Sources
Mercury is a global pollutant, released into the atmosphere primarily from human activities, such as burning fossil fuels, mining, and industrial activities, as well as some natural sources, such as volcanoes. Once in the atmosphere, mercury can be transported long distances, and eventually be re-deposited via rain and snow. Sources that release mercury to the atmosphere are known as non-point sources, because it is difficult to find the specific source once mercury has been transported through the atmosphere. Point sources are individual sites, such as a specific industrial plant, that release large amounts of mercury that can be traced back to the source. Recent regulation has reduced the amount of mercury entering the atmosphere from human sources, but in the Northeast, mercury has accumulated in forest soils and watersheds.
Mercury pollution (Lake Champlain Basin Program)
Mercury in the environment
When mercury is deposited in aquatic ecosystems, it can be transformed into methyl mercury. Methyl mercury is efficiently taken up by small organisms at the bottom of the food chain. When these organisms are eaten the predator stores the mercury accumulated over the lifetime of the prey, meaning that the predator has a higher concentration of mercury in its tissues than its prey. This process is known as biomagnification, and causes mercury concentrations to increase up the food chain, sometimes reaching very high concentrations in top predators. These high concentrations are sources of mercury exposure to humans and wildlife, and are the cause of fish consumption advisories for some species of fish and locations.
Fish and shellfish advisories (USEPA)
Mercury: From Source to Seafood, a ten-minute informational video created by the Dartmouth Toxic Metals Superfund Research Program
Why collect dragonflies?
Monitoring mercury in the environment is complex, involving measurements of multiple forms of mercury in the air and water. A simpler way to find out how much mercury is entering the food chain is to pick a single species or group of species, known as a biosentinel, and measure the mercury in their tissues. Those measurements can be used to draw conclusions about mercury levels in the surrounding ecosystem. Dragonfly larvae are a useful biosentinel because – among aquatic insects – they are high-level predators that are accumulating mercury as they feed, but they are far smaller and easier to collect and process than other relevant species like fish. They are resilient and are present in many different water bodies, so they can be used to compare different locations.