Spatial and seasonal distribution of chromophoric dissolved organic matter and dissolved organic carbon in the Middle Atlantic Bight

Spatial and seasonal distributions of chromophoric dissolved organic matter (CDOM) absorption, fluorescence (excited at 355 nm; Fn) and dissolved organic carbon (DOC) concentration in coastal waters of the Middle Atlantic Bight (MAB) were acquired. A linear relationship was observed between CDOM absorption at 355 nm [aCDOM(355)] and Fn on the shelf of the MAB (aCDOM(355)≤1 m−1), but this relationship curved downward for higher aCDOM(355) and Fn values in the Chesapeake Bay (CB) and the Delaware Bay (DB) source waters (aCDOM(355)≥1 m−1), suggesting the presence of different end members with higher Fn/aCDOM(355) ratios, the occurrence of a photochemical/biological processing of the CDOM as it moves down the estuaries or the involvement of both of these processes. Strong evidence was acquired for the photobleaching of CDOM in the surface waters of the MAB during summertime stratification, as indicated by the highly nonlinear dependence of aCDOM(355) and Fn on salinity in surface waters, the concomitant increase of the CDOM spectral slope parameter (S) and the much lower ratio of aCDOM to DOC (aCDOM/DOC) observed in surface waters. The aCDOM/DOC ratio decreased by over an order of magnitude with increasing salinity, owing primarily to two reasons: (1) the substantial photobleaching of CDOM in the surface waters of the shelf and (2) the very different content of CDOM and DOC in the freshwater and oceanic end members. These results indicate that CDOM represents only a small portion of the DOC pool in offshore waters and that the sources and sinks of CDOM and DOC are uncoupled despite the often-observed correlation between CDOM and DOC in the coastal environment.