MULTIPLE NEGATIVE CONSEQUENCES OF SULFATE POLLUTION OF FRESHWATER SYSTEMS: NUTRIENTS, MERCURY, METHYLMERCURY, AND MORE

Enhanced microbial sulfate reduction can fundamentally shift a freshwater ecosystem toward a eutrophic, mercury-contaminated system with altered flora. Although sulfate is benign under aerobic conditions, experimental and field survey results demonstrate that in freshwaters with organic-rich sediments the addition of dissolved sulfate causes a cascade of interrelated and potentially deleterious ecosystem consequences. These consequences can occur irrespective of the pH of the system, even when sulfuric acid (such as derived from mine drainage) is neutralized. Microbial reduction of sulfate that diffuses or advects into anoxic sediment porewater produces sulfide, which may precipitate with iron and other metals, or, in metal-poor sediments, can accumulate to phytotoxic levels. Many aquatic rooted plants have mechanisms to partially reoxidize and “detoxify” sulfidic sediments, but their growth and reproduction may also be negatively affected by porewater sulfide, leading to local extirpation (as has evidently occurred in some wild rice [Zizania palustris] waters in Minnesota). Even if sulfide does not accumulate to toxic levels, a harmful positive feedback can arise due to the “additional” decomposition of organic matter possible in anoxic environments when an elevated level of sulfate is available as a terminal electron acceptor. The constituents of organic matter, including nitrogen, phosphorus, carbon compounds, and mercury, are by this decomposition mechanism released to the water column instead of being sequestered in the sediments. Nutrient enrichment supports phytoplankton growth, and dissolved organic and inorganic carbon can alter ecosystem structure as well. Mercury is methylated by the same microbes that reduce sulfate, potentially increasing the supply of methylmercury to the food web. The release of P is exacerbated when sediment iron is tied up as a sulfide compound, which sorb P relatively poorly, driving the system toward eutrophication.