Inside the Mesocosm Facility at Umeå University, on the eastern shore of Sweden, columns of the Baltic Sea sit in black tanks. The tanks line the room’s perimeter, 16 feet tall and illuminated by lamps dangling above. The salty brew in each imitates the sea outside—miniature, semi-realistic ecosystems that allow scientists to study natural processes under controlled conditions.

Environmental chemist Erik Björn uses the mesocosm lab to examine the biological effects of an obscure consequence of climate change: an increase in the amount of soil and other organic material that washes from land into coastal water. A little more soil in the water may not seem like a big deal, but Björn suspected that these changes could increase the levels of toxic methylmercury, which bacteria naturally produce when they encounter industrial mercury in waterways. His experiment, which traced methylmercury through the bottom rungs of coastal food webs, was published today in Science Advances.

The study lays the groundwork for a better understanding of how climate change indirectly affects bird health. That’s because in study after study, methylmercury has been shown to poison birds and interfere with their reproduction. Scientists have found that mercury causes male Snow Petrels to neglect their eggs; alters White Ibis courtship behavior; and leads Common Loons to raise fewer chicks.

However, mercury is a tricky chemical to trace through a complex ecosystem. “We know what it does, that it’s bad, and [where] it comes from,” says Jennifer Provencher of Carleton University in Canada, who studies Arctic seabirds exposed to mercury. “But we know very little about how that interacts with other environmental burdens,” like global warming, ocean acidification, or pollution.

Erik Björn uses 16-foot tanks of seawater to study how climate change affects coastal food webs. Mattias Pettersson/So Jonsson

When Björn first saw the mesocosm lab, he realized the research possibilities they represented: He could use them to simulate an increase in organic matter, and then watch how they affected the food chain as far up as zooplankton. Unlike other experimental set-ups, mesocosms can hold the bacteria that create methylmercury (found in the seabed), and also the plankton communities that live in seawater. “They’re still representative of the natural environment, as opposed to just taking a bunch of lake sediment and putting it on your lab bench in a small container,” says Jeff Jeremiason, an environmental chemist at Gustavus Adolphus College with mercury monitoring experience.

The whole-ecosystem approach turned out to be key because he found that there wasn’t a big difference in how much methylmercury was produced—but there was a striking difference in how much entered the food web.

When Björn and his team added soil and organic matter to the mesocosm, phytoplankton took a real hit because sunlight couldn’t reach them through the muddied water. Without phytoplankton, the food chain shifted to one built on bacteria—a shift that mean that two-to-seven times more methylmercury made its way into plankton in the food chain. If this were to happen in the ocean, that plankton would then be consumed by fish, and bigger fish, and so on, up to seabirds and other top predators.

Each step up a food chain makes mercury more concentrated, which is why animals at the top are at particular risk of mercury poisoning. Moreover, disturbances in a food web can have unpredictable and sometimes negative consequences. “Even small, subtle changes can actually lead to much larger changes in the food chain,” Provencher says.

While some effects of mercury poisoning in birds are known, it's still an open question all the negative impacts mercury will have, and where those impacts will be the worst. Björn says that the organic matter influxes caused by climate change will be more localized than other issues, like rising temperatures and ocean acidification. “One thing that we can say about this process is that it will not be a global phenomenon; it will not happen everywhere,” Björn says. In general, birds and other animals that forage along coastlines and in lakes will face a greater risk of mercury poisoning than those that feed in the open ocean.

The study is a powerful reminder that climate change will have unforeseen consequences on the natural environment. And even as climate scientists model the big changes—like ice melt, sea level rise, and drought—smaller changes will also lead to unpredictable outcomes.

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