Drivers of Spatiotemporal Distributions of Macro- and Micronutrients (Fe, Ni, Cu, Zn) in the Northern Gulf of Alaska

The productive Northern Gulf of Alaska (NGA) ecosystem is impacted by spatiotemporal variability in nutrient distributions and ratios. Due to its advective nature and wide (50–150 km) continental shelf, the NGA may act as a nutrient source to the greater NE Pacific. Using high-resolution data from 11 seasonal (spring, summer, fall) research cruises (2018–2023), we examined regional drivers of nutrient fields. We found that variable sources of micro- and macronutrients influence their distributions and ratios. For example, relative to cellular nutrient quotas for coastal diatoms, surface dissolved Fe (dFe): nitrate (NO₃⁻) ratios indicated an Fe-replete regime near river outflows throughout the year. In contrast, far from freshwater and subsurface Fe inputs, this ratio indicated potential Fe stress or limitation within outer waters during spring. Near the seafloor, a pool of high dissolved micro- and macronutrients highlighted variable nutrient sources (sedimentary and deep offshore waters), and its location indicated its susceptibility to wind forcing. We also observed detachment of bottom boundary layers along shoaling isopycnals within the shelf-break front that can carry subsurface dFe to depths of ∼100 m: a depth that can be accessed by winter mixing. This represents a potential mechanism to deliver dFe from the shelf to offshore high nutrient low chlorophyll (HNLC) surface waters of the NGA, although shelf-basin exchange can be diminished by along-shelf flow (e.g., the Alaskan Stream). This work enhances understanding of nutrient sources in the NGA and adds to a relatively small global repository of seasonally high-resolution micronutrient data over continental shelves.