Freshwater Lake Acidification

"We're monkeying with the very chemical foundation of these ecosystems."
"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 being a little bit frightening."
"I'll probably put my money on increased variability from lake to lake. They're just going to be more extreme."
Emily H. Stanley, fresh-water ecologist, University of Wisconsin Madison

"Many fish are not able to detect their predators anymore. They can even get more bold."
"I discovered there was no information [on carbon dioxide levels in fresh water]."
"We didn’t really know what to expect. But the speed of acidification we find is quite fast."
"I think this study we’re publishing is like a door-opener. I hope there will be other scientists who will follow."
Linda C. Weiss, aquatic ecologist, Ruhr University Bochum, Germany

The Sorpe reservoir in northwest Germany, one of four freshwater reservoirs observed in a recent study that found that carbon dioxide absorbed in lakes, rivers and streams can affect entire ecosystems. Credit Mauritius Images GmbH/Alamy

It's not yet clear whether lake waters across the globe are building up carbon dioxide. A study was published in November by Dr. Stanley and colleagues setting out carbon dioxide levels in Wisconsin lakes. The study concluded that no significant change whatever was detected between the years 1986 and 2011. All lakes take in carbon dioxide from the atmosphere, some drawing it in from the soil around lakes and some lakes may have quite a lot of underwater plants that take up the gas, while others harbor microbes releasing more of the carbon dioxide.

Over  time, levels of carbon dioxide can undergo dramatic alterations, in any body of water. Time alone, over the passing decades will determine the level of its presence, as carbon dioxide continues to take its fuller presence in the atmosphere. As the levels progress upward, Dr. Stanley has speculated that the actual presence and what it fully portends will become more nuanced. What is certain at the present time is that very few ecologists have studied the presence of carbon dioxide levels in fresh water.

And that is because it has been the presence of sulfuric acid and allied pollutants that have gathered in freshwater lakes as a result of acid rain, including the environmental impacts of runoff from agricultural lands into lakes that have historically presented as a concern. Variables such as water temperature and the organic carbon contained in a lake also come into play in determining the presence and level of carbon dioxide in lakes.

From 1981 to 2015, Dr. Weiss and her colleagues determined the levels of carbon dioxide in four reservoirs in Germany. Their findings were published in the journal Current Biology, making it clear that amounts throughout that period of time had tripled. Back in the 1980s, oceanographers were continuously measuring carbon dioxide in seawater, chronicling a steady rise, lowering the pH of seawater, so it became more acidic, interfering with chemistry coral use for their calcium skeletons.

Chemical changes in water interfere with marine organisms' ability to find food and to avoid danger. The researchers, curious about the effects on freshwater life of the presence of carbon dioxide, ran experiments on water fleas. These are minuscule creatures filtering algae and microbes from water and they in turn are eaten by small fish, which in the natural order of nature's design, are then devoured by ever larger fish. Were the presence of carbon dioxide to affect the bottom of the food chain, the entire lake ecosystem could be influenced.

Nature has equipped water fleas with a unique capacity to make themselves more difficult to consume, in response to sensing chemicals in the water related to the presence of fish nearby. Some of the water fleas grow a crest on their heads, and others sprout self-defensive spikes, making them less palatable to the fish, and more difficult to swallow. High levels of carbon dioxide resulted in water fleas producing smaller crests and shorter spikes, Dr. Weiss discovered. She reasoned that the presence of carbon dioxide interferes with the water fleas' nervous system, decreasing their capacity to detect predators.

At top right, a water flea with its protective crest; at bottom right, a water flea with “neckteeth.” These defenses may be blunted by chemical changes in water. Credit Linda Weiss and Sina Becker

This new research addressing the question of how much carbon dioxide might harm freshwater life has impressed Caleb T. Hasler, a biologist at the University of Winnipeg in Manitoba. He and his colleagues studied minnows in water rich with carbon dioxide, discovering that the fish fail to respond as swiftly to alarm signals that other minnows release. His team studied two species of mussels, one which relaxed its muscles in high carbon dioxide presence, leaving its shell gaped open. While the other clamped its shell tight shut rendering it unable to filter food.

The filtering actions of mussels are responsible for maintaining clear water in a lake. Such changes as have been detected may in the final analysis impact upon entire ecosystems. "You can have lakes where the carbon dioxide increases tenfold at night", observed Dr. Hasler.