Water Column Structure and Nutrient Supply on the Northwest Atlantic Shelf: A Nitrate Isotope Study

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Abstract
The Northwest Atlantic continental shelf is a highly productive marine region with major uncertainties regarding the supply mechanisms of nutrients fueling productivity. This study uses nitrate isotopes (δ15NNO3 and δ18ONO3) from the Atlantic Zone Monitoring Program 2020 fall mission and an Optimum Multiparameter Analysis to evaluate on‐shelf nutrient transport versus biological supply pathways across three transects from the Laurentian Channel to the central Scotian Shelf. Surface waters showed an imprint of remineralized production, with partial nitrification creating an isotopically light (δ15NNO3 ≥2.7‰) surface lens extending from Cabot Strait across the coastal Scotian Shelf. A concurrent enrichment in δ15NNO3 and δ18ONO3 (15NNO3 signaling sedimentary denitrification in the deep Laurentian Channel. Conversely, subsurface nutrients on the Scotian Shelf were predominantly preformed (4.2‰)−characteristic of N2 fixation in shallow NACW−indicate a stronger NACW imprint on the central Scotian Shelf compared to the Laurentian Channel and eastern shelf. Our results confirm the importance of slope water advection in supplying subsurface nutrients to the shelf while highlighting the seasonal imprint of recycled production on near‐shore surface waters during fall.
Funding Information
  • Ocean Frontier Institute

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