Ocean fish can contain high levels of mercury, even though mercury levels in the seawater around them are extraordinarily low. Now, scientists have an explanation for what's going on.
A new study in the Pacific Ocean suggests that algae at the water's surface absorb mercury from the atmosphere and then sink to mid-depths, where they decompose and release methylmercury, a highly toxic form of the metal that poisons both fish and the people who eat them.
Mercury concentrations have increased in the Pacific by 30 percent in the last 20 years, the study found. And if emissions continue to rise as expected, the scientists predict another 50 percent jump by 2050.
"We can now explain why large predatory fish in the open oceans have methylmercury in the first place," said Dave Krabbenhoft, a geochemist with the United States Geological Survey in Middleton, Wisc. "We don't have to scratch our heads anymore."
Mercury is a byproduct of coal combustion, industrial waste and other human activities. It is also a powerful neurotoxin that can cause developmental problems in babies and heart disease in adults, among other health woes. More than 90 percent of methylmercury that gets into people in the United States comes from ocean fish and shellfish, especially tuna.
Yet, compared to lakes, oceans haven't been studied much when it comes to mercury, and there hasn't been a clear explanation for how mercury gets from the air into ocean fish. One reason is that ocean waters contain such low amounts of methylmercury that scientists have struggled to collect reliable samples.
Krabbenhoft and colleagues were the first to employ new, highly sensitive techniques to get water samples at 16 sites in the Pacific Ocean, form Alaska to Hawaii. At each site, the researchers analyzed samples every 100 meters (328 feet) or so, down to a depth of 1,000 meters.
When the team graphed their results, they saw that methylmercury levels were highest at a depth where oxygen was being depleted the fastest. In this zone, microbes are busy decomposing dead algae, which sink to a level where water is dense enough to stop their descent.
That depth ranges from 300 meters (984 feet) to 800 meters (2,620 feet), depending on the site. Tuna swim to those depths, Krabbenhoft added, but not deeper.
"This is the first discovery of a specific depth in the ocean at any particular spot where the maximum amount of methylmercury is produced," said Krabbenhoft. "That's a really significant finding."
Previous theories proposed that oceanic mercury came either from volcanoes that sit tens of thousands of meters below the surface, or from river runoff.
Now, the most plausible theory for how mercury gets into fish in the middle of the ocean, Krabbenhoft said, is that algae absorb mercury at the surface, especially off the coast of Asia, where power plants burn lots of coal. Then, circulation patterns carry the algae in a counter-clockwise pattern across the Pacific to North America and back again. As the algae die, sink and decompose along the way, they release methylmercury, which works its way up the food chain into fish and the people who eat them.
Compared to previous research, the scientists reported an alarming rise in ocean mercury levels in just the last two decades. Their results appeared in the journal Global Biogeochemical Cycles.
"It was totally surprising that they were able to detect an increase in mercury concentration in the Pacific Ocean," said Vincent St. Louis, a biogeochemist at the University of Alberta. "That's a big body of water."
By clearly linking human-produced emissions with rising mercury levels in fish, St. Louis added, the work might also help convince policy-makers that there's a problem with allowing those emissions to rise unchecked.