Presenter: Thomas Daley (Bowdoin College)

Description:
Western Boundary Current Systems (WBCS) can support large marine ecosystems; however, the underlying dynamics are distinct from the wind-driven upwelling paradigm and thus deserve separate attention from Eastern Boundary Upwelling Systems. Early studies have paved inroads to the relationship between current-/eddy-induced upwelling and nutrient transport. Yet, a spatially explicit and quantitative assessment is necessary to identify the geographic locations and governing dynamics for environment monitoring and projection purposes. As a case study, we investigated coastal upwelling and primary production in the Southeast (SE) US shelf region. We combined nitrate concentration data from the World Ocean Atlas 2018 and vertical water velocity data from the Estimating the Circulation and Climate of the Ocean (ECCO2) project to analyze upwelling velocities and calculate vertical nitrate fluxes. We identified two specific upwelling zones east of South and North Carolina in 32-34^oN and 35-37^oN, respectively. Vertical nitrate fluxes averaged over these regions are as high as 0.4 µmol m^-2 s^-1 -- comparable to the California Current System. At 300 m depth, vertical nitrate fluxes exist year-round. When shelf circulation becomes favorable in Fall (i.e., shoreward current at depth), nutrient supply from offshore deepwater fuels mid-shelf primary production. Net primary production (NPP) derived from satellite data (CbPM, Oregon State University) ranges from 750 to 1100 mg C m^-2 day^-1, consistent with that computed from nutrient fluxes using Redfield ratios. Interannual variations (1992-2020) show changes in nutrient fluxes by a factor of three along with a decadal contrast. NPP on the SEUS shelf is significantly correlated with nutrient fluxes in the South Carolina Upwelling Zone. Our work highlights the complexity of upwelling in WBCS; a modeling study is needed to better quantify the underlying dynamics of the SEUS shelf system and its response to climate change.