Remote assessment of benthic substrate composition in shallow waters using multispectral reflectance

We investigated the utility of quantifying percent coverage of benthic substrate constituents from surface multispectral reflectance measurements. Six substrates were considered: kelp, eelgrass, clay, silt, mineralic sand from a temperate environment, and turtlegrass and carbonate sand from a tropical environment. Each had a unique albedo spectrum that contributes differently to the upward light field in an optically shallow environment. Simplifications to the radiative transfer equation yield an analytic solution for surface reflectance in optically shallow environments. The objectives were to test the inverse model to predict bottom albedo from measurements of surface reflectance, diffuse attenuation, and water depth in the turbid water of eastern Long Island Sound, Connecticut, and clear waters off Exuma, Bahamas. A linear mixing model was used to deconvolve the derived albedo spectra into contributions by the six constituents. The inverse and deconvolution models accurately identified the dominant substrate in the six homogeneous habitats (single point determinations) and predicted the gradient in substrate composition along transects. This approach has applications to benthic survey mapping, habitat assessment, and habitat monitoring. Coastal, estuarine, and inland waters support a diverse assortment of benthic communities and ecosystems, many of which possess both commercial and ecological value. Climatic disturbances, erosion, industrial pollution, and dredging may alter the physical characteristics and composition of benthic ecosystems, frequently resulting in species diversity changes. These have broader impacts on local fisheries, industrial and private development, and water quality. Periodic assessment of the abundance and health of the submerged marine and intertidal communities is needed to improve our scientific understanding of perturbation responses by benthic ecosystems and our ability to manage delicate marine resources (Snedaker and Getter 1985; Dennison et al. 1993; Clark 1996). A number of techniques exist for monitoring the benthos of shallow waters. SCUBA-based surveys provide great accuracy and high resolution (Luckhurst and Luckhurst 1978; Schneider et al. 1987) yet are limited by the time and manpower necessary to monitor large bodies of water and long stretches of coastline. Acoustic techniques, such as the ultrasonic signal processor (Roxanne), have been successful in determining the geological composition of the sea floor (e.g., silts and clays, sand, gravel, and rock; Chivers et al. 1990; 1 Current address: Science Systems and Applications, Incorporated, Goddard Space Flight Center, Code 970.2, Greenbelt, Maryland 20771. 2 To whom correspondence should be addressed. Current address: Bigelow Laboratory for Ocean Sciences, 180 McKown Point, West Boothbay Harbor, Maine 04575.