Optical properties of boreal lake waters in Finland and Estonia

A ten-year (1995-2005) research programme SUVI (Suomi-Viro) on the optics of Finnish and Estonian lakes has been completed. The objectives were to examine the light conditions, to map optically active substances (OAS), and to develop remote sensing methods. Altogether 14 Estonian and 7 Finnish lakes representing different types of water, from oligotrophic to hypertrophic and dystrophic were included. We performed extensive analyses of concentrations of three main OAS, light attenuation and Secchi depth. They varied among lakes and with seasons, but no systematic temporal change could be detected during the 10-year period. We studied the underwater light field using a spectrometer and two PAR quantum sensors, and elaborated three versions of optical classification of lake waters based on: (1) apparent optical properties and the amount of OAS, (2) irradiance reflectance spectra, and (3) light attenuation coefficient and the predominant OAS. We developed two models for determining the diffuse attenuation spectra of light, using data for (1) one wavelength and (2) three wavelengths, and elaborated a semiempirical model for quantitative description of the underwater light regime. We developed a bio-optical model for interpretation of remote sensing data that allows simulation of the reflectance spectra based on the concentrations of OAS. Winter expeditions were undertaken to examine the optical properties of ice and snow and the light conditions in the water beneath the ice cover. Gas pockets are the main optically active impurity in snow and ice. The transparency of ice is similar to that of lake water. The ice cover lowers the light level and makes the light more diffuse in the near surface liquid water layer.