Temporal and spatial distributions of sediment mercury in restored coastal saltmarshes

Abstract Saltmarsh restoration, through de-embankment, is implemented across Europe and North America with the aims of restoring habitat, sustainably protecting coastlines, and improving water quality. Yet, there is very little understanding of the effects of de-embankment on sediment physico-chemical characteristics and how these characteristics may influence the behaviour of redox-sensitive contaminant metals such as mercury (Hg). The overall aim of this study was to provide baseline data on Hg biogeochemistry in restored coastal saltmarshes and in particular: 1) to assess the spatial variability of sediment total mercury (THg) and methylmercury (MeHg) concentrations over three spatial scales, 2) to examine the association between Hg speciation and indicators of saltmarsh development (loss on ignition, sediment bulk density and moisture content) and 3) to explore how these sediment characteristics and Hg biogeochemistry change with time following de-embankment and hence saltmarsh development. Total Hg and MeHg concentrations at all sites are moderate to high and THg variation is controlled by proximity to external Hg sources. MeHg concentrations and physico-chemical sediment characteristics are variable at different spatial scales in natural and restored sites. MeHg concentrations and physico-chemical parameters varied predominantly at the intermediate (~ 15–50 m) spatial scale in natural saltmarshes, whilst restored sites were less heterogeneous indicative of lower habitat and topographic heterogeneity suggesting that physico-chemical sediment characteristics exert a strong control on Hg methylation. In restored sites, physico-chemical sediment characteristics and MeHg concentrations reflect saltmarsh development and MeHg concentrations increase with time following de-embankment. In the first few decades following de-embankment previous land-use has a significant impact on physical sediment characteristics and MeHg concentrations are lower in restored saltmarshes probably due to poor drainage and/or limited vegetation development. However, with time both physico-chemical characteristics and MeHg concentrations in the restored and natural sites converge, suggesting that it may take decades or even centuries before restored sites have similar physical and biogeochemical conditions to their natural counterparts.