Export fluxes of dissolved, colloidal and particulate organic carbon, major and trace elements from the Ob River and its tributaries across seasons

Abstract

Riverine export fluxes of organic carbon (OC), major and trace elements remain at the forefront of environmental research in Arctic and subarctic regions, due mostly to high sensitivity of river hydrochemical parameters to climate warming and, at the same time, global importance of OC, nutrients and toxicants delivered by rivers to the Arctic Ocean. In contrast to reliable information on export fluxes of carbon and metals from the mainland to the Arctic Ocean by large and mid-size Arctic rivers, the majority of these studies quantify either dissolved (< 0.45 µm) or particulate suspended matter (PSM) fluxes, without mechanistic analyses of element speciation in most labile, low molecular weight (LMW) or colloidal form. Here we assessed colloidal versus LMW (< 3 kDa) and suspended (> 0.45 µm) export of OC, major and trace elements in a large Siberian river (Ob) and its smaller tributaries, situated within a boreal taiga / wetland zone. The main differences between Ob and its small tributaries are (i) higher dissolved OC and dissolved and particulate Fe concentrations in tributaries, due to input from surrounding bogs, (ii) much higher PSM load (clays) in the large river (Ob) compared to smaller tributaries; and (iii) strong underground water input for the Ob River, especially during baseflow, which is less pronounced for its tributaries. These major environmental factors were largely responsible for specific features of colloidal vs particulate export for the Ob River and its tributaries. Annual export fluxes normalized to the watershed area (yields) were similar for most elements in dissolved (< 0.45 µm) fraction between the Ob and the tributaries, exempting Fe, Mn, Co and Zn which were higher in tributaries essentially due to input from wetlands. Export of Mo, V, W and U was higher in the Ob River due to pronounced groundwater influence. Colloidal (3 kDa - 0.45 µm) yields were also generally higher in the tributary compared to Ob, except for those soluble elements of groundwater input (Sr, Mo, W, U). The particulate suspended (> 0.45 µm) yield of all elements, except Mn, was much higher in the Ob River when compared to its smaller tributaries. Overall, elemental fluxes of small wetland-draining tributaries of the Ob River can be considered analogous to the small rivers of the permafrost-free portion of the Arctic coast. Such small rivers are more sensitive to processes occurring at the watershed level, hence being an efficient potential sentinel for environmental changes. Fluxes in large Arctic coast rivers are more conservative and are strongly controlled by the effects of PSM interaction with river water at high flow and groundwater loading at base flow. Global significance of our findings is that elemental dissolved (< 0.45 µm) yields of small Artic rivers draining coastal wetlands and boreal forests can be adequately (within a factor of 2 to 3, comparable to inter-annul variations and uncertainties) approximated by those currently available for mid-sized and large rivers.

Keywords: Colloid; Element; Fluxes; Organic carbon; River; Trace element.