Day 99: Link shored up between atmospheric mercury, contaminated tuna

Date posted: May 3, 2009
Posted in: 100 Days of Science
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cutting-tunaCarbon dioxide, of course, isn’t the only gas we emit into the atmosphere. Methane, a highly effective greenhouse gas, belches from landfills. And about five percent of our gas emissions comprise mercury. 

We get it back: About 90 percent of human methylmercury exposure comes from ocean fish and shellfish — about 40 percent of that from tuna, a predator high in the ocean food chain. 

A new study by the U.S. Geological Survey has confirmed a fascinating (if a bit ominous) mercury cycle that connects the atmospheric source to our dinner tables.


Credit: U.S. Geological Survey

Credit: U.S. Geological Survey


The first part of the cycle is fairly intuitive: We emit mercury, though the burning of coal, oil and gas production and other means. It goes into the atmosphere and falls as rain. 

Scientists have also known for some time that mercury deposited from the atmosphere to freshwater ecosystems can be transformed (methylated) into a highly toxic form of mercury called methylmercury, given the right conditions. 

Data and modeling results from the new paper — which appears in the journal Global Biogeochemical Cycles — support the notion of a Mercury Methylation Cycle, in which much of the methylmercury in the open ocean is the result of biologically mediated transformation of mercury into methylmercury.

Most of the mercury originates from atmospheric fallout to the ocean surface and the subsequent transport of the mercury to greater ocean depths (650 to 2,200 feet, or 200 to 700 meters).

At these depths, naturally occurring bacteria decompose organic matter, mostly comprised of settling algae (commonly referred to as ocean rain) that are produced in the sunlit waters near the surface.

But the decomposition of organic matter also results in unintentional conversion of mercury to methylmercury, which is then passed up the food web through the bioaccumulation process, eventually to top predator fish like tuna.

Among the paper’s other findings:

Total mercury levels in the North Pacific Ocean water have risen about 30 percent over the last 20 years. The authors attribute the rise to increases in global mercury atmospheric emission rates, particularly from Asia. 

The authors project a 50 percent increase in Pacific Ocean mercury levels by the year 2050, based on published projections of increases in mercury emissions over the same timeframe.

The USGS, an admirably nonpartisan research organization, nevertheless makes extensive references to environmental groups trying to rein in mercury emissions. Yet another reason to be eager for alternative energy solutions that reduce our reliance on fossil fuels.

Sources: Press releases from both Eurekalert and the USGS (the USGS site has links to much more information). The new paper is available here.

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