Salt marsh channel
Why Are We Concerned About Mercury?
Mercury is an element that naturally occurs in the earth’s crust. It can be found in different forms in the environment, including elemental mercury, inorganic mercury, and organic mercury. All forms of mercury are toxic, however, of these forms, organic mercury, or methylmercury, is more toxic and more easily incorporated into living organisms. Mercury is considered a global pollutant capable of spreading far beyond its source area due to its stability in the atmosphere. As a result of its prevalence in all environments, adverse exposure effects, and potential for human exposure, mercury is number three on the ATSDR 2019 Substance Priority List. Although there are natural sources of mercury emissions to the environment, the greatest source of 
mercury in the biosphere is currently from human activities. Mercury is released directly into waterbodies or into the air and subsequently makes its way into lakes and estuaries, where some of it settles to the bottom. Lake, river, and estuary bottoms serve as major sites for the transformation of inorganic mercury to organic methylmercury by bacteria.
Methylmercury biomagnifies in aquatic food chains, increasing in concentration in higher trophic level organisms. Unlike inorganic mercury in the atmosphere, methylmercury is a small organic compound that can be easily absorbed by living organisms. It is a potent neurotoxin and the easiest form for animals to store in their tissues. Methylmercury binds to proteins and easily crosses cell membranes, including the blood-brain barrier and the placenta. 
It harms the brain, affecting memory, understanding, and movement. Studies have shown that mercury exposure in humans can result in developmental delays in children, motor impairment, cardiovascular effects, and in acute cases, death. Its effects have been studied in fish, whales, seals, and seabirds. Even the arctic, which has no known sources of mercury emissions, harbors mercury-contaminated fish, and recent studies indicate whales who feed in the arctic have high levels of mercury in their tissue. Affected wildlife develop behaviors that ultimately reduce their survival and reproduction. Studies conducted on human populations have estimated that between 200,000 and 400,000 children in the United States alone are born each year with pre-natal exposure to methylmercury sufficient to put them at risk of neurologic impairment.
Mercury in fish
People are exposed to mercury mainly through eating fish and shellfish. The majority of the mercury in fish is the more toxic methylmercury form. The EPA has established a human health screening value of 0.3 parts per million (ppm) for mercury in fish tissue, and recommends eating fish with low concentrations of mercury. Fish caught in water with less than 1 part per trillion of mercury can contain harmful levels of methylmercury due to biomagnification through the food web. In some marine ecosystems, the concentration of methylmercury increases 10 million times as it makes its way up through the food web from microscopic algae to shark and tuna.
Mercury concentrations in fish from many lakes and rivers throughout the United States exceed the mercury levels that are cause for human and wildlife health concern. All 50 states and some U.S. territories and tribes have fish consumption advisories for mercury. In addition, all states on the Atlantic and Gulf of Mexico have coastal fish consumption advisories. Our Community Engagement Core, working with partners in Vermont, produced a Mercury Fish App for Vermont and other anglers to help them select low-mercury fish.
Although much of the scientific research on mercury in fish has focused on freshwater ecosystems, most people in the United States are exposed to mercury through seafood 
consumption. There are many questions to be answered about where the mercury in the fish that we eat comes from and what fish are safe to eat. Solutions to the complex problem of mercury pollution have been impeded by conflicting information on the sources, transport, and accumulation of mercury in the environment. Our program hopes to address these questions and therefore provide the scientific basis for solutions to this pressing environmental and human health issue.