Seasonal variation of CDOM and DOC in the Middle Atlantic Bight: Terrestrial inputs and photooxidation

Abstract

Extensive surveys of the fluorescence and absorption of chromophore‐containing dissolved organic matter (CDOM), dissolved organic C (DOC) concentration, chlorophyll fluorescence, and salinity were performed during August and November 1993 and March and April 1994 along a cruise line extending from the mouth of Delaware Bay southeast to the Sargasso Sea. With shallow stratification in August, photobleaching dramatically altered the optical properties of the surface waters, with ∼70% of the CDOM absorption and fluorescence lost through photooxidation in the waters at the outer shelf. S, the slope of the log‐linearized absorption spectrum of CDOM, increased offshore and seemed to increase with photodegradation. The increase in S combined with the seasonal variation in the relationship between Chl and CDOM underscores the difficulty in developing algorithms to predict Chl concentrations in turbid coastal waters with ocean color data. Despite the photooxidation of CDOM, the seasonal variation in the CDOM fluorescence‐absorption relationship and fluorescence quantum yields was <15%. When using appropriate methods, the airborne lidar approach for remote determination of CDOM absorption coefficients seems to be a very robust technique. The photooxidation of CDOM in August also affected the relationship between CDOM and DOC concentration in the surface waters, although for the rest of the year the relationship was reasonably linear. The results of a simple model suggest ∼10% of the DOC in the mixed layer was directly converted photochemically to dissolved inorganic C (DIC).