Remote sensing is a valuable tool for rapid identification of benthic features in coastal environments. Past applications have been limited, however, by multispectral models that are typically difficult to apply when bottom types are heterogeneous and complex. We attempt to overcome these limitations by using a spectral library of remote sensing reflectance (R rs ), generated through radiative transfer computations, to classify image pixels according to bottom type and water depth. R rs spectra were calculated for water depths ranging from 0.5 to 20 m at 0.5-to 1.0-m depth intervals using measured reflectance spectra from sediment, seagrass, and pavement bottom types and inherent optical properties of the water. To verify the library, computed upwelling radiance and downwelling irradiance spectra were compared to field measurements obtained with a hyperspectral tethered spectral radiometer buoy (TSRB). Comparisons between simulated spectra and TSRB data showed close matches in signal shape and magnitude. The library classification method was tested on hyperspectral data collected using a portable hyperspectral imager for low light spectroscopy (PHILLS) airborne sensor near Lee Stocking Island, Bahamas. Two hyperspectral images were classified using a minimum-distance method. Comparisons with ground truth data indicate that library classification can be successful at identifying bottom type and water depth information from hyperspectral imagery. With the addition of diverse sediments types and different species of corals, seagrass, and algae, spectral libraries will have the potential to serve as valuable tools for identifying characteristic wavelengths that can be incorporated into bottom classification and bathymetry algorithms.Remote sensing has long been used to analyze terrestrial features, such as soil mineral content, foliage density and type, and surface elevation (Curran et al. 1992; PalaciosOrueta and Ustin 1998;Rollin and Milton 1998;Kokaly and Clark 1999). Satellite and airborne sensors are well suited to terrestrial observations in the visible and infrared range. These sensors are more limited, however, when used over oceans or lakes because of the low reflectance values of deep water (giving relatively poor signal-to-noise ratios) and the complexity of combined water and bottom signals in shallow water (Jerlov 1976). Most applications of marine remote sensing to date have been estimations of phytoplankton biomass and sea surface temperatures (SSTs). In these applications, it is generally assumed that all light from the ocean is either spectrally reflected from the upper several meters of the water column or thermally emitted from the first few millimeters at the surface. For biomass and SST applications in shallow water, visible radiation reflected from the bottom Acknowledgments

Microbial communities often produce copious films of extracellular polymeric secretions (EPS) that may interact with sediments to influence spectral reflectance signatures of shallow marine sediments. We examined EPS associated with microbial mats to determine their potential effects on sediment reflectance properties. Distinct changes in spectral reflectance signatures of carbonate sediments from the Bahamas were observed among several sediment sites, which were specifically chosen for their presence of microbial mats and adjacent nonmat sediments. The presence of mats greatly reduced sediment reflectance signatures by ϳ10%-20%, compared with adjacent nonmat areas having similar sediment characteristics. Decreases in reflectance near 444 and 678 nm could be attributed primarily to absorbance by photopigments. However, additional nonspecific decreases in reflectance occurred across a wide spectral range (400-750 nm). Experimental manipulations determined that nonspecific reflectance decreases were due to EPS that are produced by biofilm-associated microorganisms of the mats. Microbial EPS, isolated from natural mat sediments exhibited small but nonspecific absorbances across a broad spectral range. When EPS was in relatively high concentrations, as in microbial mats, there was a ''biofilm gel effect'' on sediment reflectance properties. The effect was twofold. First, it increased the relative spacing of sediment grains, a process that permitted light to penetrate deeper into sediments. Second, it resulted in a more efficient capture of photons because of the change in refractive index of EPS gel itself relative to seawater. The relatively translucent EPS of biofilms, therefore, influenced the magnitude of reflectance across a broad spectral range in marine sediments. Downwelling light, on interaction with carbonate sediments, produces variable upwelling reflectance and scatter- AcknowledgmentsWe thank the staff and scientists of the Caribbean Marine Research Center at Lee Stocking Island, Bahamas, for use of their facilities and for support in carrying out field work. We thank Lisa Drake (Old Dominion University) for help in collecting samples. Finally, we acknowledge the very helpful comments of anonymous reviewers who greatly improved the quality of the manuscript.

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This study was conducted in subtidal areas around Lee Stocking Island, Bahamas, to investigate how microalgal biomass and community structure affect hyperspectral reflectance of sediments. Hyperspectral reflectance was measured on the surfaces of sediment cores collected from several types of carbonate sediments and habitats. Subsequently, photosynthetic and photoprotective pigments within the microalgae colonizing the top 5 mm of the sediment cores were quantified by high-performance liquid chromatography (HPLC). Results of pigment analyses indicate that both microalgal biomass and community structure varied within and among sampling sites. Examination of spectral reflectance revealed differences both in the magnitude of overall reflectance between 400 and 710 nm and in the magnitude of absorption features. Second derivative analysis of reflectance spectra was used to identify nine narrow wavebands that correspond to wavelengths most affected by in vivo absorption by specific pigments. Results of linear regression analyses of the ratio of second derivatives at 676 nm to reflectance at 676 nm versus chlorophyll a plus chlorophyllide a indicate that total (living plus senescent or dead) microalgal biomass can be estimated from measurements of hyperspectral reflectance. Estimates of microalgal biomass can also be made based on the ratios of second derivatives at 444 nm to reflectance at 444 nm. Concentrations of other pigment groups can be estimated from second derivatives at 492 and 540 nm. These relationships between hyperspectral reflectance of sediments and benthic microalgal pigments suggest that remote sensing reflectance might be useful for distinguishing major differences among benthic habitats in some optically shallow areas.