Porewater inputs drive Fe redox cycling in the water column of a temperate mangrove wetland

Iron is a vital micronutrient within coastal marine ecosystems, playing an integral role in the scale and dynamics of primary production and carbon cycling in the world's oceans. We investigated the relative importance of in situ Fe(II) production from photochemical, microbial and thermal Fe reduction in the surface water column as well as advective porewater inputs in a temperate saline wetland in Australia containing mangrove and salt-marsh vegetation. The diel average concentration of Fe(II) (0.63 +/- 0.21 mu M, accounting for > 70% of the total dissolved Fe present in surface water) was much higher than commonly reported in oxygenated marine waters despite high dissolved oxygen concentrations (81-112% saturation), pH (7.7-7.8) and salinity (33-36) that favor Fe oxidation. In situ production of Fe(II) in the surface water column was primarily driven by microbial processes rather than photochemical and thermal reduction, with a maximum production rate of 4.9 x 10(-3) nM s(-1). Advective porewater Fe(II) inputs to the wetland averaged over a diel cycle (3.0 x 10(-1) nM s(-1)) were an order of magnitude greater than the combined Fe(II) production rate from autochthonous water column processes (1.0 x 10(-2) nM s(-1)). A bottom up model based on the estimated individual fluxes was used to explain the high Fe(II) concentrations measured during a 24 h time series experiment. Combined, different lines of evidence suggest that advective porewater exchange provides significant quantities of Fe(II) to the estuarine wetland.