Variability of bio-optical parameters in two North-European large lakes

The bio-optical properties of some North-European large lakes were examined during 1995-2005 using field data and laboratory measurements. The key variables were optically active substances (OAS: chlorophyll, total suspended matter and dissolved organic matter), Secchi depth, and the spectrometric and diffuse light attenuation coefficients. Our main study sites were Lake Peipsi and Lake Vortsjarv in Estonia, both eutrophic with mean Secchi depth below 3 m. The measured water parameters were compared with those obtained from two clear-water Swedish lakes, Lake Vanern and Lake Vattern. This comparison describes the bio-optical differences of the water in eutrophic and oligotrophic lakes. The variability of water parameters in the turbid Estonian lakes was rather high, e.g. the chlorophyll content varied from 1.8 to 102 mg m(-3) and the diffuse light attenuation coefficient from 0.92 to 6.5 m(-1). The change in water properties depends on the season and the biological activity of phytoplankton. We found no apparent long-time trend in water properties. Regression analysis showed that in the turbid Estonian lakes the optical properties were well correlated with chlorophyll and suspended matter, but not with dissolved organic matter. The highest determination coefficients (between 0.73 and 0.89) were obtained when the optical parameters were correlated with all three OAS together (multiple regressions). Our results concerning the variability and interconnections among bio-optical parameters in two Estonian large lakes illustrate the effect of OAS and light field on the ecological conditions of lakes in general.