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Freshwaters release methane, even when they dry out

Freshwater releases methane – even when it dries out

Due to climate change, water bodies will increasingly emit greenhouse gases. Credit: Pexels from Pixabay

Fresh water is an underestimated source of greenhouse gases. In a study published in Science of the total environment, researchers at the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) have now shown that even dry bodies of water can release significant amounts of methane. An overview of the causes and magnitudes of methane emissions from freshwater and a look at future developments in climate change make this clear.

Methane is produced when organic matter decomposes in the absence of oxygen. For example, it can be released during the extraction of coal, oil or natural gas. It is produced in the stomachs of cows, as well as in inland waters and oceans.

Different Methane Production Processes in Water Bodies

“Among the freshwater species that release greenhouse gases, reservoirs and lakes are major emitters,” explains IGB researcher Professor Hans-Peter Grossart. “This is because organic matter from dead plants and animals sinks more to the oxygen-poor sediment of lakes and covers more than it sinks in flowing water. This methane release is enhanced by higher temperatures. The methane then rises from the bottom to the water surface in small gas bubbles and is released into the atmosphere.”

For a long time, researchers assumed that methane is only formed in waters where there is no oxygen. “Recent studies show that this greenhouse gas is also produced in the oxygen-rich water column: For example, several species of phytoplankton – cyanobacteria, diatoms and haptophytes – emit methane during their photosynthesis,” said IGB researcher Dr. Mina Bizic, who shared the knowledge about methane formation by phytoplankton a scientific article

Methane is also produced on dry sediment

Methane is produced even where there is no water at all: bodies of water that dry up are known to be a source of greenhouse gases such as carbon dioxide. However, until now little was known about whether and how much methane is released from these areas. A research team led by Radboud University in the Netherlands has estimated global methane emissions for dry areas of lakes, ponds, reservoirs and rivers in different climate zones. The researchers also determined the environmental factors that control these emissions.

Hans-Peter Grossart, who was involved in the study, stated: “Methane emissions from dry inland waters were consistently higher than emissions measured in adjacent slope soils in all climatic zones and in all aquatic systems except streams. Worldwide, dry inland waters emit 2 .7 million tonnes of methane per year, according to projections, and emissions are likely to increase.”

The type of water body itself and the climatic zone did not affect the amount of methane released. The soil organic matter content of the water body combined with the local temperature and humidity were the main influencing factors. Especially at the start of the drying process and during the so-called ridge flushing, the moment when the water returns to the reclaimed area due to, for example, heavy rainfall, a lot of methane is created.

Higher methane emissions due to climate change

Climate change processes can further boost methane emissions. For starters, water bodies are getting warmer. In addition, the oxygen content in lakes is declining worldwide. Hans-Peter Grossart was involved in a Nature study who quantified the oxygen deficiency for 400 lakes in different climatic zones: the oxygen content of the studied water bodies decreased by 5.5 percent over the past 40 years at the surface and by 18.6 percent in the deep zone.

“Phytoplankton will also emit more methane in the future simply because more of it will be present in water bodies,” Mina Bizic predicted. This is because increasing nutrient loads and warming of water bodies are considered to be the main drivers of the recent increase in phytoplankton blooms. In addition, phytoplankton blooms can increase the creation of oxygen-free, so-called dead zones. This in turn stimulates the classic methane formation in the case of oxygen deficiency.

“Methane emissions from dried-up areas will also increase as a result of more frequent extreme weather events – droughts and heavy rainfall – because especially large amounts of greenhouse gases are emitted during these changes,” adds Hans-Peter Grossart. †

What can be done?

Measures to improve water quality help to curb methane formation from water bodies despite climate change. “If less nutrients are brought into water bodies, less organic matter will be formed. In addition, less phytoplankton blooms will occur,” says Mina Bizic.

Measures that keep water in the landscape and stabilize groundwater are also helpful, as many lakes are fed by groundwater. Water bodies therefore dry up, not only due to increased evaporation, but often also due to falling groundwater levels. The construction of wetlands and peatlands ensures that both water shortages and water surpluses are compensated. Peatlands have another advantage: “An ecologically intact peatland acts as a long-term store of carbon. When it dries out, it releases greenhouse gases.

A drained swamp emits an average of 15 tons of CO . from2 per hectare per year. Methane is certainly produced in a semi-natural peat area. However, the methane emissions from a reclaimed peat area are usually higher, also due to the high methane emissions from the numerous drainage ditches. Therefore, protecting peatlands is always climate protection,” explains Dr. Dominik Zak, visiting researcher at IGB and peat researcher at Aarhus University in Denmark.

Fjords can emit as much methane as all deep oceans worldwide

More information:
José R. Paranaíba et al, Cross-continental importance of CH4 emissions from dry inland waters, Science of the total environment (2021). DOI: 10.1016/j.scitotenv.2021.151925

Provided by Forschungsverbund Berlin eV (FVB)

Quote: Freshwater releases methane even when they dry out (2022, June 15) retrieved June 15, 2022 from https://phys.org/news/2022-06-freshwaters-methane.html

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