CDOM variations in Finnish lakes and rivers between 1913 and 2014.

In lakes and rivers, the concentrations of dissolved organic carbon (DOC) and coloured dissolved organic matter (CDOM) are closely related. We analysed three large spectrophotometer data sets of Finnish inland waters from the years 1913-1914, 1913-1931 and 2014 for long-term changes in optical properties. The first data set consists of absorption spectra in the band 467-709nm of 212 filtered water samples, the second one contains 11-19years of data for seven rivers, and the third one contains 153 sites with high resolution spectra over the band 200-750nm. These data sets were supplemented with more recent monitoring data of DOC. The sites represent typical optical inland water types of north-eastern Europe. The results did not show any consistent large-scale changes in CDOM concentrations over the 101-year time period. The statistics of the absorption coefficients in 1913 and 2014 were almost identical, at 467nm they were 1.9±1.0m-1 in 1913 and 1.7±1.2m-1 in 2014, and the shape of the CDOM absorption spectrum was unchanged, proportional to exp(-S·λ), S=0.011nm-1 and λ is wavelength. Catchment properties, primarily lake and peat-land percentages, explained 50% of the variation of CDOM concentration in the lakes, and hydrological conditions explained 50% of the variation of CDOM in the rivers. Both illustrate the importance of catchments and hydrology to CDOM concentrations of boreal inland waters.