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Minnesota’s lakes are running low on oxygen

This story originally featured on Nexus Media News, a nonprofit climate change news service.

On a sweltering morning in July of 2021, thousands of dead fish washed onto the northeastern shores of Pokegama Lake, 60 miles north of Minneapolis. 

Deb Vermeersch, an official with the Minnesota Department of Natural Resources, was called in to investigate. 

When she arrived, she saw a quarter-mile stretch of sand covered with the rotting carcass of walleye and Northern pike, which thrive in deep, cool waters, as well as crappies, sunfish and suckers—all warm water dwellers. “They were already pretty decomposed because of the warm water,” Vermeersch recalls. 

Because so many different types of fish had died, Vermeersch and her colleagues knew it wasn’t a species-specific parasite, a common cause of fish kills. They zeroed in on the culprit: dangerously low oxygen levels.

Oxygen is disappearing in freshwater lakes at a rate nine times that of oceans due to a combination of pollution and warming waters, according to a study published in Nature earlier this year. Lakes like Pokegama are warming earlier in the spring and staying warm into autumn, fueling algae blooms, which thrive in warm waters, and threaten native fish.

Minnesota, with its 14,380 lakes and temperatures that have risen faster than the national average, is a unique laboratory for studying how climate change is affecting temperate-zone lakes around the world. The state sits at the intersection of four biomes––two distinct prairie ecosystems and two ecologically different forest systems. This means scientists here are able to study how lakes in different ecosystems fare on a warming planet, and look for ways to stave off the worst effects of climate change. 

“If you start losing oxygen, you start losing species.“

“What’s going on at the surface is that warmer water holds less oxygen than cool water,” says Lesley Knoll, a University of Minnesota limnologist and one of the authors of the Nature report. She says that longer, hotter summers are interfering with two key processes that have historically kept lakes’ oxygen levels in check: mixing and stratification. In temperate climates, water at the surface of lakes mixes with deep waters in the spring and the fall, when both layers are similar in temperature. As the surface water warms during the summer, the water forms distinct layers based on temperature––cool water at the bottom, warm at the top. This is known as stratification. In the fall, when the surface waters cool again, the water mixes for a second time, replenishing oxygen in deeper waters. But as climate change makes surface water warmer, and keeps it warmer for longer, that mixing doesn’t happen when it should.

“As you have that stronger stratification, the water in the deep part of the lake is cut off from the oxygen at the top part of the lake. If you start losing oxygen, you start losing species,” says Kevin Rose, a biologist at Rensselaer Polytechnic Institute in New York and a coauthor of the Nature study.

Knoll, Rose and a team of 43 other researchers studied 400 temperate lakes from around the world. They found that, on average, surface waters warmed by 7 degrees Fahrenheit and have lost roughly 5 percent of oxygen since 1980; deep waters, which haven’t warmed much, have still lost an average of almost 20 percent of their oxygen. (Thanks to the state’s long-held lake monitoring programs, almost a quarter the lakes in the study were in Minnesota.)

Warming lakes emit methane

Fish kills aren’t the only reason scientists are concerned about lakes losing oxygen. In extreme cases, when deep waters go completely void of oxygen, something else happens: Methane-emitting bacteria begin to thrive.

“As lakes warm, they will produce more methane and most of that has to do with stratification,” says James Cotner, a limnologist at the University of Minnesota.

Lakes normally emit carbon dioxide as a natural part of breaking down the trees, plants and animals that decay in them, but plants in and around fresh water also absorb it, making healthy lakes carbon sinks. 

Lakes have historically emitted methane, too––about 10 to 20 percent of the world’s emissions––but the prospect of them releasing more of the greenhouse gas has Cotner and his colleagues alarmed. Methane is about 25 times more potent than CO2 when it comes to trapping heat in Earth’s atmosphere.

Cotner is leading a team of researchers who are studying what conditions allow methane-emitting bacteria to prosper in lakes and how conservationists can respond. 

“The key questions are understanding how much and when carbon dioxide and methane are emitted from lakes, and what are the key variables that can tell how much will be emitted. Certainly, oxygen is a big part of that, but stratification and warming also plays a role,” says Cotner. 

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Pollution plays a big role

It’s not just longer, hotter summers that are causing lakes to lose their oxygen. Polluted agricultural runoff (pesticides and fertilizers) and logging have long plagued Minnesota’s lakes. It’s a problem that’s getting worse worldwide as climate change pushes agriculture further away from the equator and into new territory, says Heather Baird, an official with Minnesota’s Department of Natural Resources.

