Rainwater as a source of Fe(II)-stabilizing ligands to seawater
Limnol. Oceanogr., 53(4), 2008, 1678-1684 | DOI: 10.4319/lo.2008.53.4.1678
ABSTRACT: Rainwater hydrophobic extractable dissolved organic matter (EDOM) contains ligand(s) that completely prevent Fe(II) oxidation for at least 4 h after rain is mixed with seawater. The EDOM Fe(II) complex is at least of comparable strength to the ferrozine complex, indicating that this rainwater Fe(II) ligand is among the strongest Fe(II) ligands ever observed in natural waters. In addition to the strong class of ligands that prevent oxidation, rainwater EDOM also contains weaker Fe(II) ligands that slow oxidation of Fe(II) in seawater. Rainwater EDOM is not a single molecule but rather a complex mixture of relatively hydrophobic compounds, even in marine rain with minimal continental influence. When EDOM from the nearby Cape Fear River was extracted using the same method as for rainwater, the river EDOM could not prevent or even slow the oxidation of Fe(II) on mixing with seawater. Therefore, rainwater EDOM is fundamentally different than surface-water EDOM with respect to the strength of Fe(II) ligands. The stability of EDOM complexed Fe(II) most likely affects the bioavailability of rainwater-derived Fe in the surface ocean because the length of time atmospherically deposited Fe remains dissolved in seawater is critical to its role as a phytoplankton nutrient.