Effects of Spartina alterniflora invasion on mercury speciation in vegetated sediments of the wetland in Changjiang River Estuary, China

To investigate the changes in mercury (Hg) speciation in vegetated sediments of the wetlands in Changjiang River Estuary, China, following the invasion of Spartina alterniflora, we determined total mercury (THg), methylmercury (MeHg), toatal organic carbon (TOC), reduced sulfur (S) and grain size in core sediments (0-40 cm) vegetated with different plants in wetlands. The results showed that: (1) the mean concentrations of THg were 49.9-100.9 µg/kg in sediments vegetated by S. altermflora, Phragmites communis, Scirpus mariqueter and Scirpus tabernaemontani had a significant positive correlation with the fraction of fine particles (<16 цт) and TOC content (r²=0.85, p<0.01; r² =0.58, p<0.01), indicating that the distribution of Hg levels in sediments could be dominated by the spatial differentiation of the mineral-organic complexes in fine particles. The invasion of S. alterniflora promoted the deposition of fine particles, and thus could facilitate Hg storage indirectly in wetland sediments. (2) The mean concentrations MeHg and average values of MeHg/THg (%) in vegetated sediments were 0.3-1.4 µg/kg and 0.4%-1.4%, respectively. The profiles of MeHg and MeHg/THg exhibited decrease with increasing depth across all sites. There was no significant difference in the content of MeHg and the values of MeHg/THg in vegetated sediments dominated by S. alterniflora, P. australis and S. mariqueter, indicating that the impact of S. alterniflora invasion on Hg methylation may be limited. In addition, there was no significant positive correlation between MeHg/THg and THg, TOC and acid volatile sulfur (AVS). S-K edge XANES further revealed that the reduced sulfur (S) (e. g., thoil and sulfide) changed greatly in depth profiles. These results suggested that the changes in reduced sulfur could have limited impacts on MeHg production. The values of MeHg/THg was higher in surface horizon (0 — 8 cm) than in deeper horizon, indicating that the higher rates of Hg methylation in surface sediments and the degradation of fresh organic matter (e. g., algae and plant litter) could be the key biogeochemical process on controlling MeHg production in surface sediments of wetland in Changjiang River Estuary.