Seasonal variability and controls on chromophoric dissolved organic matter in a large river-dominated coastal margin
Limnol. Oceanogr., 54(6), 2009, 2233-2242 | DOI: 10.4319/lo.2009.54.6.2233
ABSTRACT: The spring-to-summer seasonal transition in the spatial and vertical distribution of chromophoric dissolved organic matter (CDOM) optical properties was examined during surveys conducted in March, May, July, and August of 2005 in the waters influenced by the Mississippi-Atchafalaya River system of the northwestern Gulf of Mexico. Strong seasonal river influences and a well-correlated inverse relationship between CDOM absorption at 412 nm and salinity during spring of 2005 suggests conservative mixing in the water column between the riverine freshwater sources and the oceanic end members. Trends in the CDOM absorption and the spectral absorption slope (S) and deviations from the end-member conservative mixing line in surface and bottom waters during the summer suggest apparent sources and sinks of CDOM. Elevated CDOM in lower-salinity surface waters at many stations appears to be linked to high concentrations of dissolved oxygen (DO; >10 mg L-1) and elevated chlorophyll concentrations. Effects of photo-oxidation in surface waters were observed as increasing S and enhanced CDOM loss in the ~27-32 salinity range and were attributed to seasonal stratification and the increased solar radiation during the summer. In the bottom waters, elevated CDOM levels in the ~32-36 salinity range were associated with low DO and relatively higher bacterial abundance; this suggests a potential CDOM source due to microbial remineralization of organic matter in the hypoxic zone in bottom waters. Seasonal dynamics associated with CDOM loss in surface waters and CDOM gains in bottom waters may be another pathway influencing hypoxia in coastal waters.