Ponds made to control floods can spew climate-warming gases, finds study

Ponds made to control floods can spew climate-warming gases, finds study

As Earth’s climate has been warming, the risk of urban flooding has grown, too. Building ponds to collect excess rainwater can limit that flooding. But in some places, those ponds may actually add to global warming by releasing greenhouse gases into the air. That’s the finding of a new study.

Earth’s changing water cycle has made many cities and their suburbs wetter. These urban areas have a limited ability to soak up rain. Instead of porous ground, precipitation hits roads, concrete parking lots, buildings and other features. Ponds can collect the water that runs off. But where nature hasn’t provided enough ponds, cities have been building them. Florida alone has built about 76,000 such stormwater ponds. Many other areas also rely heavily on these ponds for flood control, notes Audrey Goeckner. She’s a graduate student at the University of Florida in Gainesville and lead author of the new study.

Explainer: Earth’s water is all connected in one vast cycle

Recently, her team compared the uptake — and later release — of carbon-based gases at five stormwater ponds. All were in Manatee County, Fla. That’s on the state’s central west coast.

Along with a lab technician, Goeckner collected samples from the bottoms of the ponds. The samples had a layer of muck above sandy material. Older ponds had deeper layers of muck. Back at the lab, the research team measured the carbon-based compounds in the samples.

In general, older ponds released less of these carbon-based chemicals into the air than younger ponds did. At the same time, “the rate that [older ponds] were storing carbon in the sediment increased,” Goeckner says. In contrast, younger ponds seem to emit more carbon than they stored away.

“It’s just really important to take into account how these human-made ecosystems are playing a part [in the environment],” she concludes. Some ponds mostly store carbon, it seems. Others mostly release it into the air. Her team shared its findings March 9 in Communications Earth & Environment.

a woman wearing a ball cap, blue shirt, jeans and boots stands with one foot on a canoe parked at the edge of a pond
Audrey Goeckner prepares to row out in a stormwater pond to collect sediments and measure emissions of carbon-based greenhouse gases.COURTESY AUDREY GOECKNER, UF/IFAS

Where does the carbon go?

It’s important to understand what a stormwater pond is, Goeckner says. “It’s not just a hole in the ground with water.” Each quickly houses a complex ecosystem. Below its surface, she explains, “There’s a lot that goes on.”

Rains running into these ponds often carry leaves, grass, soil and other carbon-rich materials. They also can ferry a host of chemicals into these ponds. For instance, they can wash fertilizers off of lawns. Or traffic pollution off of streets. Rains can even wash pollutants out of the air.

a woman poses in a canoe holding a tall tube filled with sediment

Shortly after a pond is built, plants and microbes will move in. They’ll begin grabbing carbon-based chemicals from the water for growth and energy. Along the way, they will bind up much of that carbon in their tissues. They do it in much the same way that trees remove carbon dioxide, or CO2, from the air.

But pond organisms don’t live as long as trees. As soon as they die, microbes in the water will begin breaking down the dead cells — and their carbon — for reuse. The carbon will tend to be released as CO2 and methane (CH4). Both are potent greenhouse gases. Those gases will rise to the surface and out into the air. Sometimes the methane will suddenly burp up to the surface in bubbles.

Not all of a pond’s carbon will enter the air. Some will sink to the bottom and collect in the muck. And some carbon may flow out if there’s an outlet to a stream or water-treatment plant. The researchers used a device at multiple spots at each pond to measure the CO2 and methane entering the air. The team did this every two weeks from June 2019 through March 2020.

Offgassing of the ponds could reach 2,900 grams (6.4 pounds) of CO2 equivalents per square meter (roughly, per square yard) of surface area per year. Meanwhile, they found, carbon storage for the ponds ranged from 22 to 217 grams (0.8 to 7.7 ounces) per square meter of surface area each year.

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That range of values shows the ponds did not all respond the same way. Older ponds tended to store more carbon and release less. Why?

