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Everglades Marshes Contain Mercury That Can Poison Birds. But There's A Fix: More Water

Egrets nest in Everglades National Park, like these, and in marshes throughout the Everglades where mercury can hamper breeding.
Egrets nest in Everglades National Park, like these, and in marshes throughout the Everglades where mercury can hamper breeding.

South Florida may have retired most of its noxious smokestacks, but the pollution they produced, along with stacks still belching around the planet, continues to pose a threat to Everglades wading birds.

In a study encompassing more than 20 years of nesting data, University of Florida researchers found that mercury contained in the pollution can hamper breeding in egrets, by as much as half.

“That's huge,” said Peter Frederick, a UF wildlife ecologist and co-author of the study published in the journal Environmental Science & Technology. “Think about what would happen if the human population, half of us, didn't breed. My God, we'd be in negative population growth immediately.”

In recent years, nesting has been up, thanks in part to increased water in southern marshes. Two years ago, record rain led to an explosion of nesting that produced the most fertile nesting season since the 1940s.

Mercury pollution, Frederick said, is also down due to the elimination of many smokestacks and a change in the production of batteries, which accounted for about 70 percent of the mercury from incinerated trash seeping into the atmosphere.

Better farming practices have also helped clean up sulfate in water, which helps convert mercury to its toxic cousin, methylmercury.

“We have essentially brought the Everglades back into a realm that is much more like the 1950s,” Frederick said.

That’s good news. But mercury can persist in marshes, trapped in sediments. And it can get worse when conditions are overly dry.

Frederick said he first noticed a possible connection between mercury and nesting in the 1990s. When mercury levels were low, birds happily mated and built nests, he said. But when mercury rose, nesting dropped.

To figure out the connection, the research team collected birds to document the relationship in the lab. The team studied ibises and found a similar drop in lab conditions. But it turns out ibises, which nest in huge numbers in fewer places, make more difficult test subjects in the wild. Egrets, however, nest in smaller numbers in far more places.

“So we could use these as little monitoring stations,” Frederick said.

Scientists have also long known that wetlands can become hotspots for this toxic mercury because mercury gets methylated by the microbes in marshes, at the soil and water surface, he said.

“The Everglades is sort of a worst case scenario for turning mercury, the elemental mercury — the stuff in a thermometer — into something that’s really dangerous, which is methylated mercury,” he said.

The methylmercury gets absorbed by fish and then consumed by wading birds. The chemical can then interfere with the birds' hormone receptors and hamper breeding.

They weren't even doing courtship because they didn't have enough testosterone to do it,” Frederick said.

As they looked at the data spanning 21 years, Frederick said he and his colleagues noticed a pattern. In seasons when Everglades water conditions were good, when wet seasons left marshes plenty wet, nesting boomed.

They suspect two things happened: wet marshes kept more mercury trapped and produced more fish. So even though birds may eat fish contaminated with mercury, their overall health made it possible for them to tolerate the effects of the toxins, he said.

The findings also support an important management decision for marshes: Everglades restoration, Frederick said. The plan is now expected to cost $16 billion to undo damage from decades of flood control and return water flow to southern marshes to protect water supplies, revive Florida Bay, halt land loss and help Florida battle impacts from climate change.

Scientists now know revising water may also help contain the harm from mercury.

“Twenty years down the road from now, people are going to look back on this and say, 'Wow, the Everglades was like Yellowstone and the reintroduction of wolves,'” he said. “They're going to say ... 'They did a lot of studies. They were pretty sure they knew what water was going to do. And my gosh, it did.’”
Copyright 2020 WLRN 91.3 FM. To see more, visit WLRN 91.3 FM.

Birders photograph Great Egrets, wood storks and other wading birds foraging in Everglades National Park.
Everglades National Park /
Birders photograph Great Egrets, wood storks and other wading birds foraging in Everglades National Park.

The Comprehensive Everglades Restoration Plan, now expected to cost about $16 billion, aims to restore some of the historic flow of water to marshes.
Everglades National Park /
The Comprehensive Everglades Restoration Plan, now expected to cost about $16 billion, aims to restore some of the historic flow of water to marshes.