Optical investigations of Estonian and Finnish lakes

Sixteen lakes in Estonia and five lakes in Finland were investigated during 1992–95, using mainly optical methods. A total of 14 expeditions were undertaken, but the number was different for each lake. Vertical profiles of solar irradiance (spectral and integral), temperature and dissolved oxygen in the water column were measured, and Secchi disk depths were determined. Chlorophyll a and suspended matter concentrations were determined in the laboratory from water samples. Spectrophotometrical processing of the water samples (unfiltered and filtered water) was carried out to describe the beam attenuation coefficient spectra and to determine the spectral influence of yellow substance in the water. Passive optical remote sensing measurements were made from on board a boat. The data obtained show a rather high variability in water characteristics, although the lakes investigated are situated in the same climatic region. Several lakes receive human impact, and this has led to a decrease in water transparency and an increase in eutrophication. Despite rather low values for attenuation depth in lakes, water quality can be estimated using optical remote sensing data.