Effect of coloured dissolved organic matter on the attenuation of photosynthetically active radiation in Lake Peipsi

Abstract

On the basis of underwater radiation measurements and laboratory analyses of water samples, the effect of coloured dissolved organic matter (CDOM) on the light field in the largest Estonian lake, Lake Peipsi, was investigated. CDOM variation and its optical properties were compared with two other large European lakes, Vänern and Vättern in Sweden, as well as with 41 small lakes in Estonia, Sweden, and Finland. The light absorption coefficient at 380 nm for filtered water varied from 4.1 to 16.7 m-1 with the highest values close to the inflow of the Suur-Emajõgi River. CDOM values in L. Peipsi are in the same range as measured earlier in small Estonian and Finnish lakes. Various optical water types ("brown", "moderate", "turbid") were sampled over L. Peipsi. It is shown that the use of GF/F filters results in on average 4% higher absorption values than the use of filters with pore size 0.2 µ m, and the effect might be up to ± 20% in clear waters. Comparison of spectral measurements of diffuse attenuation coefficient with three-band spectro-meter's data showed high consistence of datasets. This allows supplementing the existing database of optical properties of lakes in Estonia with new data from L. Peipsi.