Mercury cycling in restored coastal wetlands.

Abstract

Saltmarsh restoration is being implemented across Europe and North America yet, there is little understanding of the effects of de-embankment on Hg biogeochemical cycling. The aim of this thesis is to understand the controls upon Hg dynamics in coastal sites, with specific emphasis on the effects of ecosystem restoration on MeHg production. This is the first study in the UK to examine the effect of saltmarsh restoration on Hg methylation. Field observations were used to assess broad-scale Hg dynamics and physico-chemical controls on MeHg production. A laboratory experiment was conducted to explore the short-term effects of saline inundation on MeHg production. Recently de-embanked sites have lower MeHg concentrations, probably due to poor drainage and limited vegetation development. Physical sediment properties are less heterogeneous in restored sites, which are reflecting lower habitat and topographic heterogeneity. Previous land-use has a significant impact on physico-chemical sediment characteristics and these characteristics change over time to reflect saltmarsh development. There was evidence to show that it takes decades for restored sites to attain similar physico-chemical characteristics to their natural counterparts. This could have significant implications for wider biogeochemical cycling in restored saltmarshes, and long-term implications for the delivery of biogeochemical ecosystem services. This aspect of the research was completely novel, providing the first evidence of the spatial and temporal variation of Hg and MeHg concentrations in restored saltmarshes in the UK. Laboratory experiments showed that MeHg production was greatest in fluctuating saline conditions (cumulative exposure of 1830 pg g-1) compared to all other treatments. For example, the anoxic-saline treatment had a cumulative exposure of 460 pg g-1. Findings indicated that peaks of MeHg could be produced immediately following tidal inundation and that MeHg could potentially increase over time as the site develops to a tidal regime more comparable to a natural saltmarsh. Previous 3 studies have indicated that permanently flooded soils produce high MeHg concentration however this is the first study to show that fluctuating saline conditions could produce a large pulse of MeHg, an important consideration for coastal managers. Surface sediments in restored coastal wetlands appear to be areas of significant MeHg production. MeHg concentration was found to be well correlated with indicators of sulphate reducing bacteria (r=0.536, p<0.001), however most importantly, evidence was found for biogeochemical relationships with MeHg concentration, particularly the association of MeHg and indicators of iron reduction (r=0.561, p<0.001). Therefore, where MeHg is normally restricted by sulphide production, high levels of MeHg can be formed through other pathways. Coastal areas are not generally considered to be areas of concern for MeHg production because high chloride and sulphide concentrations have been shown to inhibit Hg methylation. However, this research shows that other pathways can also be responsible for Hg methylation (i.e. iron reduction) and therefore coastal sediment can be significant contributors to Hg methylation.