Effects of De-Embankment on Pore Water Geochemistry of Salt Marsh Sediments

Abstract

Salt marshes form part of the widely distributed intertidal landscape. The salt marshes of the East Frisian barrier island Langeoog (NW Germany) belong to the barrier-connected salt marsh type and were protected by a summer dike that was removed in 2004. In this study, pore water and sediment data were combined to investigate the effects of the de-embankment along a transect including sites on seawater-influenced grassland, high and low salt marsh, and transition zone tidal flat–low salt marsh. Pore waters were sampled with in situ pore water samplers for 13 mo and analysed for trace metals (Fe, Mn), nutrients (NH4 , PO43−), dissolved organic carbon, and sulphate. Additionally, on site measurements of pH and salinity were carried out. Pore water ultrafiltration experiments with 5000 Da MWCO (molecular weight cut off) complemented the water analyses. Sediment samples were taken from hammer corings and were analysed for bulk parameters (total carbon [TC], total inorganic carbon [TIC], total organic carbon [TOC], total sulphur [TS]) and selected major elements (Si, Al, Fe, Mn). Additionally, reactive iron and manganese were analysed.Sediments along the study transect are characterised by quartz dominance and very low TC (TOC and TIC) and TS contents. The iron content is comparable to other salt marsh sediments. The high percentage of reactive iron (up to 40%) indicates that salt marsh sediments form an important iron source for pore waters, as confirmed by high pore water concentrations of dissolved iron (up to 583 μM). Dissolved iron in pore waters most likely results from reduction and dissolution of oxidised iron minerals by organic ligands or Fe(II) organic complexes. Iron complexation by humic substances and siderophores in combination with circum-neutral pH values keep iron in solution. Therefore, the studied salt marshes presumably form an important iron reservoir, which may account for elevated pyrite contents frequently observed in Holocene coastal peats.Flooding of the salt marsh during a storm surge resulted in a considerable increase in pore water iron (8-fold), manganese (21-fold), phosphate (5-fold), and ammonia (7-fold) concentrations. These results show that seawater restoration (de-embankment) should be handled very carefully, especially with regard to nutrient release and subsequent changes in pore water quality.