The Baltic Sea is characterized by a low salinity with a gradient from the Baltic Proper with > 20 to nearly fresh water conditions in the northern part. Thus, the changing refractive index has to be taken into account, when the Fresnel reflection is computed. Another characteristic is the high concentration of humic matter (Gelbstoff, yellow substance), with an inverse gradient from north to south, which is correlated with salinity (Hojerslev, 1996). The third important factor is cyanobacteria, which may occur during the summer season in massive concentrations at the surface. In such cases the atmospheric correction will fail and a special algorithm has to be applied. Furthermore, the volume relationship of a concentration unit becomes meaningless because the vertical structure is not known.