Decadal Variability in Subsurface Nutrient Availability on the Scotian Shelf Reflects Changes in the Northwest Atlantic Ocean

Subsurface nutrients on the Scotian Shelf, an ocean region at the convergence of the subpolar and subtropical western boundary currents (i.e., Labrador Current (LC) and Gulf Stream (GS)), are chiefly modulated by upstream shelf and slope waters. Yet little is known about long-term fluctuations in the advective transport of nutrients to the shelf. Here, we synthesized nutrient and hydrographic data from 1975 to 2020 to characterize decadal changes in upper (60-150 m) and lower (150-200 m) subsurface nutrient availability across the shelf. Hydrographic and nutrient anomalies highlight a transition from a warm, saline, high-nutrient GS-dominated system in the 1980 to colder, fresher, lower-nutrient LC-dominated conditions in the late 1980 and early 1990. In the mid-1990, an increase in excess silicate and phosphate (relative to nitrate) marks a shift in the LC contribution toward a strengthening of the inner shelf component relative to the outer shelf-edge branch. Since 2010, rapid increases in subsurface temperature and salinity indicate the transition back to a GS-dominated system, albeit with a distinct decline in dissolved nutrients relative to the previous warm period, which diverges from the nutrient increase generally associated with GS water. Declines in lower subsurface nutrient concentrations since 2010 are likely driven by a density-related shift in the GS source water toward a less dense, lower-nutrient GS contribution. This shift is evidenced by a decrease in excess phosphate amidst minor declines in excess silicate, reflecting the characteristic density-related patterns of excess nutrients in GS waters.