A study intentionally polluting a lake to measure mercury accumulation in organisms from tiny zooplankton to large fish has revealed a surprisingly rapid recovery following pollution cessation. The study, published in Nature by 26 Canadian and US scientists, was led by Paul Blanchfield with Fisheries and Oceans Canada, the International Institute for Sustainable Development (IISD) Experimental Lakes Area (Winnipeg, MB), and Queen’s University (Kingston, Ontario).

Mercury occurs naturally in the earth’s crust. However, when emitted into the air through mining and burning fossil fuels, it precipitates onto land and waterways, where microorganisms convert it to more toxic and biologically active methylmercury. A compound that accumulates in tissues over time, methylmercury biomagnifies up the food chain as big fish eat little ones, creating a health hazard for fish-eating humans.

Following air pollution over centuries, a legacy environmental burden of methylmercury has accumulated despite tightening of emissions controls internationally spurred by the Minamata Convention. Exposure from that legacy burden has made it difficult to know – if we stop mercury pollution, how quickly will ecosystems recover? That was the question investigated at the IISD-Experimental Lakes Area, a remote boreal region of northwestern Ontario renowned for manipulating whole lakes.

The team added inorganic mercury each year for seven years (2001-2007) to lake number 658 and its surrounding watershed. Mercury additions over seven years elevated levels to those similar to more polluted regions. They could precisely track mercury flow through the ecosystem during and after additions because their experimental deposits to the lake, wetland, and upland areas had used unique isotopic signatures (spikes). To the lake, from the back of a boat, they added 202Hg. Using a crop duster, they added 198Hg to wetlands and 200Hg to uplands adjacent. After seven years of application, they stopped adding spiked mercury, then measured the amounts of mercury in water, surface sediments, zooplankton, aquatic invertebrates, perch, shiners, whitefish and pike for another eight years (2008-2015).

Most of the impacts came from the spiked MeHg added directly to the lake.  Additions to the adjacent watershed mostly remained there. After mercury additions stopped, methylmercury concentrations declined quickly. Within the first three years of the “recovery” phase, the spiked MeHg had decreased significantly in sediments, zooplankton, midge larvae (genus Chaoborus), and water. By eight years later, spiked mercury levels had declined by 76% in the pike population and 38% in the longer-lived whitefish population.

Toxicologists often sacrifice animals for analysis. But removing fish from small populations impacts behaviour and survival dynamics for those left behind, so this study avoided removals by taking repeated annual muscle biopsies of tagged pike and whitefish. These tiny muscle plugs enabled examination of mercury levels in individual fish over time for comparison with population-level impacts.

Spiked mercury burdens in individual pike continued increasing in the early recovery phase with little to no loss 6–8 years after additions ceased, highlighting prolonged retention of MeHg in tissue. However, because younger fish gradually replace older fish in healthy populations, there was swift recovery of the pike population as a whole, whose average spiked MeHg burden was reduced by 50% in less than five years.

Interestingly, says Blanchfield, “northern pike, which is the top predator, had concentrations of the isotope twice as much as the whitefish, but its [population level] loss of mercury was actually twice as fast as the whitefish because they are a much shorter-lived species.” Overall, the results were striking, says Blanchfield. He did not expect to see such dramatic and speedy ecosystem reductions in spiked mercury levels.

Aquatic toxicologist Roxane Karimi, a research scientist at Stony Brook University (Long Island, New York) who was not involved in the study, applauds the innovative research as answering an important question: when you reduce mercury emissions, do fish mercury levels decrease, how quickly, and by how much? Karimi, who has studied mercury ecotoxicology for two decades, says, “If we turn off the spigot of mercury emissions… you’d expect mercury levels to decrease, eventually.” However, the process by which mercury transforms into methylmercury and then biomagnifies up the food chain is a complex multi-step process, she explains. “So the length of time it takes fish mercury levels to go down was an open question.”

Stressing that the study is significant and good news, Karimi says, “We’ve run out of excuses to not curb mercury emissions. We know it will make a difference.”

Blanchfield concurs. “The broader implications are that policies that reduce the amount of mercury coming into lakes and ecosystems will be really beneficial.”