Nitrogen cycling in the warming Arctic Ocean

• Main Author: Doncila, Antonia
• Other Authors: Ganeshram, Raja, Tuerena, Robyn
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: The University of Edinburgh 2022
• Subjects: Arctic Ocean; stable isotopes; nitrogen cycling; nutrients; global warming; Biogeochemistry; Arctic; Barents Sea; Fram Strait; Global warming
• Online Access: https://hdl.handle.net/1842/38813; https://doi.org/10.7488/era/2067

This PhD thesis is investigating fixed nitrogen cycling at a Pan-Arctic scale, in order to better understand the sensitivity of Arctic biogeochemistry to ongoing warming and to identify future implications for long-term carbon fixation, nitrogen mass balance and oxygenation of the Arctic Ocean. Using stable isotopes of nitrate (δ¹⁵N-NO3 and δ¹⁸O-NO3), alongside an extensive suite of biogeochemical and hydrographic data, this project provides spatially and temporally integrated measurements of N supply, uptake and recycling in the Central Arctic Ocean (Nansen, Amundsen and Makarov basins) and the Atlantic inflow regions (Barents Sea, Fram Strait), outlining the processes that control nutrient budgets and fluxes on a Pan-Arctic scale. This study proves the paramount role of bordering Arctic shelves in the reprocessing and redistribution of nutrients throughout the entire Arctic Ocean. Stable isotope data demonstrates that the entire nutrient pool in the Central Arctic halocline has been regenerated and laterally advected from the surrounding shelves. However, nutrient supply and uptake across the Central Arctic basins exhibits an east-west gradient dictated by location of the Transpolar Drift (TPD). Regions situated on the TPD path (i.e. Amundsen and Makarov basins) exhibit surface NO3-depletion and a halocline isotopic signal (δ15N-NO3 ~6.5±0.1‰, δ18O-NO3~-1±0.2‰) consistent with lateral advection of partially nitrified and denitrified shelf bottom waters, traced back to the Siberian shelves. The strong salinity stratification in Amundsen and Makarov basins impedes surface nutrient recharge from the halocline, restricting biological uptake to the NO₃-depleted meltwater layer (top 20-30m). Using an isotopically constrained mass balance model, this study shows that the TPD is not an additional source of (riverine) NO3 to Amundsen/Makarov basins and cannot support a future increase in Central Arctic productivity because any riverine nitrogen inputs concentrated in the TPD are lost to denitrification on the shallow, ...