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

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 (δ¹⁵N_NO3 and δ¹⁸O_NO3) 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 (δ¹⁵N_NO3 ≥2.7‰) surface lens extending from Cabot Strait across the coastal Scotian Shelf. A concurrent enrichment in δ¹⁵N_NO3 and δ¹⁸O_NO3 (<8.5‰, <7.4‰) alongside decreasing nitrate concentrations further indicated phytoplankton assimilation over the deep central shelf (overlying Emerald Basin), a process not observed across Cabot Strait or coastal shelf stations. Subsurface nutrients in Cabot Strait are highly regenerated (<43%), with increased bottom water δ¹⁵N_NO3 signaling sedimentary denitrification in the deep Laurentian Channel. Conversely, subsurface nutrients on the Scotian Shelf were predominantly preformed (<86%), sustained by Cold Intermediate Water from the Gulf of St. Lawrence and warm North Atlantic Central Water (NACW) from offshore. Derived water mass contributions and a distinct isotopically light subsurface layer offshore (δ¹⁵N_NO3 of >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.