Reservoir/Dam construction and microbial transformation of mercury into more toxic methylmercury

Let's start by saying metals are problematic pollutants. As such, microbial and environmental chemistry can transform metals into both more toxic or less toxic forms - but the underlying  element remains. Some such as hexavalent chromium (CrVI) can be converted by aerobic microbes into less toxic Cr III improves the environment. But there are other microbial transformations that can pose a problem. A recent article in the NY Times on the Muskrat Falls Dam in Canada shows how a green source of hydroelectricity can impact downstream fisheries. The article 

In the case of Muskrat Falls, construction of the dam builds a deep reservoir of water. At the reservoir bottom is organic debris and sediment that undergo slow anaerobic degradation. The reservoir also receives elemental and mercury II from natural sources such as the atmosphere (volcanoes, ocean volatilization) and underlying rocks. Additional mercury comes from human activities such as metal processing, mining, and mainly burning of coal. Most mercury inflow from the environment is in the "less" toxic but still toxic elemental form. When the dissolved mercury enters the anaerobic zone, microbes growing on the organic sediment convert elemental mercury into the more toxic methyl form that is more adsorbed by plants, fish, and all animals eating the fish. Adsorbed methyl mercury bioaccumulates and can cause mercury poisoning in populations depending upon contaminated animals as a food source.

It should also be noted that methyl mercury can be converted by UV radiation and aerobic microbial processes back into elemental form - which is part of the natural mercury cycle. It is the increased amount in mercury entering the cycle in an ecosystem that creates an abundance of methyl mercury.

Below is a great graphic of the mercury cycle from the USGS.

From https://www2.usgs.gov/themes/factsheet/146-00/