NASA satellite monitoring of water clarity in Mobile Bay for nutrient criteria development

Water clarity controls the loss of sunlight reaching the underwater habitats. Because many organisms living in estuarine and coastal waters rely on photosynthesis, water clarity needs to be incorporated into protective water quality standards for these valued ecosystems. To develop the protective standards, a better understanding of causes and effects of water clarity variability at local and regional scales is needed. To that end, NASA remote sensing data are being used to monitor water clarity (measured by light attenuation) and the constituents that decrease water clarity (chlorophyll a, total suspended solids, and colored dissolved organic matter) in the estuarine and coastal systems of the northern Gulf of Mexico. The NASA measurements are intended to augment and extend temporal and spatial coverage of water clarity monitoring conducted by the Federal and State environmental agencies in the same areas. The main objective is to develop a methodology for and to demonstrate the feasibility of producing long-term (1984 to present) time series of the water clarity parameters based on combined satellite measurements from the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments deployed on the Aqua and Terra spacecraft and from the Thematic Mapper (TM) and Enhanced Thematic Mapper Plus (ETM+) instruments from the Landsat 4/5 and Landsat 7 spacecraft, respectively. Aqua and Terra MODIS provide daily coverage dating from 2000, while Landsat TM/ETM+ data extend back to 1984, although with frequency of only once per 8 to 16 days. NASA Earth science research results that improved instrument calibration and data processing techniques have enabled merging the time series of observations from Landsat and MODIS. Algorithms for the retrieval of water clarity parameters from satellite data selected for this project are based on the inherent optical properties of water: absorption and scattering of light. The algorithms are refined based on comparison with field data collected during water quality monitoring in Mobile Bay, Alabama. Results of this project will support future interagency collaborative efforts to develop numeric nutrient criteria for estuarine and coastal waters in the Gulf of Mexico and will contribute to addressing the Gulf of Mexico Alliance priority issue of reducing nutrient inputs to coastal ecosystems.