Man is significantly contaminating oceans with mercury

Background:
Mercury is a toxic trace metal, creating a global health risk for humans and animals amplified by its bioaccumulation in organisms and its biomagnification along the food chain. Mercury emissions to the environment have increased significantly as a result of anthropogenic activities such as mining and fossil fuel combustion (see the graph below). Model calculations have estimated that anthropogenic emission of Mercury has increased the oceanmercury inventory by 36-1313 million moles since the 1500s.  Not only are such estimations orders of magnitude apart but also largely untested owing to a lack of historical data and natural archives for retrospective analysis.




The new study:
An international team of researchers now found there’s more mercury from human sources – mostly burning fossil fuels and mining for gold – than scientists had thought.

The study assessed inorganic mercury, which is the form mainly emitted both by natural and anthropogenic activities and gets converted in the oceans into the toxic methylmercury found in seafood.  Researchers collected thousands of water samples during eight research cruises to the North and South Atlantic and Pacific oceans between 2006 and 2011. To determine how mercury levels had changed over time, they compared samples of seawater from depths down to 5 kilometres with water closer to the surface, which had been more recently exposed to mercury pollution from land and air. The study is published this month in the journal Nature.

“Everywhere on the planet is contaminated with mercury to some extent,” said Carl Lamborg of the Woods Hole Oceanographic Institute in Massachusetts. His study found that mercury concentration in ocean water varied by depth, generally higher at the surface and mid-level depths than in deep water. But in the North Atlantic, high concentrations reached even deeper than 1000 meters.

The researcher estimate the total amount of anthropogenic mercury present in the global ocean to be 290 ± 80 million moles, with almost two-thirds residing in water shallower than a thousand metres. In general, mercury levels between the surface and 100 m deep were more than triple pre-industrial times levels, Lamborg said. Between 100 and 1000 m deep, they were about 150 percent greater than the levels from more than a century ago. But they were only about 10 percent higher at depths greater than 1000 m, except for the North Atlantic.

The study is important and will hopefully help scientists eventually understand the processes and the depths at which inorganic mercury species are converted into toxic methyl mercury and subsequently bioaccumulate in marine food webs.