During the passage of a cold front in March 2002, bio-optical properties examined in coastal waters impacted by the Mississippi River indicated that westward advective flows and increasing river discharge containing high concentrations of nonalgal particles contributed significantly to surface optical variability. A comparison of seasonal data from three cruises indicated spectral models of absorption and scattering to be generally consistent with other coastal environments, while their parameterization in terms of chlorophyll (Chl) a concentration showed seasonal variability. The exponential slope of the colored dissolved organic matter (CDOM) averaged $0.0161\pm 0.00054{\text{nm}}^{-1}$ and nonalgal absorption averaged 0.011 nm⁻¹ with deviations from general trends observed due to anomalous water properties. Although the phytoplankton specific absorption coefficients varied over a wide range [0.02 to $0.1{\text{m 2}}^{}{\left(\text{mg \hspace{0.17em} Chl}\right)}^{-1}$ at $443\text{\hspace{0.17em} nm}$] being higher in offshore surface waters, values of phytoplankton absorption spectra at the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) wave bands were highly correlated to modeled values. Particulate scattering characteristics were similar to observations for other coastal waters, while backscattering ratios were on average lower in phytoplankton-dominated surface waters $\left(\text{0 .011}\pm \text{0 .003}\right)$ and higher in low Chl near-bottom waters $\left(0.0191\pm 0.0045\right)$. Average percent differences in remote sensing reflectance $R_{\text{rs}}$ derived from modeled and in-water radiometric measurements were highest in the blue wave bands $\left(52\%\right)$ and at locations with more stratified water columns. SeaWiFS estimates of Chl and CDOM absorption derived using regional empirical algorithms were highly correlated to in situ data.

Empirical band ratio algorithms for the estimation of colored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) for Sea-viewing Wide Field-of-view Sensor (SeaWiFS), Moderate Resolution Imaging Spectroradiometer (MODIS) and MERIS ocean color sensors were assessed and developed for the northern Gulf of Mexico. Match-ups between in situ measurements of CDOM absorption coefficients at 412 nm (a_CDOM(412)) with that derived from SeaWiFS were examined using two previously reported reflectance band ratio algorithms. Results indicate better performance using the R_rs(510)/R_rs(555) (Bias = −0.045; RMSE = 0.23; SI = 0.49, and R² = 0.66) than the R_rs(490)/R_rs(555) reflectance band ratio algorithm. Further, a comparison of a_CDOM(412) retrievals using the R_rs(488)/R_rs(555) for MODIS and R_rs(510)/R_rs(560) for MERIS reflectance band ratios revealed better CDOM retrievals with MERIS data. Since DOC cannot be measured directly by remote sensors, CDOM as the colored component of DOC is utilized as a proxy to estimate DOC remotely. A seasonal relationship between CDOM and DOC was established for the summer and spring-winter with high correlation for both periods (R²~0.9). Seasonal band ratio empirical algorithms to estimate DOC were thus developed using the relationships between CDOM-R_rs and seasonal CDOM-DOC for SeaWiFS, MODIS and MERIS. Results of match-up comparisons revealed DOC estimates by both MODIS and MERIS to be relatively more accurate during summer time, while both of them underestimated DOC during spring-winter time. A better DOC estimate from MERIS in comparison to MODIS in spring-winter could be attributed to its similarity with the SeaWiFS band ratio CDOM algorithm.

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.