Colored dissolved organic matter in coastal waters influenced by the Atchafalaya River, USA: effects of an algal bloom

Abstract

Colored dissolved organic matter (CDOM) optical properties were studied in the northern Gulf of Mexico coastal waters influenced by the Atchafalaya River in July 2005 during a period of low river discharge using both field and satellite ocean color data. Although a general conservative CDOM absorption-salinity relationship between the riverine and oceanic end members were observed, deviations in surface CDOM absorption from this relationship (CDOM loss) and average higher spectral slope S of the log-linearized absorption spectra in comparison to bottom waters suggested photodegradation of surface CDOM. Excess CDOM absorption in bottom waters at many stations were associated with low oxygen or hypoxic conditions suggesting a potential CDOM source due to remineralization of organic matter in the bottom waters. At a few stations anomalously high CDOM absorption could be linked to biological production associated with algal blooms. Absorption spectra of a surface sample at one such station deviated from standard exponential form in the 300-400m UV range probably due to the presence of photo-protective pigments. CDOM derived from the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) using a regional algorithm provided a synoptic view of surface CDOM variations and the extent of river influences into the northern Gulf of Mexico.