Colored Dissolved Organic Matter Absorption Research Articles

Seasonal seawater optical properties of the U.S. Middle Atlantic Bight

Optical properties of seawater in the U.S. Middle Atlantic Bight (MAB) were measured during four cruises spanning 1993–1994. Samples were collected in August and November 1993 and March and April 1994 along a cruise track extending from Delaware Bay to the Sargasso Sea. Each sample was analyzed for colored dissolved organic matter (CDOM) and particulate absorption and chlorophyll a (chl a). In situ attenuation and reflectance were also measured at selected stations using underwater spectroradiometers. The overall seasonal and spatial ranges for CDOM and particulate absorption (442 nm) and chl a concentration were 0.02–0.41 m−1, 0.01–0.49 m−1, and 0.07–9.4 mg m−3, respectively. CDOM absorption ranged from twofold to threefold between seasons on the MAB shelf, whereas particulate absorption ranged from threefold to sevenfold at the same locations. The results indicate that although CDOM absorption often dominated water column light attenuation at chl a absorbing wavelengths (e.g., 442 nm), chl a could be estimated from in situ reflectance spectra with an average error of ±20% using a relationship previously determined for coastal and open ocean waters. It appears that reflectances at 442 nm (R442) within the MAB shelf are typically low and invariable and most of the chl a sensitivity is contained in the ratio denominator (R554), resulting in relative insensitivity to CDOM absorption. Furthermore, CDOM absorption could not be explicitly determined using reflectance ratios (R412/R490); rather, the reflectance ratios correlated more closely with total (CDOM plus particulate) absorption.

Particulate and dissolved spectral absorption on the continental shelf of the southeastern United States

Visible absorption spectra of particulate and dissolved materials were characterized on the continental shelf off the southeastern United States (the South Atlantic Bight), emphasizing cross‐shelf and seasonal variability. A coastal front separates turbid coastal waters from clearer midshelf waters. Spatial and seasonal patterns were evident in absorption coefficients for phytoplankton, detritus, and colored dissolved organic matter (CDOM); spectral shape parameters for CDOM and detritus; and phytoplankton chlorophyll‐specific absorption. The magnitude of CDOM absorption reflected seasonal differences in freshwater discharge and the salinity of the midshelf waters. In the spring of 1993 (high discharge), CDOM absorption at 443 nm was >10 times that of total particulate absorption between 12 and 50 km offshore (0.28–0.69 m−1 versus 0.027–0.062 m−1) and up to 10 times the CDOM absorption measured in the previous summer (low discharge). Phytoplankton chlorophyll‐specific absorption in the blue increased with distance from shore (from <0.03 m2 mg−1 in inner shelf waters to ∼0.1 m2 mg−1 at the most seaward stations in summer) and, for similar chlorophyll concentrations, was higher in summer than in the winter‐spring. These spatial and seasonal patterns in phytoplankton chlorophyll‐specific absorption can be attributed to a shift in phytoplankton species composition (from predominantly diatoms inshore to a cyanobacteria‐dominated assemblage midshelf in summer), pigment packaging, and higher carotenoid:chlorophyll with distance from shore.