"Ocean mercury on the increase"

From Nature:

Mercury levels in the Pacific Ocean are rising, a new study suggests.1 The increase may mean that more methylmercury, a human neurotoxin formed when mercury is methylated by microbes, accumulates in marine fish such as tuna.

The research comes as researchers and policymakers, who have tended to focus on atmospheric concentrations of the element, are looking for a fuller picture of the mercury cycle. US guidelines on methylmercury in fish are currently under review.

It remains unclear exactly how atmospheric mercury — whether dumped directly into oceans or carried there through rivers or coastal deposits — is methylated and eventually taken up by fish, which are a major source of human exposure to methylmercury. But the new data, collected by Elsie Sunderland of Harvard University and colleagues, also provide a possible mechanism for mercury methylation within the ocean.

The researchers collected samples from the eastern North Pacific, an area also monitored by research cruises in 1987 and 2002.2 They estimate that methylated mercury accounts for as much as 29% of all mercury in subsurface ocean waters, with lower concentrations occurring in deeper water masses. The group's modelling indicates that atmospheric deposition of mercury could lead to a doubling of the total ocean mercury concentrations recorded in the mid-1990s by 2050.

Sunderland's team also found a relationship between levels of methylated mercury and organic carbon. Particles of organic carbon from phytoplankton or other sources may provide surfaces on which microbes could methylate mercury in the ocean, the researchers suggest. That methylated mercury could then be released back into the water.

"We don't have a causal mechanism yet, but it does seem to be linked to the biological pump in the ocean," says Sunderland. Previous findings in the southern and equatorial Pacific, she adds, observed similar high methylmercury concentrations where biological activity was highest. That connection has implications for climate change and the mercury cycle: warmer, more productive oceans, with more phytoplankton and more fish, might increase the amount of methylated mercury that ends up on human plates.

The researchers also hypothesize that waters in the western Pacific could be picking up mercury deposited from increasing atmospheric emissions in Asia, and then moving to the northeast Pacific. The ocean may only now be responding to higher mercury loads from past atmospheric deposition, Sunderland says...