Transformation of dissolved organic matter and related trace elements in the mouth zone of the largest European Arctic river: experimental modeling

The relationship between dissolved organic matter (DOM) and heterotrophic bacterioplankton controls the ecological status of freshwater ecosystems and the fate of metals in aquatic environments. To improve our understanding of physicochemical and biological processes controlling the DOC and related trace elements in the Arctic estuary, we conducted experiments of the biodegradation and physicochemical coagulation of DOM and associated trace elements. For the experiments, water from the estuarine zone of the Severnaya Dvina River in Russia, the largest pristine European Arctic river, was mixed with sterile river water from the freshwater zone, sampled during spring flood in June and summer base flow in August. We revealed a lack of transformation of allochthonous organic matter in spring and measurable degradation of more autochthonous DOM in summer. According to their behavior during incubation experiments, several groups of trace metals were distinguished, comprising (1) nonconservative Fe, Mn, Al, trivalent (Ga, Y, rare earth elements) and tetravalent (Ti, Zr, Hf, Th) hydrolysates, subjected to significant removal from solution, especially at low salinities (0.5-2.5%); and (2) divalent metals such as Ba, Cd, and Pb, subjected to desorption from suspended particulate matter. The trace elements present in the form of oxyanions and neutral molecules (B, Si, As, Sb, Mo) and alkalis (Rb) exhibited highly conservative behavior with negligible removal from the freshwater-seawater mixtures. The spring flood, providing the largest contribution to the annual riverine flux to the ocean, demonstrated high stability of freshwater dissolved organic carbon (DOC) and divalent metals (Cu, Zn, Ni, Cd) to both physicochemical coagulation and biodegradation. This may imply a less significant transformation of the riverine DOC and metal micronutrients delivery to the Arctic Ocean compared to a traditional estuarine removal scheme.