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 mgm-3 and the effective amount of yellow substance from 1.8 to 32mgL-'. 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 Irl = 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.

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. Key wordslake optics, optical properties of water, optically active substances in water, underwater light climate.

Problems in the interpretation of passive optical remote sensing data obtained by telespectrometric measurements on hoard a research vessel (or aircraft) are discussed. Two methods are considered: (1) The correlation method. where correlative relationships between the remotely sensed spectra and concentrations of optically active substances in the water are determined and corresponding regression formulae found; (2) the similarity method. where the remotely scnscd spectrum is compared with the multitude of spectra obtained by means of model calculations. The application of these methods and analysis of the results are madc using our remote and in situ data. It is found that the correlation method is far from being general (the regression parameters depend on the location. season and weather conditions), but it has the advantage of heing applicable without the need to describe the aquatic environment by a theoretical model. The similarity method is much more general but involves difficulties in including the optical properties of the aquatic environment in the theoretical model, cspccially the backscattering properties of several types of suspended matter in the water bodies. Some aspects of detecting oil-slick pollution on the sea surface by means of passive optical remote sensing methods are discussed and corresponding examples are shown

Problems in the interpretation of passive optical remote sensing data obtained by telespectrometric measurements on board a research vessel (or aircraft) are discussed. Two methods are considered: (1) The correlation method, where correlative relationships between the remotely sensed spectra and concentrations of optically active substances in the water are determined and corresponding regression formulae found; (2) the similarity method, where the remotely sensed spectrum is compared with the multitude of spectra obtained by means of model calculations. The application of these methods and analysis of the results are made using our remote and in situ data. It is found that the correlation method is far from being general (the regression parameters depend on the location, season and weather conditions), but it has the advantage of being applicable without the need to describe the aquatic environment by a theoretical model. The similarity method is much more general but involves difficulties in including the optical properties of the aquatic environment in the theoretical model, especially the backscattering properties of several types of suspended matter in the water bodies. Some aspects of detecting oil‐slick pollution on the sea surface by means of passive optical remote sensing methods are discussed and corresponding examples are shown.

No abstract