In northern Minnesota, potatoes now grow where pine forests have thrived for years. Phosphorus, a common fertilizer, now runs off from the soil into the region’s lakes, Baird says. Though small amounts of phosphorus occur naturally in lake ecosystems, too much of it feeds harmful algae blooms. 

Those blooms, which thrive in warm, nutrient-rich water, set off a chain of events that remove oxygen from deep lake waters.

“When phosphorus builds in lakes and creates algae blooms, those blooms eventually die. As they do, they sink. Deeper down, bacteria break down the algae, using up the remaining oxygen at those lower depths,” said Baird.

A quarter of Minnesota lakes now have phosphorus levels that are so high that the state advises against swimming, fishing or boating in them. Fueled by these nutrients, algae blooms take over, covering the lake in sometimes toxic residue that thrives in warm, nutrient-rich water, as was the case in Pokegama Lake earlier this year. The protists choke out aquatic life, especially fish that thrive in cold, deep waters. This is all exacerbated by warming air temperatures. 

The 75 percent rule

Researchers and conservationists in Minnesota are now studying the best ways to protect temperate-climate lakes from the worst effects of climate change. They have found that preserving 75 percent of deep-water lakes’ watersheds appear to keep fish stocks healthy. 

“Having a forested watershed helps keep better water quality by filtering out nutrients, which in turn can buffer against the impacts of climate change, to a point,” Knoll said. However, she added, as temperatures continue to rise, “that 75 percent may not be high enough anymore.” 

Knoll and state conservationists are focusing their research and efforts on deep, cool lakes that have a better chance of staying oxygenated than warmer, shallower lakes, like Pokegama.

July 2021, when the Pokegama Lake fish kill occurred, was the hottest month ever recorded on Earth. Parts of Minnesota were also experiencing the worst drought in 40 years, a trend some climatologists expect to persist in future summers. 

Vermeersch, the Minnesota fisheries supervisor, said it’s unclear what this will mean for the future of lakes like Pokegama. “Hopefully it’s not going to be a linear thing,” she said, adding that fish kills are “probably going to happen more often,” depending on a combination of factors. “When you get lakes like Pokegama that are shallow and already impaired, I think we are going to see more and more conditions like this.”

Correction (December 23, 2021): The story previously identified the wrong Pokegama Lake in Minnesota. The one that experienced the fish kill in July is 60 miles away from Minneapolis, not 140 miles away.

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How big did ancient millipedes get? Bigger than you’d like.

A fossil found in sandstone near the England-Scotland border contains the largest millipede ever found—and the discovery was completely by accident. 

In January 2018, Neil Davies, an Earth scientist at the University of Cambridge, had taken a group of PhD students on a “social trip” to Northumberland, England, where he had previously gone on holiday. The group noticed some rocks had crashed onto the beach where they were walking. One of those chunks happened to contain a paleontological surprise. 

“The way the boulder had fallen, it had cracked open and perfectly exposed the fossil, which one of our former PhD students happened to spot when walking by,” Davies said in a statement. “It was a complete fluke of a discovery.” 

Davies and his colleagues were at first unsure about what they had found. In May 2018, they extracted the fossil and brought it back to Cambridge for analysis. The specimen is just the third known example of an Arthropleura, a genus of giant millipede that roamed the Earth during the Carboniferous Period, between 359 million and 299 million years ago. But that’s not all: This Arthropleura fossil is also the oldest ever found, dating back to 326 million years ago, as well as the largest. It measures a whopping 30 inches by 14 inches. 

That suggests a pretty impressive beast. The millipede itself was likely around 8.5 feet long and nearly two feet wide, and probably weighed about 110 pounds. The team’s results were published in the Journal of the Geological Society.

[Related: This eyeless millipede shattered the record for most legs]

“Finding these giant millipede fossils is rare, because once they died, their bodies tend to disarticulate,” Davies told BBC. This particular specimen is likely just part of a molted exoskeleton, rather than a piece of a millipede’s corpse. Such a sparse fossil record means that these bugs largely remain a mystery. To this day, “we have not yet found a fossilised [millipede] head,” Davies added, “so it’s difficult to know everything about them.”

For example, researchers are unsure how many legs these millipedes had. Current best guesses are either 32 or 64—a paltry set compared to the maximum 1,300 legs recently found on some living millipedes. Scientists also don’t know what these giant bugs ate to sustain their lumbering bodies, though they seem to have thrived due to an abundance of resources and little competition. But later, in the Permian Period, they went extinct—either because of a changing climate or due to new reptile species outcompeting them for food. To uncover the mysteries still lurking in giant millipedes’ history, researchers will need more examples of them to fill out the fossil record.

