Adaptation of an algorithm for chlorophyll-a estimation by optical data in the oligotrophic Gulf of Eilat

Downwelling irradiance and upwelling radiance from the water surface were measured in the Gulf of Eilat (Red Sea) on 55 separate occasions in a pelagic station in the period from January 1994 to December 1996. The reflectance was calculated in seven of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) spectral channels throughout the visible spectrum. Concurrent with the optical measurement, water samples were taken to the laboratory and the concentrations of chlorophyll-a (Chl-a) were determined. The ratio of the reflectance in the blue (R443) to the reflectance in the green (R550) was regressed against the Chl-a concentration, and showed that the best-fit of the three datasets (1994, 1995 and 1996) assumed an exponential form, as found in previous work in oligotrophic waters. The coefficients determined in the current study were, however, different from those published in the literature; the reason for this probably originates in the composition of water constituents in the studied area. Small cells dominate the phytoplankton in the pelagic waters in the Gulf of Eilat, and the concentration of suspended and soluble materials of terrestrial origin is very small. The suggested algorithm enabled the prediction of Chl-a concentration within 0.08 mg m^-3. It was shown that calibration of the coefficients, based on empirical data, is important for increasing its prediction accuracy.