Manganese and iron release from mangrove porewaters: a significant component of oceanic budgets?

Mangrove porewater can be highly enriched in dissolved manganese (Mn), iron (Fe), and other trace metals. As a result, porewater exchange may release dissolved metals to surface waters. This study assessed dissolved Mn exchange with the coastal ocean in four mangroves ecosystems, and whether porewater exchange represents a major driver of the oceanic exchange along a latitudinal gradient in Australia (from 28° S to 12° S). Dissolved Fe was also determined but concentrations were below detection in most surface water samples, preventing any flux estimates. Average concentrations of Mn in porewater were approximately an order of magnitude greater than surface waters at all sites, resulting in average porewater-derived Mn fluxes of 441 kmol km^{− 2} year^{− 1} at the four sites. Time series surface water observations indicate that average Mn concentrations decrease at lower latitudes. The average dissolved Mn export rate from the four mangrove systems to the coastal ocean was 88 kmol km^{− 2} year⁻¹. Porewater-derived Mn inputs were greater than surface water exports, which may be explained by dissolved Mn precipitation, oxidation or flocculation at the sediment water interface. While the removal of Mn at the sediment-water interface brings about uncertainties in the estimated porewater fluxes, it has no impact on estimated surface water exports to the coastal ocean. If our surface water export estimates are representative of the global mangrove area (140,000 km²), mangroves may deliver 12 Gmol year⁻¹ of dissolved Mn to the coastal ocean. These fluxes are greater than the estimated flux from global riverine (5.4 Gmol year⁻¹) and atmospheric (11 Gmol year⁻¹) sources, demonstrating that mangroves may be a major player in the oceanic cycle of Mn.