Temporal Variations in Bio-Optical Properties of Four North Estonian Lakes in 1999-2000

To investigate the seasonal variation of bio-optical parameters in lakes of different type regular measurement campaigns were carried out in four North Estonian lakes in 1999–2000. Some parameters (relative transparency of water (zSD), vertical profiles of downwelling vector irradiances in the photosynthetically active region of spectrum (PAR, 400–700nm), vertical profiles of temperature and dissolved oxygen) were measured in situ in the lakes. Concentrations of chlorophyll a (Cchl), suspended matter (CS) and yellow substance (Cy,e) and spectra of light “spectrometric” attenuation coefficient (c*λ) were determined from water samples in the laboratory. Relying on the spectra of c*λ the averaged over 400–700 nm values were computed (c*(PAR)). The lakes under investigation represent a large proportion of the variety of lakes in Estonia with respect to their trophic status and phytoplankton abundance. Lake Paukjarv, which is close to oligotrophic type, is located in a forest 70 km from Tallinn. Lakes Ulemiste and Harku are within the borders of Tallinn, and Lake Maardu is in the vicinity of the city. Although lakes Ulemiste and Harku belong both to the hypertrophic nonstratified lake type, the species composition in these lakes differes greatly. Lake Maardu is a dyseutrophic lake. In lakes Harku, Ulemiste and Maardu, the annual variation of light attenuation is strongly influenced by changes in phytoplankton. In oligotrophic Lake Paukjarv the values of optically active substances and light attenuation coefficients are small, and their annual variations are irregular. Our analysis of the database collected in the course of 87 expeditions shows that despite large optical and biological variability in the lakes investigated some general relationships exist between the measured variables throughout the whole set of lakes. The optical characteristics (c*(PAR), zSD and diffuse light attenuation coefficient) correlate better with phytoplankton-related variables (Cchl, CS) whereas the content of mostly allochthonous humic compounds is less interrelated with other variables. The correlation coefficient lnc*(PAR) vs. lnzSD for all lakes together was 0.98, which confirms the suitability of c*(PAR) for the characterization of water transparency.