Penetration of UV radiation into Finnish lakes with different characteristics

Penetration of UV radiation into Finnish lakes was evaluated through field measurements and/or by determining the optical properties of the lake water. The spectral measurements of air and underwater irradiance (280-400 nm) were performed in early September, 1998, in four lakes (Lake Konnevesi, Tuomiojärvi, Jyväsjärvi, and Kopru) representing a variety of humic (DOC 8.1-16.0 mg l-1) and chlorophyll (2.3-14 micrograms l-1) concentrations. A linear relationship was found between vertical attenuation coefficient (Kd) and DOC (r2 > 0.75), whereas no relationship between Kd and chlorophyll a was observed (r2 < 0.20). The depth where the UV-B irradiance is 10% of the value just beneath the surface (z10%) ranged 5-19 cm. Kd based on field measurements, and absorption coefficient (ad) derived from spectrophotometric scanning measurements of filtered lake water showed a linear relationship (r2 = 0.93). Kd values for two lakes where field measurements were not made (DOC 4.2-5.2 mg l-1, chlorophyll a 1.7-1.9 micrograms l-1) were obtained from the relationship between Kd and ad, giving the 10% UV-B penetration depth of 1.3 m to Lake Kuorinka and 0.21-0.24 m to Lake Palosjärvi.