1 1 CDOM variations in Finnish lakes and rivers between 1913 and 2014 1 2 Lauri Arvola 1 , Matti Leppäranta 2 , Cecilia Äijälä 2 3 1 Lammi Biological Station, University of Helsinki, Lammi, Finland 4 2 Department of Physics, University of Helsinki, Helsinki, Finland 5 6 7 Abstract 8 In lakes and rivers, the concentrations of dissolved organic carbon (DOC) and coloured dissolved 9 organic matter (CDOM) are closely related. We analysed three large spectrophotometer data sets 10 of Finnish inland waters from the years 1913–1914, 1913-1931 and 2014 for long-term changes 11 in optical properties. The first data set consists of absorption spectra in the band 467-709 nm of 12 212 filtered water samples, the second one contains 11-19 years of data for seven rivers, and the 13 third one contains 153 sites with high resolution spectra over the band 200-750 nm. These data 14 sets were supplemented with more recent monitoring data of DOC. The sites represent typical 15 optical inland water types of northern Europe. The results did not show any consistent large-scale 16 changes in CDOM concentrations over the 101-year time period. The statistics of the absorption 17 coefficients in 1913 and 2014 were almost identical, at 467 nm they were 1.9 ± 1.0 m –1 in 1913 18 and 1.7 ± 1.2 m –1 in 2014), and the shape of the CDOM absorption spectrum was unchanged, 19 proportional to exp(–S·l), S = 0.011 nm –1 and l is wavelength. Catchment properties, primarily 20 lake and peat-land percentages, explained 50% of the variation of CDOM concentration in the 21 lakes, and hydrological conditions explained 50 % of the variation of CDOM in the rivers. Both 22