How does vegetation mediate mercury cycling in tidal wetlands? The roles of wetland plant physiological and decay processes

The biogeochemistry of tidal wetlands is highly conducive to the production of methylmercury (MeHg), a neurotoxin of concern for human and wildlife health.  In addition to hydrogeomorphic regulation, processes associated with plant growth and senescence (decay) can modify mercury (Hg) bioavailability through a number of soil, water and atmospheric interactions.  Experimental and comparative studies in the Hg-contaminated San Francisco Bay-Delta have demonstrated the significant influence of plant structure and growth, rhizosphere activity, and decomposition on Hg availability.  This presentation reviews the relative influence of emergent and aquatic plant biomass on Hg cycling in tidal marshes, both as living and decomposing tissues. We synthesize data from similar studies in three tidal marsh regions where Hg bioaccumulation is of concern -New Jersey’s Hackensack Meadowlands, Louisiana’s Barataria Basin, and the San Francisco Bay-Delta region.  Rates of MeHg production are a function of both microbial activity and inorganic Hg(II) availability.  Microbial activity is strongly influenced by plant-regulated rhizosphere conditions, whereas inorganic Hg(II) availability is influenced by the conditions and chemistry associated with biomass decay, sediment oxidation-reduction chemistry associated with the biogeochemical cycling of sulfur and and iron, and differences in dissolved organic matter quantity and quality.  In wetland sediments where MeHg production rates are limited by microbial activity, processes associated with the plant rhizosphere will have a greater influence than will decay processes.  Conversely, in wetland sediments where rates of MeHg production are limited by Hg(II) availability, the influence of vegetation biomass decay will dominate.