The influence of inundation and lake morphometry on the dynamics of mercury in the water and plankton in an Amazon floodplain lake

Seasonal flooding changes chemical and morphometric characteristics of the tropical floodplain lakes, affecting stratification which can influence mercury dynamics. We investigate the influence of flooding on the mercury dynamics in an Amazon floodplain lake. Three points on the lake, the Solimões River, and the connecting channel to the river were sampled along the annual flood pulse. During high-water, the lake was deep (12.7 m, on average) and the area farthest from the river was stratified with low oxygen and high methylmercury concentrations ([MeHg]) at depths below 7 m (0.37 ng l−1, on average). The two sites closest to the river were destratified and the [MeHg] was lower (0.077 ng l−1, on average along all depths). At low-water, the lake was shallower (3 m, on average) and destratified, with higher oxygen concentrations, and no difference in [MeHg] along the water column at all lake sites. Independent of season, the [MeHg] in the connecting channel was higher than those in the river. The [MeHg] in phytoplankton varied in response to changes in biomass, showing a biodilution effect. Variations in lake morphometry controlled the stratification and [MeHg] in the lake during the high-water. Floodplain lakes export hypolimnetic MeHg to their associated rivers during high-water.