In Switzerland, near the shore of small Lake Murten, lies an ancient Roman city called Aventicum. Today, apart from the city’s intact amphitheater, which occasionally hosts rock concerts, Aventicum is mostly ruins. But in the early part of the first millennium, at the height of the Roman Empire, the city was a bustling place home to about 20,000 people, and archaeological evidence shows that the Romans who lived there cleared the landscape around the city for farming and to expand their city.
As the landscape changed, something else happened: Lake Murten became polluted, according to new research published in Earth and Planetary Science Letters. Nutrients eroded from the newly churned land found their way into the lake, where they caused the lake to become devoid of oxygen—something that can suffocate creatures living in a lake.
Today humans pollute many of the planet’s lakes in much the same way. One example is Lake Erie in North America, where runoff carrying nutrients from fertilizers used in agriculture flow into and pollute the lake, causing, like with Lake Murten so long ago, the waters to become oxygen starved.
“Anthropogenic soil erosion is a problem of global concern,” said Mischa Haas, a geologist at the Swiss Federal Institute of Aquatic Science and Technology who led the research into Lake Murten. The work, he notes, reminds us that anthropogenic influences on the environment did not begin with the industrial era.
The work also offers something else: clues to how long it can take for a lake polluted by nutrients to recover and to return to a natural state. Pollution by the Romans in Lake Murten continued right up to the collapse of the Roman Empire, which began around 300 CE. As the empire collapsed, so did Aventicum, and the pollution of Lake Murten stopped.
The results show that the lake did not quickly bounce back to natural conditions. “We found that it took centuries after severe human land use for the recovery of the lake environment to happen,” Haas said.
The erosion of soils can transport nutrients like phosphorus and nitrogen to freshwater lakes. These nutrients are key foodstuffs for primary producers like algae, and when there is an influx of these nutrients into a lake, algal populations can explode. This process, called eutrophication, was on brilliant display in Lake Erie in 2014, when an algal bloom visible from space turned the lake water green and made the tap water in the city of Toledo, Ohio, unsafe to drink for days.
When the algae that create such blooms die, they in turn become food for a lake’s bacteria. These bacteria decompose the algae, but they also consume oxygen, and when there is a lot of dead algae for bacteria to eat, there will be a lot of bacteria breathing oxygen. The result, explained Haas, is oxygen-deficient waters.
Without oxygen, organisms that live in a lake—especially in a lake’s bottommost waters—vanish, Haas explained. These creatures include things like worms and crustaceans, which, when alive and wriggling, dig into and churn lake sediments about. But when they suffocate because of a lack of oxygen, the lake sediment remains undisturbed. This means sedimentary structures known as varves—alternating layers of sediment that, like tree rings, deposit on an annual basis at the bottom of a lake—remain untouched.
To peer into the history of Lake Murten, Haas and his team studied a sedimentary core taken from the bottom of the lake and, using radiocarbon dating, dated the different sections of the core. What they found is that between about years 0 and 170 CE, varves occur throughout the core. To Haas and his team, the presence of varves points toward a deoxygenated—and thus eutrophic—lake.
And back up on land, the ruins of Aventicum appear to support what the lake sediment core data reveal. Archaeologists note that a wall surrounding the city likely took about 100,000 trees to build. According to Hans Paerl, an aquatic ecologist at the University of North Carolina at Chapel Hill’s Institute of Marine Sciences who was not involved in the new work, such large-scale deforestation would have caused nutrient-rich soils, unanchored by trees, to sluice into the lake. Those soils would have contained more than enough nutrients to flood into Lake Murten, leading to large-scale eutrophication. “It probably enhanced erosion tremendously,” he said.
But by 300 CE, as Aventicum diminished along with the Roman Empire, varves start to disappear in the sediment core. This, Haas explained, is the signal that the lake’s ecosystem had begun to recover.
A Layered Puzzle
According to the team’s findings, it took about 50 years for bottom-dwelling organisms in Lake Murten to bounce back, seen by the disappearance of varves. But the team also discovered something a bit off in their radiocarbon dates: A chunk of the varves seen during the time of eutrophication had carbon dates that were much older than dates from layers below them. In some cases, the carbon dates were hundreds of thousands of years older than expected.
Haas explained that the sediments had likely undergone “preaging,” meaning that older ages calculated through carbon dating of varves likely came from large amounts of eroded soils on land. The carbon in those soils, he explained, was much older than the remains preserved in the lake sediment.
Lake sediments moved back to their natural accumulation only 300 years after the fall of Aventicum, the team notes in the paper. In other words, carbon—and thus other nutrients—from eroded soils stopped infiltrating lake sediments en masse 300 years after Aventicum dwindled, implying that the vegetation following deforestation had finally regrown to hold soils in place.
The history of Lake Murten reveals that even once a civilization collapses, it takes centuries for lake conditions to recover. What lessons does this history hold for modern lakes? In addition to Lake Erie, there are many freshwater lakes—like Lake Okeechobee in Florida and the Caspian Sea—burdened with pollution related to heavy, human-driven nutrient influxes.
Since our modern civilization doesn’t seem to be going anywhere anytime soon, the only surefire way to help these lakes recover is to mitigate those influxes, Paerl explained. “Reducing nutrient input is key,” he said. “The quicker we deal with restricting excessive nutrient input, the better.”
—Lucas Joel ([email protected]), Freelance Journalist