Optical signatures of dissolved organic matter in the Siberian Rivers during summer season

Rivers are the main way of mobilizing, transforming and transporting of dissolved organic carbon to the ocean, thus largely determining the features of the carbon cycle on the Arctic shelf and in the Arctic Ocean as a whole. Seasonal variations in dissolved organic matter (DOM) fluxes from land to sea have been well documented in the Siberian river's deltas, yet remarkably little is known about how DOM varies with hydrograph within the rivers main stems. The distribution of DOM and its optical characteristics as well as the CO2 fluxes between river water and the atmosphere in the main channel and tributaries of Siberian rivers such as the Lena, the Ob and the Anadyr in summer (July) were studied. The aim of this study was to investigate spatial changes in dissolved organic carbon (DOC) concentrations and DOM composition based on its absorption properties along the riverine main stem, to compare the dynamics of DOC concentration and DOM properties in different Siberian rivers and to reveal the main factors determining its variability, to evaluate the relationship of DOM with carbonate chemistry of riverine waters. The observed spatial variability of the investigated parameters in the river channel was strongly influenced by the river hydrograph along with landscape and geographical features of its catchment area. The phase of river runoff differed significantly in the same time period in different river sections, thus largely determining the quantitative and qualitative variations in the composition of DOM. In the Ob River the inflow of water from its largest tributary, the Irtysh River, is also an important factor of spatial variations. During the summer low-flow period, less aromatic DOM was found in river waters; it was more photodegraded with a relatively low molecular weight of compounds, compared to the high-water period. Such dynamics is one of the factors that alter the carbonate chemistry of waters as well as the magnitude and direction of CO2 fluxes between river water and the atmosphere.Higher DOC concentrations and higher aromatic DOM were found in the Ob River waters compared to the waters of 'permafrost' rivers - the Lena and the Anadyr rivers, which was caused by higher soil organic carbon content and its greater availability determined by the state of permafrost located in the river's watersheds. A strong correlation between DOC concentrations and the colored fraction of DOM was established, thus confirming the possibility of using the latter as a proxy of DOC concentrations in Siberian rivers. The unique information obtained on the properties and variability DOM, which enters the rivers in different flow phases along the main stem, allows a more accurate evaluation of the effect of changing climate on biogeochemical cycles in the Arctic.