We collected water samples from the river surface in the center of the Olenekskaya Channel near Samoylov Island using a pre-rinsed HDPE 1 L bottle. During the open water period (June to October), water was sampled from a small boat, and during ice-covered period (November to May), through a hole drilled through the river ice. Some samples during the river ice break-up (between May and June) as well as some samples during the ice freeze-up in October were taken from the shore due to the inaccessibility of a more centered location on the river channel. For CDOM absorption (aCDOM(λ)), the sample was filtered through a 0.45 μm cellulose acetate filter which had been rinsed with 20 mL sample water. CDOM samples were collected in pre-rinsed 50 or 100 mL amber glass bottles that were stored in the dark at 4°C until analysis. aCDOM was measured at Lomonosov Moscow State University, Russia using a PE-5400-UV by ECROS LLC (Russia) spectrophotometer. The absorbance (A) was measured between 200 and 800 nm in 1 nm steps using a 1, 2 or 5 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.
