Spatial and temporal variability of trace metals (Fe, Cu, Mn, Zn, Co, Ni, Cd, Pb), iron and copper speciation, and electroactive Fe-binding humic substances in surface waters of the eastern Gulf of Mexico

The concentrations of bioactive trace metals (Fe, Cu, Mn, Zn, Co, Ni, Cd, and Pb), Fe-and Cu-binding organic ligands, and electroactive Fe-binding humic substances were measured in surface waters across the West Florida Shelf in the eastern Gulf of Mexico in June 2015 and in February-March 2018. Seasonal differences in dust deposition were associated with increased concentrations of Fe, Mn and Pb in offshore surface waters in June 2015 compared to Feb-Mar 2018. Total concentrations of Fe-binding ligands offshore were similar between seasons, and this ambient ligand pool acted to stabilize a portion of Fe delivered via dust deposition within the dissolved fraction. Apparent photoreduction of Cu-binding organic ligands in offshore summer surface waters led to bioavailable Cu2+ concentrations that could potentially inhibit the growth of some cyanobacteria species. The concentrations of Fe-binding humic-like ligands were better correlated with Cu-binding ligands than Fe-binding ligands, suggesting that terrestrially derived photoactive ligands may have a significant influence on the cycling of Cu-binding ligands across the West Florida Shelf. Intrusion of the Loop Current into the study region was observed in June 2015 and appeared to entrain bands of low salinity water from the northern Gulf of Mexico. This process brought elevated trace metals and Fe-binding humic-like ligands to the outer West Florida Shelf, which may then be delivered to the North Atlantic via the Loop Current-Florida Current-Gulf Stream system.