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Until recently, optical processes in shallow water, where a large portion of solar photons penetrate to the ocean floor, has received little attention outside of a relatively small number of modeling and remote sensing investigations. In the open ocean, scales of variability in relation to optical attenuation length often permit the treatment of the inwater light field as a one-dimensional, depth-dependent problem. In shallow waters hosting productive benthic ecosystems, such as coral reefs or seagrasses, the in-water light field is often three-dimensional in character. In the past decade, quantitative investigations of benthic optical properties and the resulting shallow-water light field have been conducted, fueled by a variety of new sensors designed specifically to address the shallow water problem. Recent publications, as well as the papers contained in this volume, illustrate the rich diversity and interdisciplinary nature of shallow-water optical problems and highlight important issues that should attract closer attention in the future. When sunlight penetrates the ocean surface and propagates down into the water column, portions of the electromagnetic energy are absorbed and scattered at rates that are determined by, in addition to pure water, the concentrations of colored dissolved and particulate matter that make up the water mixture. The ocean radiative transfer problem and, to a lesser extent, the nature of optically important matter in ocean water is well understood in areas where the depth of the ocean floor is greater than the depth of sunlight penetration. In this situation, variability in the subsurface light field under a specified illumination condition and surface wave field (Walker 1994) is largely determined by the distribution of optically important matter dissolved and suspended in the water column, and light intensity generally decreases with depth in accordance with Beer’s Law (Jerlov 1976; Kirk 1983; Mobley 1994). In shallow water, where the depth is much less than the potential for light to penetrate, a large fraction of the subsurface light reaches the ocean floor, where portions of the light energy are absorbed, reflected back into the overlying water column, or re-emitted as fluorescence. The subsurface light field in shallow water is not only a function of the properties of the water mixture, but also of the depth and properties of the ocean floor. Depending on water depth and benthic optical properties, light intensity might decrease more rapidly than expected, remain constant throughout the water column, or even increase with depth (Maritorena et al. 1994). Although the fundamental radiative transfer processes do not change in response to water depth, the environmental context does, and this affects the assumptions and boundary conditions necessary to solve the radiative transfer problem. In the ocean volume, it is most often appropriate to treat the water column as plane-parallel and infinite in horizontal extent because the geometric scales of constituent variability are much greater than the length scales of optical propagation. This is typically the case in simulations of ocean pri1

Two coccolithophore blooms in the Gulf of Maine were studied in 1988 and 1989. Each bloom was about 50,000 km* in area and confined to the top 20 m of the water column. Maximal cell concentrations were -2,000 cells ml-* and coccolith densities of 3 x lo5 ml-’ were observed. The coccolith : cell ratio was highest in the bloom center (region of most intense reflectance) and lowest at the bloom periphery, an indication of varying organic vs. inorganic C production. Chlorophyll concentrations were generally low within the bloom and no relation could be observed between major nutrients and coccolithophore abundance. Backscattered light was profoundly affected by coccolith density and was slightly wavelength-dependent. We calculated total backscattering as well as backscattering (bh) caused exclusively by coccoliths and derived the algorithm relating coccolith density to backscattering. Although cells were efficient light absorbers, coccoliths showed negligible light absorption. Diffuse attenuation was lowest in the green and blue-green part of the visible spectrum. At the center of the bloom, coccoliths contributed ~75% of the backscattering signal and > 50% of the beam attenuation signal. The most accurate way to estimate coccolith concentrations via remote sensing is to measure water-leaving radiance in the green wavebands. The coccolithophore Emiliania huxleyi (Lohm) Hay et al. Mohler (class Prymnesiophyceae) is thought to be the most abunI Present address: Lockheed Engineering and Sciences, P.O. Box 58561, Houston, Texas 77258. Acknowledgments Many thanks to Capt. Donald Bradford and the crew of the RV Argo Maine for ship handling and help with sampling. Shiptime for leg 1 of the 1988 cruise was provided by Charles S. Yentsch. Howard Gordon provided the light scattering photometer as well as the software for calculating total backscattering. Robert Evans and Jody Splain arranged for the transfer of AVHRR data from Miami to Bigelow Lab. Janet Campbell and Thor Aarup analyzed the satellite data during the cruise and relayed the information to the ship. Stephen Groom calculated the visible band reflectance from the AVHRR data. Jeffrey Brown, Tracy Skinner, and Albert Chapin were instrumental in completing many measurements at sea and in the laboratory. Dave Townsend coordinated the CTD measurements. Elin Haugen and R. R. L. Guillard provided an inverted microscope for the 1988 cruise. Christodant calcifying organism on earth (Westbroek et al. 1985). Of all coccolithophore species, E. huxleyi is numerically dominant and can be found from tropical to subarctic regions of the Atlantic, extending into waters with temperatures