BIO-OPTICAL INVERSION MODELING OF NEARSHORE WATERS IN THE USVI USING SUBMERSIBLE BEAM ATTENUATION METERS AND HYPERSPECTRAL SENSORS
Flanagan, David Curtis
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Coral reef health in the U.S. Virgin Islands (USVI) is in decline due to land-based sources of pollution associated with watershed development and global climate change. Water quality is a good indicator of stress in these nearshore environments as it plays a key role in determining the health and distribution of coral reef communities. Conventional water quality assessment methods based on <i>in situ</i> measurements are both time consuming and costly, and they lack the spatial coverage and temporal resolution that can be achieved using satellite remote sensing techniques. Water quality parameters (WQPs) such as chlorophyll-a, suspended sediments (TSM), and colored-dissolved organic matter (CDOM) can be measured remotely using empirical band-ratio algorithms, but these band-ratio models fail to take into account the specific inherent optical properties (IOPs) of the water body that control the reflectance spectrum. Semi-analytical inversion models can be used to estimate IOPs and then to derive biogeochemical constituent concentrations from IOP estimates. In this study, three semi-analytical models (Roesler and Perry (1995) (RP95), GSM (2002), and GIOP (2012)) were compared to determine the algorithm that best characterizes the IOPs in the nearshore waters of the USVI using <i>in</i> <i>situ</i> reflectance measurements. The GSM produced the highest correlation with <i>in situ</i> a<sub>tot</sub>(443) (r<sup>2</sup>=0.32) and chlorophyll-a measurements (r<sup>2</sup>=0.27) compared to RP95 and GIOP models. All of the models evaluated performed poorly in estimating b<sub>bp</sub>(443) due to contamination of the Rrs signal from benthic albedo. The GSM model was applied to Landsat 7 ETM+ and Landsat 8 OLI satellite imagery spanning 2000-2018 to develop a time series of the spatial changes in biogeochemical concentrations in the coastal waters of the USVI and indicated slightly higher concentrations of chlorophyll-a during the summer months.