Eddy‐Induced Iron Transport Sustains the Biological Productivity in the Gulf of Alaska

Abstract The surface waters of the subarctic northeastern Pacific Ocean contain high concentrations of macronutrients such as phosphate and nitrate, which can potentially drive photosynthesis and biological uptake of carbon dioxide. Regional biological productivity, however, is limited by a lack of necessary micronutrients such as iron. In the Gulf of Alaska, there is a strong biogeochemical gradient from highly productive and nitrogen‐limited coastal waters to the weakly productive and iron‐limited open ocean. It has been hypothesized that the cross‐shelf transport by ocean eddies transports the iron‐rich coastal waters to the offshore regions. An eddy‐permitting ocean biogeochemistry model is developed to simulate the role of mesoscale ocean eddies in shaping nutrient cycling and regional biological productivity. Model skills are first evaluated using in situ and satellite observations. Through a case study and sensitivity experiments, the physical and biogeochemical structures of the mesoscale eddies are examined, informing the mechanisms behind cross‐shelf transport of dissolved iron. Anticyclonic eddies generated along the continental shelves during the late winter contain an elevated level of dissolved iron just below the surface of the mixed layer. The biological productivity in the late spring and early summer in the offshore regions is supported by these anticyclonic eddies within about 300 km from the coastline. A sensitivity experiment is performed to purposefully suppress the mesoscale features, showing a reduction in the biological productivity accompanied by a reduction in eddy activity in this region. Resolving mesoscale eddies is crucial for correctly representing the biological productivity in this region.