22 July 2015
By Nanci Bompey
The amount of mercury in the Arctic Ocean is declining as the region rapidly warms and loses sea ice, according to a new study. The finding suggests that fish, marine mammals, polar bears, whales and humans in the Arctic might potentially be consuming lower amounts of toxic methylmercury as the region warms.
Mercury is released by human activities, like burning coal, and carried by winds to the Arctic, where it is removed from the atmosphere by precipitation or directly deposited into the ocean. Differences in the amount of mercury in the water and the air causes elemental mercury to move from the ocean to the atmosphere, a process called oceanic evasion. Mercury in the water can also be converted to methylmercury, a toxic substance affecting the brain and nervous system. Methylmercury is taken up by small aquatic organisms and eventually moves its way up the food chain.
The new study suggests that warmer springtime air temperatures in the Arctic are suppressing the deposition process that moves mercury down from the atmosphere to the ocean. Declining sea ice cover in the summer and fall leads more open ocean to be in contact with the air, which stimulates the transfer of mercury up from the water to the atmosphere in the late summer and early fall, according to the new study.
The result is that more mercury is escaping from the ocean to the atmosphere and there is less mercury in the Arctic Ocean than there was a decade ago – a trend that could continue as the region continues to warm and lose sea ice, the new study suggests.
“Because climate scenarios suggest that climate warming in future decades will increase surface air temperatures and decrease sea ice extent, we think it might drive substantial declines in Arctic Ocean mercury in the future,” said Long Chen, a doctoral candidate at the Ministry of Education Laboratory of Earth Surface Processes in the College of Urban and Environmental Sciences at Peking University in Beijing, China, and lead author of the new study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union.
In their study Chen and his colleagues set out to determine what was driving the varying trends in atmospheric mercury observed over the past decade. The amount of mercury in the atmosphere at northern mid-latitudes – including over the North Atlantic Ocean from Ireland to Canada — has been declining. But that hasn’t been the case in the Arctic, which has seen weaker or insignificant declines in atmospheric mercury since 2000.
The new study is the first to compare the two regions to work out the contributions of different factors, Chen said. The study’s authors used a model to simulate the observed mercury trends in the air and water from 2000 to 2009 to figure out which environmental variables, like air temperature, sea ice extent or wind speed, were contributing to these trends.
They found that less mercury is making its way from the ocean into the atmosphere at northern mid-latitudes than in past decades, driven by a decline in mercury in the waters of the North Atlantic Ocean. The new study does not address why mercury concentrations in the ocean are declining – a question that scientists are still trying to figure out. But the authors say the decline in atmospheric mercury at northern mid-latitudes is not likely a result of declining mercury emissions in North America and Europe as these may not be less than the increased emissions coming out of Asia.
The researchers found that the decline in atmospheric mercury seen at northern mid-latitudes extends to the Arctic. However, instead of seeing a decline in atmospheric mercury throughout the year, like at northern mid-latitudes, atmospheric mercury in the Arctic increases from April through May, and again from August through October – leading the researchers to conclude that these increases are likely a result of climatological variables. In the Arctic, rapid warming and declining sea ice extent are leading more mercury to move from the ocean to the air, according to the new study.
As a result, mercury in the waters of the Arctic Ocean declined by about 0.67 percent per year from 2000 to 2009, according to the study. If air temperatures continue to warm and the sea ice extent continues to decline in future decades, as some climate scenarios predict, the amount of mercury in the Arctic Ocean would continue to drop, the findings suggest.
The decline in mercury in the ocean might possibly lead to lower mercury levels in fish and other Arctic animals, but because the process of mercury accumulation in animals is complex, more studies are needed to understand if this is the case, according to Chen.
Steve Brooks, associate professor in the department of mechanical, aerospace and biomedical engineering at the University of Tennessee Space Institute in Tullahoma, said the study pulls together all of the known factors affecting the mercury cycle to get an idea of what is going on in the Arctic.
The reduction in ocean mercury predicted by the model should be seen as “good news”, said Brooks, who was not involved in the new study. It means that there is less mercury in the ocean that can wind up in the food chain, he said.
The ocean is where mercury gets into the zooplankton, then the fish, then the seals and the polar bears, and is magnified up the food chain to the point where it begins to damage the health of animals, Brooks explained. It gets to the point where polar bears in the Arctic have high enough mercury levels to interrupt their foraging and reproduction, he said.
“Mercury in the ocean is really where the rubber meets the road,” Brooks added.
— Nanci Bompey is the public information manager at AGU.