Estimation of the water quality of the Baltic Sea and lakes in Estonia and Finland by passive optical remote sensing measurements on board vessel

In the years 1993, 1995 and 1996, passive optical remote sensing measurements taken on board ships or boats were carried out at 16 lakes in Estonia and five lakes in Finland, and also in some regions of the Baltic Sea. Simultaneously, the Secchi disk depth was measured and water samples were taken, from which chlorophyll a and suspended matter concentrations were determined in the laboratory. Using an Hitachi spectrophotometer, the attenuation coefficient spectra for filtered and unfiltered water were obtained, and the effective amount of yellow substance was estimated. The properties of the waters under consideration varied within rather wide limits, the Secchi disk depth changing from 0.4 to 8.5 m, chlorophyll concentration from 0.65 to 45 mg m^?3 and the effective amount of yellow substance from 1.8 to 32 mg L^?1. Applying the correlation method for interpretation of the optical remote sensing data, we derived algorithms for estimating the water properties in the Estonian and Finnish lakes and in the Baltic Sea. The correlation coefficients between remote sensing and other water characteristics are in the limits |r| = 0.61-0.84. This shows that despite difficulties caused by a small thickness of the‘informative’water layer and shadowing of the influence of some substance on the remote sensing spectrum by other substances in turbid, multicomponent waters, the passive optical remote sensing method is applicable for estimating the water transparency and quality in lakes and inland seas. However, the method is not suitable for (i) determining the value of a very small amount of some substance in the water if the concentrations of other optically active substances are remarkably higher or for (ii) investigating the water-bodies with a large amount of yellow substance, where extremely strong absorption of light in the water leads to an‘abnormal’shape of the remote sensing reflectance spectra. Our results confirm also that remote sensing algorithms derived for the open ocean waters are in most cases not applicable for lakes and inland seas.