The build-up of the neurotoxin methylmercury in Arctic lakes has been a longstanding mystery. Now, a new study has helped explain this compound’s concentrations in some of the world's most fragile environments. In the process, it has sounded a warning about high-latitude large dams.
Elemental mercury is poisonous enough, but its effects are most serious as methylmercury (CH3Hg+), since it crosses the blood-brain barrier. Unfortunately, methylmercury is building up at frightening rates in the high Arctic, with elephant seals concentrating the toxin in the most vulnerable coastal locations.
While the source of this mercury is known – a combination of human emissions, primarily from burning coal and natural sources – ecologists have been bewildered why this mercury is being converted to its most toxic form, and not sinking safely to the ocean floor.
So the announcement in the Proceedings of the National Academy of Sciences of an answer to this question is highly significant. The discovery was made as a result of an investigation into the impact of the Muskrat Falls hydroelectric dam on the estuarine fjord Lake Melville, and indicates there is a cost to such projects beyond what has previously been recognized.
The dam is set to be finished by 2017. Much of the affected area will lie within the Inuit autonomous region of Nunatsiavut. After an unsuccessful effort to challenge the dam, Nunatsiavut’s government asked Harvard's Dr. Elsie Sunderland to investigate its potential effects, including the increase in methylmercury after a dam was built higher up the Churchill River, which feeds Lake Melville.
Neither the federal nor provincial government considered Sunderland’s preliminary findings sufficient grounds to stop the dam, but her research has been scientifically fruitful. Moreover, with both administrations behind the dam up for election in the next three months, the timing of their release could prove electorally significant
When Sunderland tested Lake Melville’s waters, she was astonished. "We found more methylmercury in the water than our modeling could explain," said Sunderland’s colleague Dr. Amina Schartup. "All of the methylmercury from the rivers feeding into Lake Melville and from the sediment at the bottom of the lake couldn't account for the levels in the water. There was something else going on here." The highest concentrations were 1 to 10 meters (3.3 to 33 feet) beneath the surface, a pattern also observed in the Arctic Ocean.
Sunderland observed that this is the point where fluffy organic material gets trapped by strong salinity gradients, too buoyant to sink to the salty depths and yet too heavy to float to the surface. Zooplankton feeds on this material, concentrating any methylmercury in the process. "This system is incredibly efficient at accumulating methylmercury," said Schartup. Instead of mercury sinking to the seafloor, it builds up continuously, often to be consumed by fish that may then be caught for human consumption.
Even more worryingly, Sunderland found that when soil from the area becomes saturated, it starts releasing huge quantities of methylmercury. If the dam is built, huge areas will be drowned and Sunderland fears the methylmercury released will make its way to Lake Melville, causing concentrations to skyrocket and endangering indigenous people dependent on food from the lake.