As people continue to burn fossil fuels, levels of carbon dioxide in the atmosphere have been rising. Some of that excess has dissolved into the world’s oceans. But levels of this greenhouse gas have also been rising in some lakes, a new study finds. Too much of this CO2 in the water may leave some tiny animals in the water too sleepy to fend off predators. This could be a problem because they are an important part of many lake food webs.
These animals are water fleas. Not true fleas, they are a type of tiny crustacean. (As such, they’re related to shrimp and lobsters.) They get their name from the way they appear to jump about in the water. The ones studied here were two different species of pinhead-sized Daphnia (DAFF-nee-uh). They are at the bottom of many freshwater food webs. That means they serve as a primary entrée in the diet of somewhat bigger animals.
Long-term measurements of the chemistry of lake water are rare. But researchers found data on four lakes in Germany. Those data covered the period from 1981 to 2015. They showed how much CO2 levels had risen over that time, as pH levels dropped. (pH is a measure of acidity.)
Rising CO2 in the atmosphere has increased levels of the gas dissolved into Earth’s oceans. That has made them more acidic. Studies show that this ocean acidification alters the behaviors of many species. It’s been less clear whether rising CO2 levels also were affecting lakes and other bodies of freshwater. It also was not clear how freshwater critters might be coping with any change, says Linda Weiss. She’s an aquatic ecologist at Ruhr University Bochum in Germany.
Her team compared the data from the German lakes. Over 35 years, they found, the lakes’ pH fell by an average of 0.01 per year. Carbon dioxide levels increased during that time by a yearly average of 16 microatmospheres. (That is a unit of air pressure.)
And water pH levels fall, the researchers now show, the behavior of the water fleas will change.
The scientists shared these data online January 11 in Current Biology.
To probe the pH effects on water fleas, Weiss and her team studied the crustaceans' behavior in the lab. Predators that feed on Daphnia include the larvae of phantom midges. While dining on the water fleas, those midges release a chemical. Various species of water fleas respond to the chemical by arming themselves with an array of natural defenses. Some raise forbidding neck spikes. Others grow giant “helmets” that make them tougher to swallow.
But in waters with high CO2, the fleas’ ability to sense the predators appeared dulled. They seemed to grow sleepy and unaware of the chemical signaling hungry midges.
The team tested the critters in waters containing both the scary midge chemical and three different levels of CO2. The scientists measure that gas in units known as microatmospheres (microatm). The lowest level was 2,000 microatm. Although it is considered high, this level is now common in lakes. They then compared this to two higher levels, 11,000 and 16,000 microatm. Both species of flea were less defensive at the higher levels of CO2. They displayed fewer neck spikes or developed smaller crests.
Further tests revealed that the elevated CO2 was responsible. It was not due to the more acidic pH. It’s unclear exactly why higher CO2 levels lower the Daphnias’ defenses. However, the researchers suggest the gas may act as a narcotic and blunt the water flea's senses.
The chemistry and environment of lakes can vary widely. As such, Weiss says, it is difficult to draw firm conclusions from the new findings. Many lakes are warming. And many are already saturated in carbon dioxide and now shedding the excess to the air. Others are still absorbing it and becoming more acidic.
Caleb Hasler is a biologist in Canada at the University of Winnipeg in Manitoba. He was not involved in the study. Hasler says it also is unclear how other freshwater species — including predators — might be affected by growing CO2 levels in water.
Occasional studies have looked at, as here, particular species. Maybe they were plankton. Or fish. Or shellfish. And looking at all of these, he says, “The effect seems to be highly variable.”
Still, studies such as the new one on water fleas show that measuring long-term changes in CO2 and biological impacts will be important to understanding ecosystem effects, Hasler says. And for starters, he says, “Showing that there is an impact on an important species is pretty significant.”
acidic An adjective for materials that contain acid. These materials often are capable of eating away at some minerals such as carbonate, or preventing their formation in the first place.
acidification A process that lowers the pH of a solution. When carbon dioxide dissolves in water, it triggers chemical reactions that create carbonic acid.
annual Adjective for something that happens every year.
aquatic An adjective that refers to water.
array A broad and organized group of objects. Sometimes they are instruments placed in a systematic fashion to collect information in a coordinated way. Other times, an array can refer to things that are laid out or displayed in a way that can make a broad range of related things, such as colors, visible at once. The term can even apply to a range of options or choices.
atmosphere The envelope of gases surrounding Earth or another planet.
average (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.
behavior The way something, often a person or other organism, acts towards others, or conducts itself.
biology The study of living things. The scientists who study them are known as biologists.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
chemical signal A message made up of molecules that get sent from one place to another. Bacteria and some animals use these signals to communicate.
chemistry The field of science that deals with the composition, structure and properties of substances and how they interact. Scientists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) Chemistry also is used as a term to refer to the recipe of a compound, the way it’s produced or some of its properties. People who work in this field are known as chemists.
crustaceans Hard-shelled water-dwelling animals including lobsters, crabs and shrimp.
Daphnia Also known as water fleas, these are actually small, freshwater crustaceans. They are near the bottom of the food chain, serving as a major energy source for many small fish.
data Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.
defense (in biology) A natural protective action taken or chemical response that occurs when a species confront predators or agents that might harm it. (adj. defensive)
ecology A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
ecosystem A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.
environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).
food web (also known as a food chain) The network of relationships among organisms sharing an ecosystem. Member organisms depend on others within this network as a source of food.
fossil fuel Any fuel — such as coal, petroleum (crude oil) or natural gas — that has developed within the Earth over millions of years from the decayed remains of bacteria, plants or animals.
freshwater A noun or adjective that describes bodies of water with very low concentrations of salt. It’s the type of water used for drinking and making up most inland lakes, ponds, rivers and streams, as well as groundwater.
greenhouse A light-filled structure, often with windows serving as walls and ceiling materials, in which plants are grown. It provides a controlled environment in which set amounts of water, humidity and nutrients can be applied — and pests can be prevented entry.
greenhouse gas A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.
larva (plural: larvae) An immature life stage of an insect, which often has a distinctly different form as an adult. (Sometimes used to describe such a stage in the development of fish, frogs and other animals.)
midges Any of many types of small flies that often live around water. Some are blood-sucking insects; others can derive their energy from eating plants. Frequently mistaken for mosquitoes, midges can transmit disease or move pollutants through an ecosystem.
narcotic A drug (such morphine) or some natural compound that may be prescribed to dull the senses — especially pain — or cause a relaxation to induce deep sleep. Overdoses of these agents may lead to coma, convulsions and death.
pH A measure of a solution’s acidity or alkalinity. A pH of 7 is perfectly neutral. Acids have a pH lower than 7; the farther from 7, the stronger the acid. Alkaline solutions, called bases, have a pH higher than 7; again, the farther above 7, the stronger the base.
plankton A small organism that drifts or floats in the sea. Depending on the species, plankton range from microscopic sizes to organisms about the size of a flea. Some are tiny animals. Others are plantlike organisms. Although individual plankton are very small, they form massive colonies, numbering in the billions. The largest animal in the world, the blue whale, lives on plankton.
predator (adjective: predatory) A creature that preys on other animals for most or all of its food.
pressure Force applied uniformly over a surface, measured as force per unit of area.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
Journal: L.C. Weiss et al. Rising pCO2 in freshwater ecosystems has the potential to negatively affect predator-induced defenses in Daphnia. Current Biology. Vol. 28, January 22, 2018, p. 1-6. doi: 10.1016/j.cub.2017.12.022.