Mercury Accumulation in Fish Tends to Take Place at Deeper Depths, Study Finds
A few years ago we published work that showed that predatory fish that feed at deeper depths in the open ocean, like opah and swordfish, have higher mercury concentrations than those that feed in waters near the surface, like mahi-mahi and yellowfin tuna,” said co-author Brian Popp, professor of geology and geophysics at the University of Hawaii at Manoa. “We knew this was true, but we didn’t know why.”
“We knew that organic and inorganic mercury dissolved in seawater has a nutrient-like profile, with lower concentrations at the surface and higher concentrations at depth,” added Anela Choy, a PhD candidate in oceanography at the University of Hawaii at Manoa. “We saw it in the water, and we saw it in the fish. But we couldn’t explain the gradient we saw, nor did we know exactly where and how the bioavailable organic mercury was entering the marine food web.”
Bacteria in the oceans transform atmospheric mercury into the organic monomethylmercury form that can accumulate in fish. Big predatory fish have high levels of methylmercury due to the fact that they consume a lot of smaller, mercury-containing fish. Previously, researchers discovered that the depths at which a species eats is almost as significant as its position in the food chain in identifying how much methylmercury it has acquired.
The researchers discovered that chemical reactions induced by sunlight annihilate as much as 80 percent of monomethylmercury in the upper depths of the central North Pacific Ocean. They also came to the conclusion that a lot of monomethylmercury must be produced and enter marine food webs in oxygen-deficient, deeper waters.
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