That finding is important in part because scientists expect mercury levels at intermediate depths (660 to 3,300 feet) in the North Pacific to rise in coming decades; one estimate calls for a doubling by mid-century. At the same time, oxygen-depleted regions called oxygen minimum zones, which typically occur at depths greater than 1,300 feet, are expanding in oceans worldwide, and human-caused climate change is expected to accelerate that process.
The work by Blum and his colleagues suggests that if these two trends unfold as expected, conditions will favor increased production of methylmercury by microbes known as anaerobic bacteria, which will increase the threat to the North Pacific fisheries, the world's most important source of seafood.
"The implication is that predictions for increased mercury in deeper water will result in higher levels in fish," said Blum, a professor in the Department of Earth and Environmental Sciences.
In their study, the researchers analyzed tissue samples from nine species of marine fish that feed at different depths in a region near Hawaii called the North Pacific Subtropical Gyre. The work combined biogeochemistry with direct marine ecology observations.
Blum led the effort to very precisely measure the ratios of the stable isotopes of mercury, relying on techniques his lab has developed to take advantage of a natural phenomenon called isotopic fractionation. Popp led the Hawaii group that sampled fish at various depths, measured the total amount of mercury in their muscle tissues, and determined their position in the marine food web.
Together, the researchers showed how and where methylation occurs in the open ocean
|Contact: Jim Erickson|
University of Michigan