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


  • M. D. DeGrandpre,

  • A. Vodacek,

  • R. K. Nelson,

  • E. J. Bruce,

  • N. V. Blough


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.