NEW YORK (NYTIMES) - To scientists who study lakes and rivers, it seems humans have embarked on a huge unplanned experiment.
By burning fossil fuels, we have already raised the concentration of carbon dioxide in the atmosphere by 40 per cent, and we're on track to increase it by much more. Some of that gas may mix into the world's inland waters, and recent studies hint that this may have profound effects on the species that live in them.
"We're monkeying with the very chemical foundation of these ecosystems," said Emily H. Stanley, a limnologist (freshwater ecologist) at the University of Wisconsin-Madison. "But right now we don't know enough yet to know where we're going. To me, scientifically that's really interesting, and as a human a little bit frightening."
Scientists began taking continuous measurements of carbon dioxide in the atmosphere in the 1950s, and today they have more than six decades of consistent readings. In the 1980s, oceanographers followed suit, developing carbon dioxide sensors and deploying them across the planet.
Over the past three decades, they've chronicled a steady rise of carbon dioxide in seawater. The increasing concentration can harm marine life in many ways.
It lowers the pH of seawater, for one thing, making it more acidic and interfering with the chemistry that coral, for instance, use to build their calcium skeletons. Ocean acidification also thins the shells of oysters and other animals.
Many marine organisms rely on chemical changes in water to find food and avoid danger.
"Many fish are not able to detect their predators anymore," said Linda C. Weiss, an aquatic ecologist at Ruhr University Bochum in Germany. "They can even get more bold."
Weiss first came to appreciate the impact of ocean acidification in 2010, when she spent time at a marine research station in Australia. The experience left her wondering if lakes and rivers might face a similar threat.
Her first step was to look for historical data about carbon dioxide levels in fresh water. But a literature search brought her to a surprising conclusion. "I discovered there was no information," she said.
Traditionally, scientists who have studied inland waters have focused on different questions. They've been more concerned, for example, with sulfuric acid and other pollutants in acid rain, along with the impacts of runoff from farms and yards.
Now that researchers have grown concerned about carbon dioxide levels, they've been developing ways to reconstruct their history.
The level of carbon dioxide in a lake depends on such variables as its temperature and how much organic carbon it contains. If those factors have been tracked in the past, scientists can use them to get an estimate of a lake's carbon dioxide level, too.
Weiss and her colleagues used this method to figure out the carbon dioxide levels in four reservoirs in Germany from 1981 to 2015. They reported Thursday in the journal Current Biology that the amounts tripled in that time.
"We didn't really know what to expect," said Weiss. "But the speed of acidification we find is quite fast."
The researchers wondered what effects this fast rise in carbon dioxide might have on freshwater life in decades to come. So they ran experiments on the humble water flea.
These tiny, shrimplike creatures filter algae and microbes from water. They are devoured in turn by small fish, which are eaten by bigger fish. If rising carbon dioxide were to affect water fleas, Weiss reasoned, it could influence the entire lake ecosystem.
Water fleas use a bizarre but sophisticated defence to escape predators. They can sense chemicals given off by fish in their vicinity, and in response they make themselves harder to eat.
Some species grow a massive crest on their head, while others sprout spikes. Weiss and her colleagues found that high levels of carbon dioxide caused water fleas to make smaller crests and shorter spikes.
Rather than the acidity of the water, carbon dioxide itself seems to be affecting the water fleas. When the researchers lowered the pH with hydrochloric acid, the water fleas responded normally to predators.
Weiss hypothesised that carbon dioxide interferes with the nervous system of the water fleas, blunting their ability to look out for predators.
Caleb T. Hasler, a biologist at the University of Winnipeg, said that the new research addressed an unanswered question: the amounts of carbon dioxide that might harm freshwater life.
"This paper is really important because it starts to show where those levels might be," he said.
Hasler's own recent research hints that water fleas may not be the only freshwater animals to be altered by carbon dioxide. He and his colleagues studied minnows swimming in water rich with carbon dioxide and found that the fish don't respond as quickly to alarm signals released by other minnows.
In another study, the team studied two species of mussels. One species relaxed its muscles in water high in carbon dioxide, so that its shell gaped open. The other species clamped its shell shut, so that it could no longer filter food.
These sorts of changes may send ripples out across entire freshwater ecosystems.
Mussels are vital for filtering food and keeping water clear, for example. If water fleas do a worse job of escaping predators, their population may decline, leaving less food in the long run for fish.
But it's not certain that inland waters around the world are building up carbon dioxide at the rate that Weiss and her colleagues observed in the German reservoirs.
In November (2017), Stanley and her colleagues published a study of carbon dioxide levels in lakes in Wisconsin. Between 1986 and 2011, they detected no significant change at all.
The mismatch points to the complex chemistry varying from one lake to the next. While lakes and rivers all absorb carbon dioxide from the atmosphere, some also draw in the gas from surrounding soils.
The chemistry of some inland waters causes a lot of carbon dioxide to be converted into other compounds. Some lakes and streams may support a lot of underwater plants that take up the gas, for instance, while others may have microbes can release more of it.
Making matters even more complicated, the carbon dioxide levels in any particular body of freshwater can change drastically over time with swings in temperature and other conditions.
"You can have lakes where the carbon dioxide increases tenfold at night," said Hasler.
In decades to come, as carbon dioxide levels continue to climb in the atmosphere, Stanley speculated, the picture will only get more nuanced.
"I honestly don't know where we're going," said Stanley. "I'll probably put my money on increased variability from lake to lake. They're just going to be more extreme."
Weiss agreed that it wasn't possible to draw big lessons from the preliminary data. "I think this study we're publishing is like a door-opener," she said. "I hope there will be other scientists who will follow."