Evidence of dissolved organic nitrogen assimilation in arctic coastal ecosystem

Abstract

The Arctic and the high latitudes underwent dramatic change over the past decades. In fact, the 20th century has been the warmest in the Arctic for at least the past 44 000 years (Miller et al. 2013). Observed changes notably include increased river discharge, permafrost thawing and decline of snow cover and sea-ice extent (Macdonald et al. 2005). Understanding the fate of riverine nitrogen (N) in the Arctic is thus critical to identify the link between dissolved nitrogen dynamic and coastal primary productivity in order to apprehend upcoming changes in the Arctic seas. In this study, we used dual isotope of nitrate to identify the different sources of nitrogen and, in conjunction with water isotopes, look for the presence of denitrification, assimilation and nitrification over the Laptev Sea shelf. We then used the first Arctic δ15NDON data to trace the origin of the DON found within the Laptev Sea shelf and identify which active processes control the DON dynamic over the Laptev Sea shelf. Our interpretation of the dual-isotopes of nitrate points toward a N source that was mostly remineralized on the shelf or advected from the Arctic. However, some extremely enriched δ18ON values found at depth (up to 70 m) highlight the presence of atmospheric nitrate below the surface layer. We hypothesized that this atmospheric nitrate was exported from the surface by the density current created during winter mixing and sea-ice formation processes. This hypothesis is supported by the relationship between the δ18ON and the fraction of brine calculated from δ18Ow and salinity, which indicate that the atmospheric nitrate was associated with high-brine content samples. The DON concentration and its 15NDON was characterized by a significant relationship with the fraction of river water. Plot of 15NDON against the inverse DON concentration suggests that a fractionation process is probably the main driver of the 15NDON. The isotopic effect is related to N-assimilation (3 to 8 ‰) and may indicate that a significant portion of the riverine DON (62%) is being assimilated within the Laptev Sea shelf. Finally, our results suggest that DON input from the Lena is characterized by a depleted 15N signature (2.1‰), suggesting that between 29-35 and 69-81% of this DON could be from atmospheric origin as N2-fixation or nitrogen oxide production by lightning.