Sea ice loss drives a regime shift in Arctic Ocean nitrogen biogeochemistry

The Arctic Ocean (AO) is changing very rapidly. Retreating sea-ice and the subsequent increase in light availability has significantly increased AO Net Primary Production (NPP). However, recent studies postulate that nutrients (and not light) now control NPP dynamics. We present observations from the Fram Strait (1998-2023) where this transition is revealed around 2009 as a sharp decline in fixed-nitrogen concentrations in the Polar Surface Water and an accompanying increase in Si:N ratios. We suggest that this represents a regime shift where fixed Nitrogen (N) has emerged as the main limiting factor for NPP in the contemporary AO. This reduction of N levels in the last decade may have resulted from increased benthic denitrification (BD) on the shelves. We investigate this by combining modelled BD rates and Lagrangian trajectories, which show a sharp increase in BD around 2009, with increasing contributions from the Chukchi and East Siberian shelves. We attribute this biogeochemical response to a drastic reduction in sea ice and circulation shift around this time. We suggest that Arctic climate change has led to a regime shift where low N levels resulting from increased loss of fixed N on the shelves now exert a tighter control on Arctic NPP.