Synoptic water clarity assessment in the Florida Keys using diffuse attenuation coefficient estimated from Landsat imagery

Abstract

The diffuse attenuation coefficient, K (m− 1), is a measure of the effective attenuation of light in the water column. It characterizes water clarity and is used as a proxy for water quality. Mapping of shallow water benthic habitats using optical means, including daytime visible satellite imagery, requires knowledge of K to correct for water column effects such as light absorption and scattering. Traditionally, K is derived from imagery using a priori knowledge of bottom types at different depths and specific locations, and assuming that light attenuates exponentially with depth. This technique is applied to three Landsat 7 satellite images (February, May, and July 2000) from the Florida Keys Reef Tract between Key Largo and Key West. Interpolated depth data, initially from NOAA vector chart data, with uncertainties of ±0.5 m, were draped over 30 m spatial resolution Landsat satellite imagery. K was derived for 27 sites where bright sand could be observed at depths between 3 and 20 m. The blue and green bands (Landsat bands 1 and 2 at 450–520 nm and 520–600 nm, respectively) provided K values consistent with time and location. Average K values for bands 1 and 2, respectively, were 0.029 and 0.043 m− 1 (Lower Keys), 0.050 and 0.072 m− 1 (Middle Keys), and 0.063 and 0.082 m− 1 (Upper Keys). The red band (band 3, 630–690 nm) provided more ambiguous and erroneous results with several negative K values, attributed primarily to three factors use of a simple atmospheric correction, the high absorption and rapid attenuation of red light in water, and low radiometric sensitivity of the Landsat ETM + sensor. Despite the fact that these observations were a snapshot in time, trends were observed for the Upper, Middle, and Lower Keys, possibly due to the influence of more turbid Florida Bay waters.