Dissolved, suspended, and colloidal fluxes of organic carbon, major and trace elements in the Severnaya Dvina River and its tributary

Abstract

The chemical status of major and trace elements (TE) and organic carbon (OC) has been studied in the Severnaya Dvina and Pinega rivers draining granitic moraine, carbonate and sedimentary deposits of the Arkhangelsk region (NW Russia, the White Sea basin of the Arctic Ocean). Sampling was performed during winter and summer baseflow seasons and during the spring flood (2007–2008). Size separation procedure included on-site filtration through 5 µm, 0.22 µm, 100, 10 and 1 kDa, and dialysis through 1 kDa and 10 kDa pore-size membranes. The organic carbon concentration in “truly” dissolved form (< 1 kDa) does not depend on rock lithology or season, being on average equal to 5.0 ± 1.4 mg/L. Our observations may suggest the presence of two types of organic matter pool: i) allochtonous large-size colloids formed by lixiviation from upper soil horizons and ii) autochthonous (aquatic) small molecular-size substances, probably linked to bacterial and phytoplankton exudates. The total dissolved concentration of colloidal TE correlates with OC and Fe contents, being highest during the spring flood and lowest in winter time. There are two different patterns of TE colloidal status during different periods of the year, depending on their association with the organic or organo-mineral constituents of the colloidal matter pool. Trace and major element concentrations in the suspended matter of the River Severnaya Dvina were measured during different seasons over three years of observation. Based on collected data, we evaluate here, for the first time, the seasonal fluxes of major and trace elements in the suspended (> 0.22 µm), total dissolved (< 0.22 µm) and colloidal (1 kDa–0.22 µm) forms. On an annual basis, there is a significant contribution (≥ 30%) of conventionally dissolved species (< 0.22 µm) to the overall flux of usually weakly mobile elements such as the divalent transition metals, Cd, Pb, V, Y, all REEs, Zr, Hf and Th. The transport of these insoluble elements occurs essentially in colloidal form. The spring flood occurring in May provides between 30 and 60% of the total annual dissolved and suspended flux of many insoluble trace elements (Fe, Co, Ni, Cr, Y, all REEs, Cd, Pb, Zr, Hf and Th). We argue that trace element fluxes in Arctic rivers are often characterized by a high proportion of total dissolved matter (< 0.22 µm) compared to suspended particulates, due mostly to the large contribution of organically-bound colloidal forms.