Seasonality of DOC Export From a Russian Subarctic Catchment Underlain by Discontinuous Permafrost, Highlighted by High‐Frequency Monitoring. Issue 10 (11th October 2021) - Gandois, L.
Abstract: Intense climate change and permafrost degradation impact northern watersheds and ultimately organic carbon transfer from terrestrial to aquatic ecosystems. We investigated the contemporary dissolved organic carbon (DOC) dynamics in a northern catchment underlain by discontinuous permafrost (Graviyka River, northern Siberia), where historical meteorological and hydrological data are available since 1936. Mean annual air temperature (MAAT), in contrast to precipitation and discharge was found to show a significant increasing trend since 1950. Using in situ sensing of fluorescent dissolved organic matter (fDOM), we estimated DOC concentrations at a high temporal frequency (1h) during 3 years (2015–2018), and calculated annual specific fluxes of 5.2–5.5 g C m 2 yr −1 . High DOC concentrations (above 10 mg L −1 ) are sustained all year, exhibiting nearly chemostatic behavior. Nevertheless, the high‐frequency survey of DOC and other water parameters revealed the seasonality of DOC origin and pathways in the watershed. The spring freshet dominates the annual export (up to 80%), but summer and autumn floods can also contribute up to 9% and 8% respectively. The high‐frequency sampling was able to capture the specific dynamic of DOC concentration during spring flood (DOC peak preceding discharge, dilution during the spring freshet) and summer and autumn floods (contribution of DOC‐rich, low conductivity water). These observations suggest a significant contribution of organic‐rich water originating in peatlands, potentially from degrading palsas. The study demonstrates both that high‐frequency sampling is essential to capture key events for DOC export, and that more long‐term monitoring is urgently needed in these rapidly evolving watersheds. Plain Language Summary: Carbon transported by rivers forms a large part of the planetary carbon budget. In a changing climate, it is important to know how much carbon is transported to rivers, especially in the Arctic, where soils are rich in organic carbon and are experiencing permafrost thaw. We installed in situ equipment for long‐term high‐frequency observations in a river in northern Siberia in order to track changes in water quality and carbon content. The climate has been becoming warmer in this region since 1950, but there has been no increase in the precipitation or water volume in the river. The waters of the studied river are rich in dissolved organic carbon throughout the year. Spring floods transport the majority of carbon, owing to high water discharge. In summer and autumn, there is less water flowing, but the carbon content in the water is still high. This water and carbon originate in peatlands. Today, these peatlands are partially frozen, but are expected to thaw in the near future. This process can increase the amount of carbon in the water, whereas the increased thawing soil depth can reduce carbon export. Therefore, more long‐term studies are needed to forecast future carbon export from northern watersheds. Key Points: Fluorescent dissolved organic matter (fDOM) high frequency sensing in a catchment underlain by discontinuous permafrost revealed dissolved organic carbon (DOC) export seasonality and captured flood events Spring floods dominate annual export, but autumn floods, coinciding with watershed refreezing are most intense for carbon export Nearly chemostatic DOC behavior suggests an important contribution of peatlands to the annual DOC flux (5.2–5.5 g C m −2 yr −1 )