Mercury accumulation in sediments of a mangrove ecosystem in SE Brazil

Abstract

The Hg accumulation in sedimentary environments of a mangrove ecosystem in Sepetiba Bay, SE Brazil, was investigated. These environments include sediments of a mangrove forest, the main tidal creek that drains the forest, and the bare seaward-edge mud flat adjacent to forest and tidal creek. Maximum Hg concentration peaks in sediments from the mud flat (184 ng g−1), tidal creek (98 ng g−1), and mangrove forest (60 ng g−1) correspond to enrichment factors of 6.1, 3.3, and 2.0 above the estimated average background level, respectively. Average inventories of Hg excess (background-corrected) concentrations were substantially different between environments, decreasing from mud flat sediments (5.2 mg m−2) to creek sediments (3.3 mg m−2) to mangrove forest sediments (0.9 mg m−2). Mercury concentration profiles indicated a consistently higher accumulation of Hg in surface layers of mud flat and tidal creek sediments, whereas mangrove forest sediments showed a higher Hg accumulation in root-rich subsurface layers, in agreement with an enrichment of Fe and organic matter contents. While Hg distribution in mud flat and tidal creek sediments appears to be largely affected by contamination, its distribution in mangrove forest substrate appears to be greatly affected by root-sediment interactions. Mercury levels in the study site were comparable to those observed in coastal sediments under moderate Hg contamination at local and regional scales. Results indicate that sedimentary environments surrounding the mangrove forest retain most of the anthropogenic Hg reaching the ecosystem. Since tidal waters have been previously demonstrated as the main source of metals to the site, it is suggested that the Hg retention in mud flat sediments precede and may avoid a higher Hg accumulation in landward environments.