SEASONAL SEAWATER OPTICAL-PROPERTIES OF THE US MIDDLE ATLANTIC BIGHT

Optical properties of seawater in the U.S. Middle Atlantic Eight (MAB) were measured during four cruises spanning 1993-1994. Samples were co llected in August and November 1993 and March and April 1994 along a c ruise track extending from Delaware Bay to the Sargasso Sea. Each samp le was analyzed for colored dissolved organic matter (CDOM) and partic ulate absorption and chlorophyll a (chl a). In situ attenuation and re flectance were also measured at selected stations using underwater spe ctroradiometers. The overall seasonal and spatial ranges for CDOM and particulate absorption (442 nm) and chi 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 abso rption ranged from twofold to threefold between seasons on the MAB she lf, whereas particulate absorption ranged from threefold to sevenfold at the same locations. The results indicate that although CDOM absorpt ion often dominated water column light attenuation at chi a absorbing wavelengths (e.g., 442 nm), chl a could be estimated from in situ refl ectance spectra with an average error of +/-20% using a relationship p reviously determined for coastal and open ocean waters. It appears tha t reflectances at 442 nm (R(442)) within the MAB shelf are typically l ow and invariable and most of the chi a sensitivity is contained in th e ratio denominator (R(554)), resulting in relative insensitivity to C DOM absorption. Furthermore, CDOM absorption could not be explicitly d etermined using reflectance ratios (R(412)/R(490)); rather, the reflec tance ratios correlated more closely with total (CDOM plus particulate ) absorption.