Tidal exchange of dissolved metal(loid)s and organic matters across the sediment-water interface in a salt marsh-mangrove ecotone

Coastal wetlands are considered as efficient barriers for pollutants such as dissolved metal(loid)s between land and sea. However, there remains a challenge in understanding how tidal hydrology regulates the source-sink patterns of metal(loid)s across the sediment-water interface, especially in the salt marsh-mangrove ecotone that commonly exists in subtropical coastlines. In this study, we conducted detailed time-series observations of dissolved metal(loid)s (Cr, Cu, Zn, Ba, Pb, Hg, and As) and organic matter (DOM) characteristics (bulk con-centrations, aromaticity, and molecular size) in southeast China over a spring-neap tidal cycle. The results demonstrated that both DOM and metal(loid)s fluctuated with tide levels, and higher concentrations exited in groundwater than that in the surface water. The concentrations of bulk DOM and chromophoric DOM both in surface water varied oppositely with tide levels, suggesting the potential role of groundwater discharge at low tides. DOM aromaticity also had tidal time scale fluctuations, but the molecular weight of DOM was insensitive to tide changes. The dispersion fluxes were one to four orders of magnitude lower than advective fluxes with the ranges of 0.14-24.8 mu g m- 2 d-1 for Cr, 6.2-22.4 mu g m- 2 d-1 for Cu, 14.7-387.4 mu g m- 2 d-1 for Zn, 0.7-25.4 mu g m- 2 d-1 for As, 259.2-2308 mu g m- 2 d-1 for Ba, 0.1-6.0 mu g m- 2 d-1 for Pb, 0-1.7 mu g m- 2 d-1 for Hg and 3.1-116 mg m- 2 d-1 for DOM, respectively. Mangrove sediments tended to be the sources of DOM and all dissolved metal(loid)s, but the salt marsh was only the source of Cu, As, and DOM. These results have implications on the geochemical cycling of DOM and dissolved metal(loid)s in the salt marsh-mangrove ecotone which has contrasting pattern of surface water-groundwater exchange.

Automated summary

This study conducted detailed time-series observations in southeast China and found that the concentrations of dissolved metal(loid)s and organic matter varied with tide levels, with higher concentrations in groundwater than in surface water. The concentrations of organic matter and metal(loid)s in surface water fluctuated oppositely with tide levels, suggesting the role of groundwater discharge. These findings have implications on the geochemical cycling of dissolved metal(loid)s and organic matter in the salt marsh-mangrove ecotone.