Mercury speciation and Hg stable isotope ratios in sediments from Amazon floodplain lakes-Brazil

Hg concentrations and isotope ratios were measured to better characterize the mercury dynamics related to Hg cycling in Amazon floodplain lakes. We collected sediments, suspended particulate material (SPM), and plankton from floodplain lakes and compared then to sediments from rivers and soils of the central Amazon basin by measuring concentrations of total Hg (THg) and methylmercury (MMHg), and mercury isotope ratios. Concentrations of THg and MMHg in the lake sediments ranged to 69-109 ng g(-1) and 0.62-4.78 ng g(-1), respectively. A positive correlation between THg and MMHg in soils and sediments suggest that inorganic Hg in this system is highly bioavailable and readily converted to MMHg. delta Hg-202 values ranged from -1.40 parts per thousand to -0.89 parts per thousand and Delta Hg-199 from -0.34 parts per thousand to -0.18 parts per thousand. These values were comparable to those measured in riverine sediments (-2.14 parts per thousand to -1.23 parts per thousand and -0.51 parts per thousand to -0.05 parts per thousand), suggesting a contribution of riverine sediments to lake sediments, at least during the season of rising waters, during which lake samples were collected. SPM on the other hand was much elevated in THg (590-1066 ng g(-1)) and showed more negative delta Hg-202 (-3.00 to -2.15), similar to those found in soils (-2.99 parts per thousand to -2.17 parts per thousand), suggesting that Hg in SPM may originate from erosion of floodplain soils.