The biogeochemistry of dissolved organic matter and nutrients in two large Arctic estuaries and potential implications for our understanding of the Arctic Ocean system

The discharge, composition, and fate of dissolved organic matter (DOM) and nutrients was investigated in two of the largest Arctic rivers, Yenisei and Ob, in order to assess their role for the biogeochemistry of the Kara Sea, and the potential of terrestrial DOM as tracer for the Arctic Ocean. Ultrafiltered DOM (UDOM) from the two river–estuary systems was characterized by high C/N ratios (34–49), depleted δ13C values (<−26.5‰), depleted δ15N values (1.8–4.2‰), and enriched Δ14C values (84–307‰). The distribution of DOM and its chemical properties indicated no major loss processes during estuarine mixing but showed considerable variability between the rivers and years, suggesting a variable input of plankton-derived DOM. The input of plankton-derived DOM was reflected in elevated neutral sugar yields, decreased C/N ratios, but variable δ13C values. In contrast, δ15N values appear to be more reliable indicators to distinguish terrestrial from plankton-derived DOM in this estuarine system. Based on δ15N values, we estimate that between 6% and 16% of the DOM in the river and estuaries is of plankton origin. Clear differences were found in the chemical composition of DOM size fractions. Plankton-derived DOM was more abundant in the higher molecular weight fractions whereas terrestrial-derived DOM dominated the lower molecular weight fraction. This suggests a more heterogeneous origin of DOM than is revealed from looking at the composition of the bulk DOM. Photooxidation experiments indicated the potential photoreactivity of river DOM from those rivers. Compiling the information from this study with previous studies indicates that river input of DOM and nutrients is not the main driver of autotrophic and heterotrophic processes in the Kara Sea ecosystem. Consistent with previous reports, we found the terrestrial fraction of river DOM to be largely conservative in the estuaries underscoring the potential of terrestrial DOM as a tracer in the Arctic Ocean, especially when combined with standard hydrographic measurements.