Rising CO2 is causing trouble in freshwaters globally, more parallel to the world's oceans than previously thought
Date: January 11, 2018
Source: Cell Press
Summary: As carbon dioxide (CO2) levels in the atmosphere rise, more CO2 gets absorbed into seawater. As a result, the world's oceans have grown more acidic over time, causing a wide range of well-documented problems for oceanic life and ecosystems. Now, researchers present some of the first evidence that similar catastrophe are happening in global freshwaters, more than it was previously expected.
Predator induced defenses in Daphnia longicephala (top row, credit: Linda Weiss) and Daphnia pulex (bottom row, credit: Sina Becker). Left shows an undefended morphotype, right shows the defended morphotype. Insert shows magnification of expressed neckteeth. These morphological features render Daphnia less susceptible to predators. When the expression of these defensive traits is hampered by high levels of pCO2, Daphnia is suspected to fall as prey more easily. This isn't isolated, freshwater animals across the globe are beginning to show signs of fragility in response to increasingly acidic waters.
The study found that freshwater ecosystems have become more acidic with rising pCO2 (partial pressure of CO2). They also show in lab studies that increases in freshwater pCO2 can have detrimental effects on at least one confirmed (so far since publishing) keystone species, a tiny freshwater crustacean; leaving them less able to sense and defend themselves against predators.The findings suggest that increasing CO2 levels may be having widespread effects on freshwater ecosystems, and we've been ignoring it, assuming wrongly damage would effect Earth's oceans more. It seems, that it effects all aquatic ecosystems in a similar way to how the oceans are currently reeling.
"Ocean acidification and chemical imbalance, 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. In fact, many locations across the Earth, freshwater biomes are doing even worse than oceanic biomes. Our assumptions could have accidentally overlooked the extinction of a large amount of documented freshwater life globally."
Studies on ocean acidification have shown that there are consequences for marine food webs, nutrient cycles, overall productivity, and biodiversity. Yet, the researchers say, surprisingly little has been known about the impact of rising atmospheric CO2 on freshwater systems. While scientists expected that there had been increased pCO2 in freshwater bodies, the data were lacking. We're not beginning to get a glimpse of the effects.
To investigate, Weiss and colleagues first looked to eighty freshwater reservoirs in Germany. Their analysis of data over 35 years, from 1981 to 2015, confirmed a continuous pCO2 increase. As in the ocean, that increase has been associated with a decrease in pH (increasing acidity).
In fact, they report a change in pH of about 0.3 within 35 years, suggesting that freshwaters may acidify at a faster rate than the oceans. An unexpected result that could mean disregard of life gone extinct under our nose.
But just what effect was increasing pCO2 having on freshwater organisms? To begin to explore that question, the researchers focused their attention on small, freshwater crustaceans called Daphnia, also known as water fleas. Daphnia are a dominant species in many lakes, ponds, and reservoirs, and they are important as a primary food source for many larger animals.If they were to vanish, many freshwater fauna in the area would rapidly decline.
When Daphnia sense that predators are around, they respond by producing helmets and spikes that make them harder to eat. Weiss' team found in the lab that rising pCO2 levels get in the way of the water fleas' ability to produce those protective features. Leading to population loss that effected the overall ecosystem's populations, from Daphnia, up.
"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. This is only an example, one cannot say for certain these results reflect all freshwater ecosystems, but there is little evidence to suggest that the acidification wouldn't effect other ecosystems as well; as in marine life.
Weiss says they were fortunate to obtain such a long data series on four freshwater reservoirs. It will now be important to gather more data representing freshwater ecosystems around the world
"We now want to know the global degree of this phenomenon, and just how much we've ignored" Weiss says.
Source: Materials provided by Cell Press. Note: Content may be edited for style and length.
Linda C. Weiss, Leonie Pötter, Annika Steiger, Sebastian Kruppert, Uwe Frost, Ralph Tollrian. Rising pCO 2 in Freshwater Ecosystems Has the Potential to Negatively Affect Predator-Induced Defenses in Daphnia. Current Biology, 2018; DOI: 10.1016/j.cub.2017.12.022