Light in shallow waters: A brief research review

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 Corresponding author ([email protected]). Acknowledgment This work was supported by the Office of Naval Research under grant N0014-98-10111. mary production (e.g., Brock et al. 1993; Platt et al. 1994) and surface mixed layer dynamics (e.g., Denman 1973; Dickey 1983) and in ocean color algorithm development (Gordon et al. 1988). This greatly simplifies the radiative transfer problem since one need only be concerned with variability in the vertical dimension. Within shallow water, the spectral composition and intensity of the light field can change significantly in response to the optical properties and orientation of benthic structures across horizontal distances that are short compared to light propagation distance. Many of the techniques and approaches to investigate light in the deep ocean are relevant in the shallow ocean, but they are insufficient to address the entire problem. In order to completely understand the details of how light is distributed within the shallow ocean, new tools and approaches must be developed that take into account the complex distribution, structure, and optical properties of benthic features associated with these ecologically and economically important biomes.