<p>Here we use a one-dimensional reaction-diffusion model to simulate the seasonal drawdown of nitrate and associated stable isotopic signatures in surface waters of the Labrador Sea during the 2022 spring phytoplankton bloom. Nitrate is depleted during the bloom while supporting primary production but is replenished through regeneration in the surface and resupply from subsurface waters. The inclusion of nitrate isotopes helps distinguish seasonal contributions from regenerated nitrate and subsurface resupply toward total primary productivity, insights that cannot be obtained from nitrate concentrations alone. Results indicate that new production, representing the portion of primary production that can support carbon export from the surface, dominated seasonal productivity, while regenerated nitrate may have supported between 4 and 38% of total production. Productivity estimates were sensitive to the assumed degree of isotopic fractionation by phytoplankton during nitrate uptake, highlighting the need for careful prescription of this parameter in this modeling framework.</p>