Nitrification increases riverine nitrate delivery to changing Arctic fjord ecosystems

Arctic fjords support important, productive ecosystems whose nutrient budgets are highly sensitive to climate-driven deglaciation. Here we present over three decades of data describing the nitrogen biogeochemistry of streamflow in the Kongsfjord and Adventfjord regions (West Spitsbergen, Svalbard) to show that microbial nitrate production by nitrification can substantially increase nitrate export to these fjords by runoff. Stable isotopes show that the nitrification of rock or “geogenic” nitrogen is particularly important, especially where shale-rich sedimentary rocks are present. In these cases, the release of nitrified nitrate into runoff may increase total riverine nitrate export by up to ~25 times, providing a potentially significant resource for utilization by downstream ecosystems. Since the influence of nitrification becomes more pronounced as ground thaw progresses, it could sustain primary production as the Arctic growing season lengthens, especially in nitrate-limited fjords. The widespread occurrence of shale bedrock across the Arctic also reveals a likely influence of geogenic nitrogen within the entire region.