River water is the main source of dissolved organic carbon (DOC) in the Arctic Ocean. DOC plays and important role in the Arctic carbon cycle and its export from land to sea is expected to increase with accelerated permafrost thaw with ongoing climate change. However, transport ways and transformation of DOC is mostly unknown. The absorption coefficient by colored dissolved organic matter (aCDOM) which can be used to estimate DOC concentration. In this study, we compiled DOC and aCDOM samples from 11 expeditions covering river, coastal and offshore waters.Water samples for DOC analysis were filtered through 0.7µm GF/F filter and acidified with 25 µL HCl suprapur (10 M) right after sampling. Samples were stored cool and dark for transport. DOC concentrations were measured using high temperature catalytic oxidation (TOC-VCPH, Shimadzu). Three measurements of each sample were averaged and after each 10 samples, a blank and a standard (Battle-02, Mauri-09 or Super-05 certified reference material from National Laboratory for Environmental Testing, Canada) were measured to sustain a quality control.Samples for aCDOM(λ) analysis were filtered through 0.22 µm Millipore GSWP filters (Gonçalves-Araujo et al., 2015, Lena 2016, Bykovksy 2017) or 0.7 µm Whatman GF/F (LD10, YS11, VB13, VB14, Lena 2014, Lena 2015) right after sampling. 100 ml filtrate was stored cool and dark in amber glass bottles until further analysis. aCDOM(λ) was measured with spectrophotometer (SPECORD 200, Analytik Jena) by measuring the absorbance (Aλ) in 1 nm intervals between 200 and 750 nm. The resulting absorbance measurements were then applied to a standard equation aCDOM (λ)=(2.303*A_λ)/L, where L is the path length (length of cuvette), to calculate the aCDOM(λ). Fresh Milli-Q water was used as reference. The cuvette length varied depending on the expected absorption in the sampled water (1 or 5 cm for river or coastal waters, 5 or 10 cm for offshore shelf waters). Resulting aCDOM spectra were corrected for baseline offsets by subtracting the absorption at 700 nm, assuming zero absorption at 700 nm.

Supplement to: Juhls, Bennet; Overduin, Pier Paul; Hölemann, Jens A; Hieronymi, Martin; Matsuoka, Atsushi; Heim, Birgit; Fischer, Jürgen (2019): Dissolved organic matter at the fluvial–marine transition in the Laptev Sea using in situ data and ocean colour remote sensing. Biogeosciences, 16(13), 2693-2713