High resolution multi-annual riverine fluxes of organic carbon, nutrient and trace element from the largest European Arctic river, Severnaya Dvina

Abstract

Assessing multi-annual riverine export fluxes of carbon, nutrients and metals from the continent to the Arctic Ocean is crucial for constraining the current status and foreseeing future changes in riverine export induced by climate change. Over 3 consecutive years (2012–2014), we measured daily to weekly concentrations of C, N, P, all major and 45 trace elements (TE) in filtered (<0.45 μm) river water at the terminal station of Severnaya Dvina, the largest European Arctic river, gauged since 1882. There were 3 main groups of elements according to their seasonal behavior, demonstrating distinct links between concentration and the discharge. Dissolved Inorganic Carbon (DIC), major anions (Cl, SO4), alkali and alkaline-earth elements (Li, Na, Mg, K, Ca, Rb, Sr, Ba), and oxyanions (B, Mo, Ge, As) and U exhibited minimum concentrations during spring flood and autumn high flow events, being significantly (p < 0.01) negatively correlated with discharge and dissolved organic carbon (DOC) concentration. These elements reflected the dominant lithology of Severnaya Dvina watershed (carbonates, gypsum) and were controlled by dilution of underground (baseflow) influx by floods. The concentration of DOC, low-soluble trace elements (Al, Ti, Ga, Y, rare earth elements, Zr, Nb, Hf, Th) and organically-bound metals (V, Cr, Co, Ni, Cu, Cd, Pb) had the maximum at the spring flood peak. The input of these elements to the river occurred via surface flow and they were significantly (p < 0.01) increasing with water discharge. Finally, N, P, Si, Mn and Fe originated from both surface and underground sources and were impacted by autochthonous biotic processes in the river channel; they exhibited a strong winter maximum and a summer minimum.The spring flood (May and June) provided 50% of overall water and DOC, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Sb, and Pb flux and 60% of low-soluble TE (Al, Y, Nb, REEs, Ti, Zr, Hf and Th). The nutrients (K, Rb, Si, P, N) exhibited around 40% share of annual export during spring flood, whereas the export of soluble mobile elements (DIC, Cl−, SO42−, Li, B, Na, Mg, Ca, Sr, Mo and U) mainly occurred during winter (35–40%) with only 20–25% of annual flow provided by spring flood.The 3-year mean export fluxes of dissolved components were 30 to 50% lower than (Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cu, Cd, and Pb) or comparable to (±30% for all other elements including DOC and for DIC) previous estimates of this river from 2007 to 2008 and the historic data of major components from 1930 to 1950. In particular, there was no trend in mean monthly seasonal and annual concentrations and export fluxes of DOC, DIC, SO4, Cl since the beginning of chemical analysis between 1929 and 1930 and in Ca, Mg, Si, and Fe since the 1950's through the duration of this study (2012–2014). The lack of seasonal and annual increases in discharge, carbon, and element concentrations in the permafrost-free Severnaya Dvina River for >120 years of discharge and over 60 years of chemical composition measurements is at odds with the reported short-term trend (based on measurements that began in 1970s) showing that element export fluxes increased in other Arctic rivers due to on-going climate warming and permafrost thaw. Novelty statementHistorical and contemporary measurements of element fluxes in the largest European Arctic river did not reveal sizeable changes in discharge and DOC, DIC, Ca, Mg, Fe, Si, SO4, Cl concentrations and fluxes over past >60 years.