Manganese and iron release from mangrove porewaters: A significant component of oceanic budgets? Ceylena J. Holloway a, ⁎, Isaac R. Santos a,b , Douglas R. Tait a,b , Christian J. Sanders a , Andrew L. Rose b,c , Bernhard Schnetger d , Hans-Jürgen Brumsack d , Paul A. Macklin a,b , James Z. Sippo a,b , Damien T. Maher b a National Marine Science Centre, School of Environment, Science and Engineering, Southern Cross University, PO Box 4321, Coffs Harbour, NSW 2450, Australia b School of Environment, Science and Engineering, Southern Cross University, PO Box 157, Lismore, NSW 2480, Australia c Southern Cross GeoScience, Southern Cross University, PO Box 157, Lismore, NSW 2480, Australia d Institute for Chemistry and Biology of the Marine Environment (ICBM), Carl-von-Ossietzky University Oldenburg, D-26111 Oldenburg, Germany abstract article info Article history: Received 21 January 2016 Received in revised form 29 April 2016 Accepted 31 May 2016 Available online xxxx 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 ex- change 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 great- er 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 ex- plained by dissolved Mn precipitation, oxidation or flocculation at the sediment water interface. While the re- moval 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 km 2 ), mangroves may deliver 12 Gmol year -1 of dis- solved 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. © 2016 Elsevier B.V. All rights reserved. Keywords: Submarine groundwater discharge Subterranean estuaries Porewater exchange Manganese Iron Radon 1. Introduction Manganese (Mn) is an essential micronutrient for marine organisms and is central component in photosynthetic processes. Iron (Fe) is con- sidered a vital biological requirement for all marine organisms (Sunda, 2002). Exports of dissolved Fe and Mn from mangroves may play an im- portant role in oceanic trace metal budgets, but the magnitude of these fluxes remain largely unknown (Sanders et al., 2012). Alternatively, mangroves can be efficient sinks of essential trace metals (Bayen, 2012; Lewis et al., 2011; Machado et al., 2002). Biogeochemical process- es such as adsorption, oxidation, reduction and precipitation often de- termine whether mangrove systems are a net sink or source of metals (Bayen, 2012; Hatje et al., 2003b; Machado et al., 2002). Due to biogeo- chemical and hydrological processes, mangroves can release accumulated Fe and Mn into adjacent surface waters (Alongi et al., 2001; Bayen, 2012; Lewis et al., 2011; Sanders et al., 2012). For example, studies in Malaysia have shown that a mangrove estuary is capable of exporting large amounts of dissolved Mn to the coastal ocean (Alongi et al., 1998). Fe and Mn are important redox reactive elements at the sediment- water interface of mangroves. In the upper oxic region, Mn is normally present as solid phase oxides and oxyhydroxides. In the deeper anoxic sediments, Mn can be reduced to a soluble form. Rapid reoxidation can occur during transport and re-exposure to oxygen (Atkinson et al., 2007; Martynova, 2013). Mn cycling in estuarine and coastal waters is complicated due to the interconnectivity of numerous biogeochemical, hydrological and physicochemical processes that determine its behav- iour (Feng et al., 2015). The redox conditions are often connected to the quality and quantity of the organic matter deposited at the sedi- ment-water interface and its decomposition rate (Marchand et al., 2011b; Martynova, 2014). In estuarine and coastal systems, trace metals Marine Chemistry xxx (2016) xxx–xxx ⁎ Corresponding author. E-mail address: ceylena.holloway@scu.edu.au (C.J. Holloway). MARCHE-03377; No of Pages 10 http://dx.doi.org/10.1016/j.marchem.2016.05.013 0304-4203/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Marine Chemistry journal homepage: www.elsevier.com/locate/marchem Please cite this article as: Holloway, C.J., et al., Manganese and iron release from mangrove porewaters: A significant component of oceanic budgets?, Mar. Chem. (2016), http://dx.doi.org/10.1016/j.marchem.2016.05.013