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Chaichitehrani, N.; D'Sa, E.J.; Ko, D.S.; Walker, N.D.; Osburn, C.L., and Chen, R.F., 2014. Colored dissolved organic matter dynamics in the Northern Gulf of Mexico from ocean color and numerical model results. Colored dissolved organic matter (CDOM) absorption and salinity relationships were assessed and used in conjunction with the salinity and current outputs of a numerical model (Navy Coastal Ocean Model [NCOM]) to study CDOM dynamics in the northern Gulf of Mexico. In situ CDOM absorption and salinity obtained from multiple field campaigns were inversely correlated seasonally (winter-spring and summer) and latitudinally (inner- and outer-shelf zones), suggesting conservative behavior of CDOM distribution. A weaker correlation, during summer in the outer-shelf zone, however, indicated stronger effects of photooxidation and lower masking effects from riverine CDOM. Applying these relationships to simulated salinity resulted in hourly maps of CDOM that revealed similarities to CDOM patterns derived from SeaWiFS satellite imagery. Further, matchup comparisons between model-derived and in situ CDOM absorption were statistically sound for the summer (bias = −0.016, root mean square error = 0.059, r2 = 0.51 SI = 0.28) and the winter-spring periods (bias = 0.033, root mean square error = 0.099, r2 = 0.52, SI = 0.21). Overlaying the model-derived CDOM maps on the simulated currents revealed the strong influence of currents on CDOM advection. Downcoast currents during the nonsummer months led to persistent advection of CDOM westward interrupted by frequent cold front events that flush CDOM-laden waters out of the coastal bays onto the inner and outer continental shelves. In contrast, the upcoast current regime, though less well organized, produces a more significant seaward advection of CDOM, likely due to the Ekman transport and subsequent entrainment by mesoscale eddies over the continental slope. [ABSTRACT FROM AUTHOR]

Chromophoric dissolved organic matter (CDOM) was measured in the spring and summer in the northern Gulf of Mexico with the ECOShuttle, a towed, instrumented, undulating vehicle. A submersible pump mounted on the vehicle supplied continuously flowing, uncontaminated seawater to online instruments in the shipboard laboratory and allowed discrete samples to be taken for further analysis. CDOM in the northern Gulf of Mexico was dominated by freshwater inputs from the Mississippi River through the Birdfoot region and to the west by discharge from the Atchafalaya River. CDOM was more extensively dispersed in the high-flow period in the spring but in both time periods was limited by stratification to the upper 12 m or so. Thin, subsurface CDOM maxima were observed below the plume during the highly stratified summer period but were absent in the spring. However, there was evidence of significant in situ biological production of CDOM in both seasons.The Mississippi River freshwater end member was similar in spring and summer, while the Atchafalaya end member was significantly higher in the spring. In both time periods, the Atchafalaya was significantly higher in CDOM and dissolved organic carbon (DOC) than the Mississippi presumably due to local production and exchange within the coastal wetlands along the lower Atchafalaya which are absent along the lower Mississippi. Nearshore waters may also have higher CDOM due to outwelling from coastal wetlands. High-resolution measurements allow the differentiation of various water masses and are indicative of rapidly varying (days to weeks) source waters. Highly dynamic but conservative mixing between various freshwater and marine end members apparently dominates CDOM distributions in the area with significant in situ biological inputs (bacterial degradation of phytoplankton detritus), evidence of flocculation, and minor photobleaching effects also observed. It is clear that high-resolution measurements and adaptive sampling strategies allow a more detailed examination of the processes that control CDOM distributions in river-dominated systems. [Copyright &y& Elsevier]

Abstract: Chromophoric dissolved organic matter (CDOM), as the light absorbing fraction of bulk dissolved organic matter (DOM), plays a number of important roles in the global and local biogeochemical cycling of dissolved organic carbon (DOC) and in controlling the optical properties of estuarine and coastal waters. Intertidal areas such as salt marshes can contribute significant amounts of the CDOM that is exported to the ocean, but the processes controlling this CDOM source are not well understood. In this study, we investigate the production of DOM and CDOM from the decomposition of two salt marsh cordgrasses, Spartina patens, a C4 grass, and Typha latifolia, a C3 grass, in well-controlled laboratory experiments. During the seven-week incubation period of the salt marsh grasses in oxic and anoxic seawater, changes in dissolved organic carbon (DOC) concentrations, dissolved nitrogen (DN) concentrations, stable carbon isotopic composition of DOC (DOC-δ 13C), and CDOM fluorescence demonstrate a significant contribution of DOC and CDOM to estuarine waters from salt marsh plants, such as Spartina and Typha species. In the natural environment, however, the release processes of CDOM from different cordgrass species could be controlled largely by the in situ oxic and anoxic conditions present during degradation which affects both the production and decomposition of DOC and CDOM, as well as the optical properties of CDOM in estuarine and coastal waters. [Copyright &y& Elsevier]

Chromophoric dissolved organic matter (CDOM) fluorescence and dissolved organic carbon (DOC) were measured in the Lingdingyang Estuary, a major component of the Pearl River Delta, China, in May 2001 and November 2002. Measurements of CDOM and DOC suggest multiple sources of dissolved organic matter (DOM) in the low-salinity region of the estuary, due to the mixing of four freshwater outlets, Humen, Jiaomen, Honqimen and Hengmen, all with different freshwater endmembers and all flowing into the head of the Lingdingyang. Our 2002 cruise expanded efforts to characterize these four major outlets, allowing for the quantification of CDOM inputs within this complex watershed.DOC and CDOM appear conservative in mid-salinity (5–20) waters, suggesting that mixing dominates production and removal processes over time scales on the order of 3 days, the residence time of water in this region of the estuary. DOC and fluorescence are linearly correlated in the mid-salinity region of the estuary north of Shenzhen, suggesting that in this region, terrestrial inputs dominate both signals. However, this relationship does not continue out onto the shelf, where water masses of differing optical properties mix. The CDOM/DOC ratio decreases with increasing salinity as would be expected through phototrasformation of CDOM into non-fluorescent DOC. Additionally, this ratio at low salinity appears to vary seasonally, with a higher value in Spring (high flow) compared to Fall (low flow). [Copyright &y& Elsevier]

Variations in the concentration and inherent optical properties of colored dissolved organic matter (CDOM) in river-dominated margins provide information on the cycling of carbon and its chemical composition. Large-scale temporal and spatial variability in CDOM concentration and optical properties were observed in the Northern Gulf of Mexico in June of 2000 and April of 2001. Terrestrial CDOM from the Mississippi and Atchafalaya Rivers dominates the region. Although the primary factor controlling CDOM in this region is the quantity of fresh water runoff, strong regional variability in fresh water sources also plays a major role. Physical complexity due to changes in circulation patterns, volume of river flow and multiple river sources makes observation of biological and photochemical effects challenging in this region. We have used two approaches to distinguish between source and transformation effects: mixing models, which include the concentration and inherent optical properties of CDOM from discrete samples, and high-resolution three-dimensional mapping of multi-spectral fluorescence. [Copyright &y& Elsevier]