly dissolved fraction (i.e. < 1000 Dalton).