Colored dissolved organic matter in the northern Gulf of Mexico using ocean color: seasonal trends in 2005

Abstract

The optical properties of colored dissolved organic matter (CDOM) were studied for the northern Gulf of Mexico coastal and oceanic waters influenced by the Mississippi-Atchafalaya river system. CDOM absorption determined from field measurements in March, May, July and August of 2005 at a station off the Atchafalaya river indicated influences related to river discharge while the CDOM spectral slope S indicated effects of CDOM photooxidation in a sample acquired in late summer. A previously derived CDOM algorithm using the reflectance band ratios of Rrs(510)/Rrs(555) were used to derive estimates of surface CDOM absorption distribution at 412 nm from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) imagery. Spatial and temporal CDOM absorption distributions from satellite for the northern Gulf of Mexico in the year 2005 indicated strong seasonal influence associated with discharge from the Mississippi-Atchafalaya river system. A seasonal increase in storms and hurricanes allowed an assessment of the effects of hurricane Rita in September 2005. SeaWiFS estimates of CDOM revealed a decrease in nearshore CDOM absorption due to the transport of low CDOM offshore waters towards the coast following hurricane Rita. Using conservative CDOM-salinity relationship for the region, SeaWiFS surface salinity derived from CDOM absorption also indicated an increase in nearshore salinity east of the hurricane track. However, an increase in CDOM and a decrease in salinity were observed in coastal waters near the location of hurricane landfall likely associated with coastal runoff following extensive coastal flooding.