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Mercury in the oceans can have serious consequences for wildlife and human health, particularly in coastal areas where most of the fishing takes place. While models have focused on mercury that has been deposited from the atmosphere in assessing mercury sources in the oceans, a new study by Peter Raymond, professor of ecology at Yale School of the Environment, published in Nature Geoscience, shows that mercury is actually primarily sourced from rivers along the coasts.
Raymond states that it "rewires" the global mercury cycle. It was previously believed that the majority of mercury found in the ocean was from the atmosphere. However, the mercury then moved to coastal areas. It is now clear that the majority of mercury moves into the ocean from rivers and out into open seas. "Policymakers currently focus mainly on controlling atmospheric emissions and deposition of mercury, but the contribution of rivers mercury to coastal oceans are not well understood," Maodian Liu, a Raymond postdoctoral fellow, says. According to him, the new findings highlight the importance of limiting mercury's entry into rivers.
Researchers also looked at changes in riverine mercury outflow cycles and found that levels worldwide were highest in September and August. They also looked at which rivers contributed the most mercury to the rivers. Ten rivers account for half the riverine mercury. The top three are the Amazon River in Brazil, the Ganges of India and Bangladesh, as well as the Yangtze and Yangtze in China.
Raymond states that while other studies have estimated riverine mercury levels, they didn't provide the same degree of precision as Raymond's. He doesn’t know which rivers have the highest mercury levels and when these levels are highest. He says that the community wasn't sure about the role of rivers and that the new research helps to prove that rivers are the biggest source of oceanic mercury.
Atmospheric mercury is mainly caused by the burning of coal. It eventually ends up in the oceans and on landmasses. Mercury that rivers transport to the ocean can be from atmospheric mercury, which has been in soils. It can also be from other anthropogenic sources such as gold mining and natural geologic sources. Researchers also believe that mercury that has been dormant for long periods of times in soils may be more easily transported to the coasts by climate change.
Raymond suggests that future research could be focused on the effects of mercury in "hotspots", where concentrations are at their highest. Also, Raymond will examine the links to fisheries in these areas. Liu says that mercury exposure is most common in fish consumption. This improved understanding of how mercury enters the oceans and the sources of mercury will ultimately inform regulations that reduce mercury content in fish.
Continue reading Scientists raise alarm over unprecedented mercury accumulation in Pacific Ocean trenches
Maodian Liu and colleagues, Rivers as the greatest source of mercury to coast oceans worldwide, Nature Geoscience (2021). Information from Nature Geoscience Maodian Liu and colleagues, Rivers as the biggest source of mercury to coast oceans worldwide, (2021). DOI: 10.1038/s41561-021-00793-2
