The U.S. Geological Survey issued a stream assessment report in November 2014. It is the first comprehensive look at methylmercury in streams within the United States. Although the Environmental Protection Agency (EPA) has conducted methylmercury assessments of water bodies in the past, their efforts have generally been concentrated on lakes. The Geological Survey report says one in four streams in the United States has fish with methylmercury concentrations exceeding EPA’s criteria for the protection of human health. EPA’s criteria for protecting human health are 0.3 parts per million methylmercury in fish tissue. The Geological Survey also said in their report that methylmercury in fish is the primary cause of fish consumption advisories for waterways.

During their study, the Geological Survey sampled 300 streams in the United States for methylmercury in water and methylmercury in fish. Large mouth bass were one of the fish they collected most for testing. Large mouth bass are what biologist call predator fish. Predator fish eat smaller fish as well as insects, frogs, snakes and other things that live in and on top of the water. From all the fishing shows you see on television, I guess it must be a very popular fish for sports fishermen. I don’t know how tasty it is to eat. But sports fisherman might want to note that in half the sites where the Geological Survey tested large mouth bass, these fish exceeded EPA’s methylmercury criteria for human health protection.

Now you might think: “Well, yeah that is probably in a bunch of streams near major urban centers.” Right? Well actually no. The Geological Survey found that in major contrast to other contaminants like lead, cadmium, arsenic and so on, methylmercury is highest in fish in undeveloped areas. The reason why methylmercury is highest in fish in rural areas is because of how methylmercury is formed. Mercury is converted into methylmercury via biotic degradation. In other words biota, like microbes and bacteria are taking up the mercury and converting it into methylmercury as they process organic material in the water. This conversion of mercury to methylmercury goes on wherever you have degrading organic material, but wetlands are kind of the hot bed of action for this process. Think about it, you have vast stretches of marsh grasses that die and then are replenished with new grass. These decaying grass mats can be really thick. You’d know this if, like me, you’ve ever waded into a wetland with some of those nutty guys who study coastal processes. Wetlands I can attest have deep organic muck. Wetlands also have something else these microorganisms like. They have very limited dissolved oxygen. These tiny little microbes reduce sulfate and low oxygen zones are where they live. So if an environment has lots of sulfate, then expect lots of these microorganisms and lots of methylmercury. Other environments that host these microorganisms include areas where water levels fluctuate, like in areas that are dammed or areas that flood a lot.

So any water basin with lots of wetlands is most likely to have higher methylmercury in fish. That is why most urban streams don’t have a lot of methylmercury. The wetlands in the urban environment have mostly been destroyed over the years. Let’s not let out a collective hurray for the lack of wetlands. Wetlands produce all sorts of other good ecosystem function, like filtering water and providing habitat and food for animals. It would be better to just keep the mercury from getting into the wetland. The only way to do that is to reduce the source of mercury. Most of the mercury in wetlands is being deposited there from the air and mercury gets in the air due to all sorts of human activities, for example burning coal in power plants.

One of the most noteworthy observations in the Geological Survey stream assessment was that the amount of actual inorganic mercury in a stream was not the driver for how much methylmercury there would be in fish. The Geological Survey looked at a number of streams in historic mining areas where mercury in the sediment and water is thousands of times greater than unmined areas. Because many of these streams have little organic material or wetlands, there is very little methylmercury in the fish. That is how important organic material and the microorganisms that degrade it are to the production of methylmercury.