Effects of microalgal communities on reflectance spectra of carbonate sediments in subtidal optically shallow marine environments

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. Focus on nearshore marine environments is increasing because of the importance of coastal areas for fisheries and recreation and because of the intensification of anthropogenic changes is these areas. Benthic microalgae are an integral part of shallow marine ecosystems at depths within the photic zone, contributing significantly to primary production of the ecosystem as a whole, oxygenation of sediments, nutrient cycling, and sediment stabilization (Paterson 1994; Yallop et al. 1994; MacIntyre et al. 1996; Nelson et al. 1999). Because benthic microalgae form a significant part of the base of the food web in optically shallow waters, changes in biomass and community structure could provide insight into the overall health of nearshore ecosystems. A number of studies designed to characterize microalgal communities near the sediment surface in intertidal habitats have been conducted (Pinckney and Zingmark 1993; Yallop et al. 1994; Brotas et al. 1995; Barranguet et al. 1998; Paterson et al. 1998). Other studies have described vertical dis-