Dissolved mercury concentrations and reactivity in mangrove waters from the Itacurussa Experimental Forest, Sepetiba Bay, SE Brazil

Abstract

Mangrove waters from the Itacurussa Experimental Forest (IEF), SE Brazil are enriched in reactive-Hg (15 ± 2.0 pM) and total-Hg (28 ± 2.5 pM) relative to open bay waters (4.5 ± 3.0 pM and 19 ± 8.5 pM, for reactiveHg and total-Hg respectively). Mercury concentrations and reactivity varied according to tidal flux in mangrove creek waters. Reactive-Hg concentrations were higher in ebb tide waters ranging from 4.5 to 8.5 pM, than in flood water (<1.0 to 3.5 pM), whereas total-Hg concentrations were higher in flood waters, from 40 to 360 pM, than in ebb water (30 to 250 pM). Low-Hg, open bay waters are enriched in Hg when flowing over mud flats prior to entering the IEF, where it mixes with pore waters containing extremely high Hg concentrations (up to 2,500 pM). Similar distributions of Hg and DOC in pore waters, suggests that DOC may facilitate Hg migration through the sedimentary column, as shown for other estuarine areas. Mud flat pore waters are the more likely source of total Hg to mangrove waters, but not for reactive Hg. These results suggest that mangroves act as a sink to total Hg, as has been shown for other trace metals at the IEF and other mangrove forests. On the other hand mangroves act as a source of reactive Hg to adjacent coastal waters. The mechanisms involved in this processes have not been studied in this paper. However, the similar distribution of DOC and Hg among the different water masses of the IEF, suggest that the dynamics of Hg-DOC complexes may play a dominant role