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The method suggested earlier for estimating the spectra of diffuse attenuation coefficient of light in the water bodies relying on the beam attenuation coefficient measured from water samples, was improved and applied to different types of lakes. Measurement data obtained in 1994-95 and 1997-98 for 18 Estonian and Finnish lakes were used. The spectra of two characteristics were available for our investigations: 1) beam attenuation coefficient estimated from water samples in the laboratory with a spectrophotometer Hitachi U1000; 2) vertical irradiance (diffuse) attenuation coefficient measured in situ with an underwater spectroradiometer LI 1800UW. A total of 70 spectra were considered. Relying on these data the parameters of our earlier model were changed. The criterion of the efficiency of the new version of our model is the coincidence of the spectra of diffuse attenuation coefficient derived from Hitachi U1000 data (Kdc) with those obtained by underwater irradiance measurements (Kdm). Correlation analysis of the model's results gave the relationship Kdm=1.0023Kdc with correlation coefficient 0.961. The respective values of mean relative difference and standard deviation were 5.4% and 0.55 m−1. This method may be useful in conditions where in situ measuring of underwater irradiance spectra cannot be performed because of weather conditions. As the measurement of the underwater radiation field is often a complicated and expensive procedure, our numerical method may be useful for estimating the underwater light climate.

Abstract. The ratio between underwater quantum irradiance (q) and irradiance (E) for derlov's [ 1976] oceanic and coastal water types and for 11 Estonian and Finnish lakes was studied. This ratio was found to depend on the depth in the water body and the transparency of the water. The ratio q/E for the photosynthetically active radiation (PAR) region of the spectrum may differ from its value in air by up to 24%. The results of the present paper can be used to convert the underwater radiation data from units of W m -2 to gmol s '• m '2 (or vice versa) and to estimate the errors caused by applying the "air" value of q/E instead of its real value in the water. derlov [1976] presents data on the ratio (E/q)350.700 for his water types L II, III, 1, and 3. The biggest difference between E/q at the surface and in the deeper layers of ocean was found for type I, where, beginning from a depth of 40 m, this difference is about 16%. We computed the vertical profiles of (q/E)4oo-7oo for Jerlov's water types I, II, III, 1, 3, 5, 7, and 9. The spectral distribution of the solar radiation just below the water surface was taken analogical to that presented by Dera [1992] for the conditions of medium air turbidity and a solar zenith angle of 40 ø. Our results (Figure 1) correspond well with Jerlov's data (taking into account that they are the reciprocal of our q/E and for a slightly different spectral range). Table 1 presents the values of (q/E)4oo-7oo for a depth of 20 m, computed by us for Jerlov's water types. These values are close to the asymptotic ones for each type. In Table 1 also, the wavelengths (Amax) are shown, for which the spectral irradiance at this depth has the maximum value, and the relative difference (in percent) between q/E at 20 7749