Sediment layers tend to be deeper in older ponds. They often have less oxygen available to fuel the breakdown of cells and tissues by microbes, Goeckner explains. This could slow the release of carbon-rich gases. Older ponds also may have more photosynthesis going on in their vegetation, she adds. So more organisms may be taking up carbon dioxide and using it to build new cells.

How important are the findings?

Explainer: CO2 and other greenhouse gases

This is not the first study to show that ponds can release greenhouse gases, notes Trisha Moore. She’s an ecological engineer at Kansas State University in Manhattan. What sets the new study apart is that it is one of the first “looking at both parts of the system” — how much carbon the ponds store and how much they release. As such, she finds, “It was just a nice example of how we need to look at a system.”

However, she cautions, this study sampled only a small number of ponds. And they were all in one state. Future research should look at what happens in ponds where the climate and vegetation are different, Moore says. Other research could test whether different pond designs might affect how much carbon their water releases. Or how might cleaning up the water before it flows into these ponds affect carbon releases? In the end, she says, pond engineers need to ask: “What can we do better?”

Power Words

More About Power Words

biology: The study of living things. The scientists who study them are known as biologists.

carbon: A chemical element that is the physical basis of all life on Earth. (in climate studies) The term carbon sometimes will be used almost interchangeably with carbon dioxide to connote the potential impacts that some action, product, policy or process may have on long-term atmospheric warming.

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.

cell: (in biology) The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall.

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.

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.

climate: The weather conditions that typically exist in one area, in general, or over a long period.

concrete: (in construction) A simple, two-part building material. One part is made of sand or ground-up bits of rock. The other is made of cement, which hardens and helps bind the grains of material together.

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. The term can also be applied to elements that make up some an artificial environment, such as a company, classroom or the internet.

engineer: A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need. (v.) To perform these tasks, or the name for a person who performs such tasks.

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 things in the vicinity of an item of interest).

fertilizer: Nitrogen, phosphorus and other plant nutrients added to soil, water or foliage to boost crop growth or to replenish nutrients that were lost earlier as they were used by plant roots or leaves.

global warming: The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

graduate student: Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).

greenhouse gases: Gases that contribute to the greenhouse effect by absorbing heat. Carbon dioxide and methane are two examples of such gases.

host: (in biology and medicine) The organism (or environment) in which some other thing resides. Humans may be a temporary host for food-poisoning germs or other infective agents. (v.) The act of providing a home or environment for something.

methane: A hydrocarbon with the chemical formula CH4 (meaning there are four hydrogen atoms bound to one carbon atom). It’s a natural constituent of what’s known as natural gas. It’s also emitted by decomposing plant material in wetlands and is belched out by cows and other ruminant livestock. From a climate perspective, methane is 20 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas.

microbe: Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

organic: (in chemistry) An adjective that indicates something is carbon-containing; also a term that relates to the basic chemicals that make up living organisms.

organism: Any living thing, from elephants and plants to bacteria and other types of single-celled life.

oxygen: A gas that makes up about 21 percent of Earth’s atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

photosynthesis: (verb: photosynthesize) The process by which green plants and some other organisms use sunlight to produce foods from carbon dioxide and water.

pollutant: A substance that taints something — such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.

porous: The description of a substance that contains tiny holes, called pores, through which a liquid or gas can pass. (in biology) The minute openings in the skin or in the outer layer of plants.

potent: An adjective for something (like a germ, poison, drug or acid) that is very strong or powerful.

precipitation: (in meteorology) A term for water falling from the sky. It can be in any form, from rain and sleet to snow or hail.

risk: The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

runoff: The rainwater that runs off of land into rivers, lakes and the seas. As that water travels through soils, it picks up bits of dirt and chemicals that it will later deposit as pollutants in streams, lakes and seas.

sediment: Material (such as stones and sand) deposited by water, wind or glaciers.

urban: Of or related to cities, especially densely populated ones or regions where lots of traffic and industrial activity occurs. The development or buildup of urban areas is a phenomenon known as urbanization.

vegetation: Leafy, green plants. The term refers to the collective community of plants in some area. Typically these do not include tall trees, but instead plants that are shrub height or shorter.

source: https://www.sciencenewsforstudents.org/

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