The U.S. Geological Survey has taken a big step toward answering long-standing questions about mercury in the oceans, with the release of a landmark study pointing to the role of human activities in releasing the contaminant and changing the makeup of the North Pacific.
The study opened the door to several key remaining questions, including whether different oceans absorb mercury differently and whether more of the metal in the water leads to increased levels of methylmercury -- mercury's highly toxic form -- in marine life.
The USGS study tested a long-held hypothesis that methylmercury originates 30,000 feet below the sea surface. The problem with that theory is that mercury probably would not be affecting seafood because harvested fish don't swim anywhere near those depths, USGS scientist and study co-author David Krabbenhoft said.
So Krabbenhoft and his team tried to determine if they could find methylmercury sources in the open ocean, away from continental margins but not at the bottom of the sea. They sampled Pacific Ocean water from 16 sites between Honolulu and Kodiak, Alaska. And they also built a computer simulation linking atmospheric mercury emissions, transport and deposition of mercury with an ocean circulation model.
Their sampling showed 2006 mercury levels were approximately 30 percent greater than mid-1990s levels.
When they first saw the increase, the researchers hypothesized that mercury emitted on land by the combustion of fossil fuels and wastes falls over the ocean. But that theory did not hold up to modeling, because the airborne emissions did not travel far enough away to fall over the middle Pacific, Krabbenhoft said.
The new theory focuses on algae. Those microscopic plants bloom in sunlit waters near the surface, die, then "rain" to greater water depths. The algae there are decomposed by bacteria and produce methylmercury. Methylated waters are then carried into the open ocean.
If that theory is true, it means human activities are in fact changing mercury concentrations in the ocean. And that, in turn, could be affecting fish, researchers say.
"Temporal trends in concentrations in the ocean could really substantially affect fish harvests from the area," said Elsie Sunderland, a Harvard University professor and a member of the USGS research team. "A lot of commercially important fish catches come from the region."
The scientists believe that mercury deposited along the Asian coast was then carried to the middle of the Pacific Ocean by currents over about three years.
"There has been a relatively recent -- within the last 10 to 15 years -- dramatic increase in mercury emissions from Asian countries that could -- and we believe does -- result in significant increases in mercury concentrations in ocean waters near the western Pacific," Krabbenhoft said in an interview.
This time frame is one of the most important findings for Canadian researcher Vincent St. Louis, a professor at the University of Alberta who was not involved with the study, because it suggests that human activities are actually capable of changing the makeup of a massive body of water in a short time.
"It's important that in huge bodies of water, they were able to detect that change," St. Louis said.
Based on computer models, the scientists predict an additional 50 percent increase in mercury in the Pacific Ocean by 2050 if emission rates continue as projected.
'Study deserves hard scrutiny'
Mary Anne Hansan, a spokeswoman for the National Fisheries Institute, said in an e-mail that it is too soon to begin drawing conclusions about mercury in fish from the study's findings.
"This study deserves hard scrutiny, especially because existing, peer-reviewed research shows no mercury increase in oceangoing fish over the last 30 years," Hansan said. "What's more, the authors of the study concede that they did not test levels of mercury absorption in fish, which renders any conclusions or forecasts about seafood incomplete and irresponsible."
The scientists agree it is too soon to make a direct link between their findings of increased methylmercury in the ocean and levels found in fish tissue. An important follow-up, they say, will try to determine whether or not there is an observed link between methylmercury levels in the ocean and in the fish that swim there.
What is most important now, Krabbenhoft said, is that the study suggests for the first time a probable link between methylmercury in the open ocean and human activity.
"We actually have a methylmercury source now, and we have a direct link to recent emissions," he said.
Comparing water bodies
Scientists also have to learn about the role nutrients within a specific ocean body play on methylmercury production. The North Pacific, where the USGS research was conducted, is relatively nutrient-rich, Krabbenhoft said. By contrast, the Southern Ocean -- between the southern tip of Africa and Australia -- is nutrient-poor.
So the team's oceanographer has for the past two months been cruising the Southern Ocean, collecting data samples for analysis.
The reigning hypothesis is that the Southern Ocean will not have the necessary nutrients to support activities such as algae production, Krabbenhoft said, which could influence which fish contain the most amounts of methylmercury.
"The idea that mercury -- and methylmercury concentrations in particular -- could be different among ocean bodies is brand-new and needs to be explored to see if we can relate fish mercury concentrations seen in the Pacific, for example, to those seen in more nutrient-poor waters," he said.
There are also plans to collect data from the Indian Ocean, which is relatively nutrient-rich, where the scientists expect to see similar results to those from the North Pacific.
Lakes and other fresh water bodies are more nutrient-rich than the oceans. It is also easier to gather data, so scientists have known for some time that mercury deposited from the atmosphere to freshwater ecosystems can be transformed into methylmercury. St. Louis, the University of Alberta scientist, said he believes the conversion of mercury to methylmercury is probably different in freshwater than in ocean water, which is another research avenue that should be explored.
Links to fish
Those additional findings could help shed light on which marine fish are relatively mercury-free and which are most susceptible to increased human emissions, the researchers note.
Uncovering the answers is compounded by the difficulties of gathering data in the open ocean and by the complex migration patterns of some fish, which can cross multiple water bodies.
At Harvard, Sunderland is working with a group to develop computer modeling to assess anthropogenic mercury emissions to concentrations that are naturally occurring. Her group is building an ocean simulation with atmospheric models and incorporating methylmercury levels to try to understand how changes in pollution will affect mercury levels globally.
Scientists say such research is critical because ocean fish and shellfish account for more than 90 percent of human methylmercury exposure in the United States. Tuna harvested in the Pacific Ocean accounts for 40 percent of that total exposure, according to a 2007 study by Harvard's Sunderland.
What's next for Congress?
On Capitol Hill, some lawmakers are hoping the USGS study and others can generate momentum for curbing industrial air pollution. Senate Clean Air Subcommittee Chairman Tom Carper (D-Del.) hopes to have a hearing next month addressing sulfur, nitrogen and mercury, according to a spokeswoman.
The benefit of moving on those three pollutants is that technology currently exists to curb them, Krabbenhoft said.
"We can and have already developed the technological capabilities that allow us to essentially trap a great deal of mercury," Krabbenhoft said. "The distinct difference between greenhouse gas releases is that we already have the technology in place to get greater than 90 percent of mercury contained."