The area of Northumberland where the fossil was found is mostly sandstone, which “is normally not brilliant for preserving fossils,” Davies told NPR. So “the fact that this has been preserved is, on the one hand, surprising. But it just suggests that actually there might be a lot more and similar things in places where people haven’t really looked for fossils before.”

The fossil will go on public display at Cambridge’s Sedgwick Museum in the New Year.

An ‘extinct’ Australian mouse has been thriving on a remote protected island

On the most westerly point of Australia lies Shark Bay, a remote island where sienna-streaked cliffs meet the sea and ancient, bulbous stromatolites speckle the coastline. Not only is this ecological wonderland a UNESCO world heritage site, but it also turns out to be one of the last habitats for the elusive Gould’s mouse—an Australian rodent thought to have gone extinct more than 150 years ago.  

But according to a study published this month in PNAS, the mouse has been around this whole time, living it up on several islands in Western Australia. The study, which was conducted by researchers at the Australian National University, used 184-year-old museum specimens to sequence the genomes of eight extinct Australian rodent species and then looked at 42 of their living relatives. 

“We compared the DNA of Gould’s mouse, thought to be extinct, to all living species of native rodents. What we found was that it was genetically indistinguishable from another living species, the Shark Bay mouse,” said Emily Roycroft, lead author of the study and postdoctoral fellow at the Australian National University. 

“Originally, we thought the Gould’s mouse only lived in New South Wales and Victoria, but after the results of our study, it’s clear that it once roamed across most of the Australian mainland.” 

Mapping out mice genomes

Australia has the highest recorded rate of mammalian extinction in the world. 

Since European colonization began in 1788, 34 land-roving mammal species have disappeared from the landscape. Of those, rodents have been disproportionately affected—they’ve comprised 41 percent of mammal extinctions since settlers arrived. 

“When we started the study, we set out to examine the relationships between extinct Australian rodents and living species, to determine the level of genetic diversity present before they became extinct,” Roycroft said. 

To do this, the evolutionary biologists extracted DNA from 87 museum specimens and mapped out the gnawing mammalians’ genomes. Understanding the genetic diversity of a population could help ecologists determine to what extent the arrival of Europeans contributed to their extinction, said Roycroft. 

One hypothesis for the mass disappearance of Australian rodents is they were already experiencing a decline due to loss of genetic diversity. Ecologists observed this when sequencing the genomes of two other Aussie animals: the endangered Tasmanian devil, which is now extinct on mainland Australia, and the fully-extinct Thylacine, a larger carnivorous marsupial also called the Tasmanian wolf or Tasmanian tiger. 

[Related: The endangered species list is full of ghosts]

Prior to colonization, these two species were experiencing a rapid reduction in genetic diversity, which indicates their numbers were already declining, leaving them more vulnerable to the threat of invaders. In other words, Europeans didn’t cause their extinction, only accelerated it. 

However the study found this was not the case for rodent extinction. In fact, there was no evidence for reduced genetic diversity in the extinct species prior to the late 18th century, which indicates that their populations were large and thriving at the time. Their rapid decline following the arrival of Europeans suggests genetic diversity doesn’t necessarily protect species from rapid, catastrophic extinction. 

“This shows how severe the impacts of European colonization have been, including introduced predators and land clearing, resulting in species that were relatively common becoming extinct in less than 200 years,” Roycroft said. 

Rodents’ role in the Australian ecosystem 

The rapid disappearance of Australia’s furry critters doesn’t just mean less scampers and squeaks—it could also have a devastating impact on almost all of the country’s ecosystems. Their presence is found in ecological niches ranging from arid deserts to the moist corners of the coastline. 

“Native rodents are important ecosystem engineers and play an integral role in Australian environments as consumers of plants, fungi, and invertebrates, and as a prey source for other native species,” Roycroft said. “The ongoing loss of native rodents from the Australian landscape has the potential to lead to broader ecosystem collapse.” 

[Related: Humans have become the biggest selection force in evolution]

Roycroft believes understanding the genome of extinct species can help inform conservation efforts for surviving species. 

“Our study shows just how much we can learn about species we’ve otherwise lost to extinction using data from museum specimens,” she said. “If we can generate this type of data from across all of Australia’s native species, not just rodents, we can learn more about the broader pattern and pace of extinctions.” 

Although it’s unlikely, future genome sequencing projects may uncover other living species once thought to have vanished from the face of the planet. But for now, we at least know the Gould’s mouse is still scuttling around the already-protected Shark Bay.

Grace Wade

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