Detecting the Zambezi River plume using observed optical properties

In April 1998 a research cruise was carried out in the Mozambique Channel taking detailed optical, water quality and salinity measurements; the Zambezi plume waters were apparent through strong colour fronts with green waters in the plume and clear blue waters offshore. A good (r2 = 0.76) negative empirical relationship was found between the salinity (S) and yellow substance, represented by the absorption of filtered samples at 440 nm (g440). An empirical relationship was found between the salinity and a reflectance ratio: S(1/2 Zsd) = 37.45-7.07(R555/R490). An optical model in which light absorption is dominated by yellow substance and light scattering by inorganic particles confirms this empirical relationship. This has been applied to a SeaWIFS image to map salinity on the Sofala Bank, demonstrating the potential to remotely determine the distribution of the Zambezi River plume. It is thought that any significant level of chlorophyll would reduce the effectiveness of the above algorithm.