Dissolved Iron Seasonal Cycle and Residence Time in the North Pacific Subtropical Gyre

Abstract The seasonal variability of dissolved iron (dFe), an essential phytoplankton micronutrient, was examined over three years on 21 cruises to Station ALOHA in the North Pacific Subtropical Gyre. Mixed layer concentrations and euphotic zone inventories of dFe were highest in winter and spring and decreased throughout the year. We estimated a mean dFe residence time in the euphotic zone (0–150 m) for each season, which was consistent at 5 ± 1 months despite 2‐fold seasonal variability in dFe inventories. The sub‐annual residence time highlights the importance of recycling winter and spring dFe sources to sustain productivity and nitrogen fixation through the summer and fall. Coincident increases in dissolved titanium and elevated particulate Ti:Al suggest dFe inputs from basalt weathering on Hawaiʻi, particularly during increased rainfall and runoff in winter. The source of dFe to Station ALOHA appears to shift from island input in winter to Asian dust deposition in spring.