The distribution and ecological impacts of iron in the surface Northwest Pacific Ocean driven by atmospheric deposition

<p indent="0mm">Iron (Fe) is a key micronutrient that controls marine primary productivity and plays a vital role in the biogeochemical cycling of carbon (C) and nitrogen (N). As a result, its availability in the surface ocean is frequently a limiting factor for biological processes, particularly in regions such as the Northwest Pacific Ocean. Atmospheric deposition is a major external source of Fe for the ocean and significantly influences the distribution of dissolved iron (dFe) within, thereby regulating marine biogeochemical cycling. Despite its importance, the spatial variability and ecological impacts of atmospheric dFe deposition remain underexplored, particularly in the Northwest Pacific, where large-scale atmospheric inputs of Fe from desert dust and anthropogenic sources complicate nutrient dynamics. Therefore, this study comprehensively investigated the atmospheric deposition-driven distribution, fluxes, and ecological effects of dFe in the surface waters of the Northwest Pacific. Long-lived thorium isotopes (²³⁰Th/²³²Th) were utilized as tracers to estimate the atmospheric dFe deposition flux, which ranged from 4.02 to 44.28 mg/(m² a). The spatial distribution of these fluxes clearly decreased from west to east and north to south. Notably, the ²³⁰Th/²³²Th-based estimates were higher than the predictions obtained using existing atmospheric deposition models, suggesting that current models underestimate the contribution of atmospheric dFe deposition in this region. Furthermore, strong positive correlations between the atmospheric dFe deposition flux and surface ocean dFe concentrations, chlorophyll a levels, and net primary productivity (NPP) were observed, emphasizing the critical role of atmospheric Fe inputs in regulating marine biogeochemical processes. In the western North Pacific Subtropical Gyre, increased atmospheric dFe deposition alleviated the Fe limitation on N-fixing organisms, thereby stimulating N fixation and introducing new N sources. This cascade of biogeochemical responses shifted the nutrient limitation condition from N limitation or Fe–N co-limitation to phosphorus (P) limitation or N–P co-limitation. By contrast, in the western North Pacific Subarctic Gyre, atmospheric dFe deposition alleviated the Fe limitation on phytoplankton growth, reflecting its direct influence on phytoplankton dynamics in Fe-limited regions. The results of this study underscore the complexity of nutrient dynamics in the Northwest Pacific, where atmospheric dFe inputs drive changes in nutrient stoichiometry and NPP. Our findings emphasize that the spatial variation of atmospheric dFe deposition not only regulates the availability of Fe but also plays a crucial role in determining the nutrient limitations on marine ecosystems. Indeed, the shift from N to P limitation in response to increased atmospheric Fe input has critical implications for various characteristics of marine food webs, including nitrogen fixation, primary productivity, and biogeochemical cycling. Overall, the results of this study enhance our understanding of the essential role that atmospheric dFe deposition plays in shaping marine biogeochemical cycles in the Northwest Pacific. However, the ecological and climate feedback mechanisms triggered by these inputs require further investigation. Future research should integrate multi-platform observational data with model simulations and field incubation experiments to refine our understanding of Fe-N-P coupling, ecological responses to atmospheric dFe deposition, and the broader climate feedback mechanisms associated with Fe inputs to this vital oceanic region.