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. Related Links: |
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