Rising CO2 levels in the atmosphere being absorbed into the world’s oceans have created a wide range of well-documented problems for marine animals and ecosystems. Now, researchers reporting in Current Biology on January 11 present some of the first evidence that similar things are happening in freshwaters too.
The study was conducted by aquatic biologists at Ruhr University Bochum in Germany. They found that some freshwater ecosystems have become more acidic with rising pCO2 levels (Partial pressure of CO2 is a measure that reflects the carbon dioxide exchange between the lake and its environment), using data spanning 35 years, from 1981-2015.
Four freshwater reservoirs in Germany were used in the study. Analysing the data covering 35 years, they confirmed there had been a continuous rise in pCO2 levels at all four freshwater bodies. A rise in CO2 levels causes a decrease in pH levels, the measure of how acid the water has become. Just remember, the lower the pH level, the greater the acidity.
What the study found
With the continuous rise in pCO2 levels in the four reservoirs, there was also a 0.3 decrease in the pH level. This is actually three times what has been measured in oceans since the industrial revolution. The researchers found that while lakes may be absorbing some CO2 from the atmosphere, like the oceans, they are getting much more of the greenhouse gas from emissions settling in the soil and washing into freshwater.
“Ocean acidification is often called the ‘climate change’s equally evil twin,’ and many current investigations describe tremendous effects of rising CO2 levels on marine ecosystems,” says Linda Weiss at Ruhr-University Bochum in Germany. “However, freshwater ecosystems have been largely overlooked. Our data indicate another pCO2 problem: pCO2-dependent freshwater acidification.”
What are the effects of more acidic freshwater?
And this is one question researchers wanted to answer, so they started out by looking at Daphnia, tiny freshwater crustaceans we call water fleas. Daphnia are an important part of the food web in many lakes, ponds, and reservoirs and are a primary food source for many larger animals, like tadpoles, newts, and fish.
To closely examine what happens to Daphnia in the presence of acidic water, the researchers examined the effect that high CO2 had on the behavior of two species of the water flea under laboratory conditions. Normally, when predators feed on Daphnia, the predators release a chemical signal that cues various species of water fleas to arm themselves with an array of defenses, including forbidding neck spikes or giant “helmets” that make the creatures tougher to swallow.
The Daphnias’ sense of danger was dulled under rising CO2 levels. The team tested the water flea’s responses in both waters with predator cues and with varying levels of partial CO2. Ad most interesting, the researchers found that the elevated CO2 was responsible, rather than the reduced pH for the dulled sense of danger in the little critters.
The researchers admit that it is unclear why CO2 leads Daphnia to lower its defenses, however, they suggest that perhaps CO2 acts as an anesthetic or narcotic, blunting the water flea’s defenses.
“High levels of CO2 reduce the Daphnia’s ability to detect their predator,” Weiss says. “This reduces the expression of morphological defenses, rendering them more vulnerable.” She adds that such effects on Daphnia may have broader effects on freshwater communities.
“We now want to know the global degree of this phenomenon,” Weiss says. The question is: “Are all freshwater impoundments prone to this kind of acidification?”
