Investigating seasonal trends in dissolved iron-binding organic ligands in the BATS region of the North Atlantic Ocean

Marine primary production is strongly influenced by the availability of iron, an essential micronutrient. Organic iron-binding ligands are increasingly recognized to play a governing role in the bioavailability and associated biogeochemical cycling of iron in seawater. The GEOTRACES program has facilitated recent high-resolution datasets of the characteristics and distributions of iron-binding ligands across ocean basins, which highlight the ubiquity of these ligands in ocean water columns. Temporal changes in iron-binding ligands in the upper ocean, including how seasonal changes in mixing and productivity influence open ocean iron speciation, remain unresolved. To assess this, iron-binding organic ligands were measured by competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV) in water-column samples collected from the Bermuda Atlantic Time-series Study (BATS) region of the western subtropical North Atlantic, the location of a long running open ocean biogeochemistry time-series and a designated baseline station for the international GEOTRACES program. Samples for dissolved (< 0.2 µm) iron-binding organic ligands were collected from the upper 2,000 m, during three BATS cruises (March, May and August 2019) as part of the BAIT (Bermuda Atlantic Iron Time-series) project. All samples were collected using trace metal clean sampling techniques and analyzed by CLE-AdCSV at the University of South Florida. Additional samples for soluble (<0.02 µm) iron-binding organic ligands were collected from the mixed layer of each depth profile. These data offer insight into seasonal trends in iron speciation in a well-studied open ocean system.