For CDOM absorption (aCDOM(λ)), the sample was filtered through a 0.45 μm cellulose acetate filter (Whatman GF/F syringe filter) which had been rinsed with 50 mL sample water. CDOM samples were collected in pre-rinsed 100 mL amber glass bottles that were stored in the dark at 4°C until analysis. aCDOM was measured at the German Research Center for Geosciences (GFZ), Potsdam, Germany using a double beam LAMBDA 950 UV/Vis (PerkinElmer) spectrophotometer. The absorbance (A) was measured between 200 and 750 nm in 1 nm steps using a 5 cm or 10 cm cuvette, depending on the expected concentration of dissolved organic matter. Absorption (a) was calculated from the resulting absorbance measurements via aCDOM(λ) = 2.303 * A(λ) * l, where l is the path length (length of cuvette in meter). Every 5 to 10 samples, the reference sample (Milli-Q water) was exchanged and a blank was measured to avoid instrument drift. Spectral slopes (S275-295, S350-400, S300-600) as well as the Slope Ratio (SR) was derived using nonlinear fitting adapted from the method of Helms et al. (2008).