Mike McHenry's claim that mercury emissions cannot significantly increase mercury concentration in oceans because the huge volume of the oceans buffers such an increase does not agree with facts (C&EN, May 25, page 2).
According to a recent study published in Global Biogeochemical Cycles, scientists from Harvard University and the U.S. Geological Survey show that mercury levels in the Pacific Ocean are expected to rise by 50% in the next few decades from power plant emissions (Global Biogeochem. Cycles, DOI: 10.1029/2008GB003425).
The subject is more complicated than the mere increase in concentration. It is well-known that some biological systems selectively concentrate certain elements. In the ocean, mercury is first concentrated by bacteria and then moves up the food chain to phytoplankton to zooplankton to fish, being further concentrated at each step and also converted to the neurotoxic form, methylmercury, in the process. It is the result of this selective concentration process in the oceans that mercury may be harmful to humans who eat the fish.
On the other hand, University of Rochester scientists have studied populations who eat large quantities of high-mercury-level fish, mainly the people of the Seychelles Islands. The results to date are that children, the most susceptible group, show no neurological damage.
The letter on mercury emissions concludes that, because the oceans are so vast, dilution would prevent anthropogenic inputs from causing significant increases in oceanic mercury levels. This simple treatment ignores the relatively long time required for oceanic mixing. Mixing through the entire ocean requires about 1,000 years, giving ample time for local sources or atmospheric deposition to raise mercury concentrations in coastal or near-surface ecosystems without affecting the entire ocean.
Norman H. Cutsall