Influence of Spartina alterniflora invasion on mercury storage and methylation in the sediments of Yangtze River estuarine wetlands

Abstract

The influence of Spartina alterniflora invasion on mercury (Hg) cycling in the coastal wetlands of China and in other parts of the world is not well understood. By combining field sampling with laboratory-scale microcosm incubation experiments, we examined the effect of S. alterniflora invasion on mercury (Hg) storage and net methylmercury (MeHg) production in the sediments of the wetlands in the Yangtze River estuary. The results showed that (1) the estimated mean annual total Hg (THg) input to the wetlands from litterfall was 8.0–26.5 μg/m2 from S. alterniflora, Phragmites australis and Scirpus mariqueter, with the highest levels (26.5 ± 9.0 μg/m2) coming from S. alterniflora due to its high primary productivity. (2) The depth-integrated (<40 cm) mean concentrations of THg in vegetated sediments ranged from 47.5 to 109.1 ng/g, which showed significant positive correlations with the fraction of <16 μm particles and the loss-on-ignition content (P < 0.01). (3) The MeHg concentrations and MeHg/THg (%MeHg) spanned 0.14–1.30 ng/g and 0.2–1.7%, respectively, and showed similar decreasing patterns with depth across all the sites, thereby reflecting the higher rates of Hg methylation in the upper surface sediments. In addition, no significant differences in MeHg concentrations were observed among the sites vegetated by the three plants (P > 0.05). (4) During plant decomposition, net MeHg production was enhanced significantly, but it was inhibited in the presence of the sulfate reduction inhibitor molybdate; these results suggest that sulfate-reducing bacteria could be important methylation mediators in sediments. Moreover, a large fraction of reduced inorganic sulfur (S) species such as iron monosulfide formed during plant decomposition, as revealed by S K-edge XANES spectroscopy. All of these results suggest that S. alterniflora invasion facilitates Hg storage in wetland sediments and affects sulfur cycling but that its effects on MeHg production are similar to those of native plants in the Yangtze River estuarine wetlands.