Short-term Changes of Apparent Optical Properties in a Shallow Water Environment: Observations from Repeated Airborne Hyperspectral Measurements

An atmospheric correction algorithm has been developed for AISA imagery over optically shallow waters in Sugarloaf Key of the Florida Keys. The AISA data were collected repeatedly during several days in May 2012, October 2012, and May 2013. A non-zero near-infrared (NIR) remote sensing reflectance (R_rs) was accounted for through iterations, based on the relationship of field-measured R_rs between the NIR and red wavelengths. Validation showed mean ratios of 0.94 to 1.002 between AISA-derived and field-measured R_rs in the blue to red wavelengths, with uncertainties generally < 0.002 sr^-1. Such an approach led to observations of diurnal changes of AISA-derived R_rs from repeated measurements over waters with bottom types of seagrass meadow, sand, and patch reef, which were driven by tides and/or winds depending on the bottom types. R_rs generally increased with decreasing tidal height and increasing wind speed, with more changes observed over sandy bottom than over seagrass as explained by changes in water turbidity (light attenuation) and bottom contributions. Some of these changes are larger than two times of the R_rs uncertainties from the AISA retrievals, therefore representing statistically significant changes that can be well observed from airborne measurements. The case study suggests that repeated airborne measurements may be used to study short-term changes in shallow water environments, and such a capacity may be enhanced with future geostationary satellite missions specifically designed to observe coastal ecosystems.