Monomethylmercury sources in a tropical artificial reservoir

Abstract

The distribution and speciation of mercury (Hg) in the water column, the inputs (wet deposition and tributaries) and the outputs (atmospheric evasion and outlet) of an artificial partially anoxic tropical lake (Petit-Saut reservoir, French Guiana) were investigated on a seasonal basis in order to appraise the cycling and transformations of this metal. The total mercury (HgT) concentrations in the oxygenated epilimnetic waters averaged 5 ± 3 pmol L −1 in the unfiltered samples (HgTUNF) and 4 ± 2 pmol L −1 in the dissolved (HgTD) phase (<0.45 μm). On average, the monomethylmercury (MMHg) constituted 8%, 40% and 18% of the HgT in the dissolved phase, the particulate suspended matter and in the unfiltered samples, respectively. Covariant elevated concentrations of particulate MMHg and chlorophyll a in the epilimnion suggest that phytoplankton is an active component for the MMHg transfer in the lake. In the anoxic hypolimnion the HgTUNF averages 13 ± 6 pmol L −1 and the HgTD 8 ± 4 pmol L −1 . The averages of MMHgP and MMHgD in hypolimnetic waters were two and three times the corresponding values of the epilimnion, 170 ± 90 pmol g −1 and 0.9 ± 0.5 pmol L −1 , respectively. In the long dry and wet seasons, at the flooded forest and upstream dam sampling stations, the vertical profiles of MMHgD concentrations accounted for two distinct maxima: one just below the oxycline and the other near the benthic interface. Direct wet atmospheric deposition accounted for 14 moles yr −1