The Deep Ocean Traps Mercury, Study Finds

"In a weird way, I'm a little bit optimistic about this," Lamborg told Live Science. "The total amount of mercury in the ocean is maybe not as big as we feared it was," he said. "If we take some steps to reduce mercury emissions, we may have a chance of seeing mercury concentrations in the ocean go down."
The problem with mercury is that even at such vanishingly small amounts, the metal can build up to toxic levels in animals over time. Mercury increases in concentration with each step up the food chain, and is found at its highest levels in long-lived, large predators such as swordfish and tuna. According to the U.S. Environmental Protection Agency, such fish have mercury levels in the parts per million, or one million million times more than the ocean's background mercury levels.
http://www.livescience.com/47222-deep-ocean-traps-mercury-pollution.html