Exploring iron processes in the California Current System through a regional dataset of dissolved iron and organic ligands measurements

Abstract. The micronutrient iron (Fe) regulates productivity across vast regions of the ocean. Along continental margins, dissolved Fe (dFe) concentrations reflect interactions between external sources and internal cycling. Benthic and riverine inputs, together with stabilization by organic ligands, counterbalance biological uptake, scavenging, and precipitation. However, the balance among these processes remains poorly constrained, particularly with respect to the roles of ligands and authigenic Fe precipitation. Here, we compile 3,990 dFe measurements from 29 studies (1987–2024) and 473 measurements of different ligand classes from 7 studies along the U.S. West Coast (USWC), a mostly Fe-limited yet highly productive upwelling system. Most dFe measurements were collected within the upper 50 m, primarily along the central USWC and during summer. dFe concentrations exhibit a strong cross-shore gradient, declining from as much as ∼10 nM near the coast to ∼0.1 nM at 100 km offshore. This pattern is consistent with riverine and benthic inputs nearshore and rapid scavenging and precipitation along the shelf-to-basin pathway. Seasonal variability differs regionally: concentrations are higher in the northern USWC during summer, consistent with upwelling, while higher concentrations in the central and southern regions during winter likely reflect riverine inputs and/or enhanced vertical mixing. Concurrent Fe and ligand measurements are strongly correlated, suggesting a central role for ligands in stabilizing dFe. Positive correlations between dFe and macronutrients highlight the importance of biological uptake and remineralization, whereas negative correlations with oxygen indicate enhanced benthic fluxes under hypoxic conditions. Despite the value of the dataset’s coast-wide coverage, gaps in spatial and temporal sampling, along with methodological differences among studies, limit a comprehensive understanding of Fe variability and underlying processes. Improved intercalibration and high-quality measurements of Fe speciation and riverine inputs are needed to better constrain particulate Fe dynamics and coastal dFe enrichment. In addition, sustained long-term time series are essential to resolve the full seasonal cycle of Fe in this highly dynamic region.