THE high level of the nerve toxin methylmercury in the Arctic is a result of global warming and the melting of sea ice, Harvard University researchers have established.
Methylmercury causes damage to the central nervous system, particularly in unborn babies.
Researchers said when fresh water from the melting Arctic sea ice met the salty ocean, the water deeper down had a higher salinity than that closer to the surface.
This stratification of salinity allowed “fluffy organic matter”, that would normally sink to the bottom of the sea, to stay in one place in the water. They referred to this as “neutral buoyancy”: the matter could not float up or down.
This layer is called “marine snow”. Bacteria stuck in this zone perform a complex chemical process that turns naturally occurring mercury into “a deadly and readily accumulated methylmercury”.
Zooplankton are attracted to this layer of marine snow and go on a feeding frenzy that can last for several weeks.
During this period, the methylmercury produced by the bacteria is taken up by the zooplankton and other organisms which absorb methylmercury from the surrounding water into their tissues.
This toxin accumulates up the food web, with the toxin becoming more concentrated as it moves higher up the food web.
The Harvard School of Public Health has established that the flooding of land for a hydroelectrical dam in an estuarine fjord in Canada will have the same effect.
A large area of land upstream of the fjord, called Lake Melville, will be flooded in 2017.
The researchers collected baseline data from Lake Melville and found more methylmercury in the water than their modelling could explain.
All the methylmercury from the rivers feeding into the lake and from the sediment at the bottom of the lake could not account for the high levels in the water.
They then found that the concentration of methylmercury in plankton peaked between one and 10m below the surface.
Further research showed that the answer for this lay in the eating habits of the plankton.
The researchers then looked at what would happen when methylmercury levels increased from the flooding of the reservoir upstream.
They collected soil samples and simulated flooding by covering the cores with river water.
Within five days the methylmercury levels had increased 14 times.
Most of the lake lies in an area governed by Inuit, who rely on the lake as a primary source of food.
The researchers said while the “clean” energy from hydroelectric power benefited the world, their research had highlighted that the costs would be born by the Inuit communities living next to the dam.