Reservoir Stratification Affects Methylmercury Levels in River Water, Plankton, and Fish Downstream from Balbina Hydroelectric Dam, Amazonas, Brazil

Abstract

The river downstream from a dam can be more contaminated by mercury than the reservoir itself. However, it is not clear how far the contamination occurs downstream. We investigated the seasonal variation of methylmercury levels in the Balbina reservoir and how they correlated with the levels encountered downstream from the dam. Water, plankton, and fishes were collected upstream and at sites between 0.5 and 250 km downstream from the dam during four expeditions in 2011 and 2012. Variations in thermal stratification of the reservoir influenced the methylmercury levels in the reservoir and in the river downstream. Uniform depth distributions of methylmercury and oxygen encountered in the poorly stratified reservoir during the rainy season collections coincided with uniformly low methylmercury levels along the river downstream from the dam. During dry season collections, the reservoir was strongly stratified, and anoxic hypolimnion water with high methylmercury levels was exported downstream. Methylmercury levels declined gradually to 200 km downstream. In general, the methylmercury levels in plankton and fishes downstream from the dam were higher than those upstream. Higher methylmercury levels observed 200-250 km downstream from the dam during flooding season campaigns may reflect the greater inflow from tributaries and flooding of natural wetlands that occurred at this time.

Spatial and temporal patterns of mercury concentrations in freshwater fish across the Western United States and Canada

“…Similarly, rivers downstream of dams may also receive elevated aqueous MeHg that can be rapidly incorporated into the base of riverine food webs. In fact, elevated MeHg signals have been detected as far as 250 km downstream from dams (Kasper et al, 2014;Schetagne et al, 2000).…”

Large‐scale degradation of Amazonian freshwater ecosystems

“…The anoxic conditions commonly found in dam reservoirs increase natural levels of MeHg. For example, MeHg levels in water, plankton, and fish downstream of the Balbina Dam on the Uatumã River have been shown to be higher when reservoir water is stratified, because stratification fosters the anoxic conditions required for methylation (Kasper et al, 2014).…”

Thermal Structure‐Induced Biochemical Parameters Stratification in a Subtropical Dam Reservoir

“…The constructed reservoirs have multifunctions such as agricultural irrigation or water supply for a downstream area (ܸcler et al, 2015), therefore, the water quality of the reservoir/lake is of great importance, as its characterization is closely linked to the safety of the discharged waters. For instance, studies on the ecological effect of manmade reservoirs on downstream rivers have proven that impounded waters greatly affected the water quality and ecosystem of downstream waters (Kasper et al, 2014;Yang et al, 2015). It was also found that reservoirs contributed mercury to the downstream river, which resulted in greater methylmercury (MeHg) transport downstream from the dam (Muresan et al, 2008).…”

“…Thermal stratification is a common phenomenon in deep lakes creating a well-mixed epilimnion, a thermocline (with a sharp temperature gradient), and a homogeneous hypolimnion, and has received special attention (Rimmer et al, 2011;Santolaria et al, 2015;Xing et al, 2014;). Thermal stratification can inhibit the transport of material between the top and bottom of the water column (Moreno-Ostos et al, 2008), which may affect a variety of water properties including physical properties (Cox et al, 1998), dissolved oxygen concentration (Ito and Momii, 2015), eutrophication status (Becker et al, 2010;Xiao et al, 2011), and environmental toxins distribution (Kasper et al, 2014;Perron et al, 2014) as well as microbial communities .…”