Size fractionation and optical properties of dissolved organic matter in the continuum soil solution-bog-river and terminal lake of a boreal watershed

The size distribution and speciation of organic matter (OM) in soil solution, bog, stream, and humic and clearwater lake in the north boreal zone (Karelia region, north west Russia) during the summer base-flow period for several years were investigated. The samples were filtered in the field using cascade filtration through progressively decreasing pore size (100, 20, 10, 5, 0.8, 0.4, 0.22, 0.1, 0.046 μm, 100 kDa, 10 kDa and 1 kDa) followed by dissolved organic carbon (DOC) analysis, UV–vis and size exclusion chromatography measurements. Surrogate parameters, such as specific UV absorbance (SUVA; absorbance at 254 nm normalized for DOC concentration in l mg−1 m−1) and the absorbance ratios E254/E436, E280/E350, E254/E365, E365/E470 and E470/E655 (ratio of spectrophotometric absorbance of the sample at two wavelengths) were applied for the characterization of OM in filtered and ultrafiltered water from soil solution, bog, river and lake. In the < 0.22 μm filtrates, there was a systematic decrease in DOC concentration, C/N ratio, SUVA (hydrophobicity and aromaticity) and proportion of colloidal (1 kDa–0.22 μm) OC along the watershed profile from peat bog soil solution, feeding humic lake, to the middle course of the stream towards the terminal oligotrophic lake. Within the filtrates and ultrafiltrates of soil solution and terminal lake, C/N increased from 100 to 140 and from 7 to 25 for 0.22–10 μm and < 1 kDa fractions, respectively. SUVA, degree of humification, hydrophobicity and aromaticity generally increased from high molecular weight (HMW) to low molecular weight (LMW) fractions, being highest in the < 1 kDa fraction. The results allowed a comprehensive view of DOM transport and transformation among various size fractions within a small boreal watershed that can serve as an analogue of small rivers discharge to the Arctic Ocean. It follows that, during the summer baseflow season, the signature of organic-rich interstitial soil solutions originating in a typical peat bog zone can be completely masked by processes occurring in adjacent bog surface waters feeding lakes, as well as in the stream itself. As such, depending on local landscape, one may expect extremely high variability in both chemical nature and MW of DOM delivered by small coastal watersheds to the Arctic Ocean during the summer baseflow period.