Monitoring the effect of watershed development and climate on coral reefs in the US Virgin Islands using satellite based sensors

Previous research in the US Virgin Islands (USVI) has demonstrated that land-based sources of pollution associated with watershed development are one of the primary causes of coral reef degradation. Combined with projections of increased storm frequency from climate change, coral reef communities are facing unprecedented pressures. Water quality is a key index that can be used to assess stress on these environments. Current assessment methods are based on in situ measurements, suggesting the need for more effective monitoring. Satellite remote sensing provides timely and spatially explicit information regarding changes in aquatic systems once the data is calibrated using in situ measurements. The challenges when utilizing high resolution satellite sensors to acquire data in USVI are accounting for atmospheric path radiance, absorption from optically complex particles in the water column (phytoplankton, suspended sediments, and colored dissolved organic matter), and bottom reflectance from various substrates. In this study, field and lab based data were collected from 18 sites within the Caribbean Sea across St. Thomas and St. John to characterize these optical water quality parameters (WQPs) and bottom reflectance of different substrates. Results show that the optical properties of these waters are a function of multiple WQPs with chlorophyll-a values ranging from 0.10 to 2.35 µg/l and total suspended matter (TSM) values between 8.97 and 15.70 mg/l. Currently, a regionally tiered bio-optical model is being developed to accurately quantify WQPs in St. Thomas and St. John using in situ hyperspectral and satellite datasets. Successful generation of this model would strongly advance tools for near-real-time and long-term monitoring at large spatial scales.