Optical properties of a region of freshwater influence (the Clyde Sea)

Optical and associated measurements from eight stations in the Clyde Sea are presented. There is a large range of the diffuse attenuation coefficient, from a minimum of 0·14 m^-1 for green light in the outer stations to 3·65 m^-1 for blue light at the most inshore station in the Clyde Estuary. The reason for this gradient of attenuation is, in part, the presence of high concentrations of yellow substance at the inshore stations. Twenty-five samples of yellow substance analysed had absorption coefficients which decayed exponentially with wavelength: the mean decay constant being - 0.018 nm^-1. The yellow substance is introduced in the freshwater runoff from the land: the evidence for this is the strong negative relationship between yellow substance and salinity. The presence of salinity (and hence yellow substance) stratification produces observable changes in the attenuation of light at the halocline. Attenuation by particulate material is of lesser importance overall but must be accounted for to produce an accurate model of light attenuation in the Clyde Sea. An r.m.s. (root mean square) difference of 0.08 m^-1 between observed and predicted attenuation coefficients is achieved using values of specific particulate absorption from the literature. A simple model of the ratio of reflectance coefficients in water whose optical properties are dominated by yellow substance is proposed. According to this the ratio of reflection coefficients in the red and one other colour should increase linearly with yellow substance concentration. The linear fit should be best if the other colour is chosen in the blue end of the spectrum. This theory is supported by the small data set presented here, and this implies that yellow substance and hence salinity, could be remotely sensed from space in these waters. (C) 2000 Academic Press.