Freshwaters release methane, even when they dry out
Freshwaters are underestimated sources of greenhouse gases. In a study published in Science of The Total Environment, researchers with the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) have now shown that even dry water bodies can release considerable amounts of methane. An overview of the causes and magnitudes of methane emissions from freshwaters and an outlook on future developments in climate change make this evident.
Methane is produced when organic material decomposes in the absence of oxygen. It can be released for example during the mining of coal, oil or natural gas. It is produced in cows' stomachs—but also in inland waters and oceans.
Several methane production processes in water bodies
"Among the types of freshwaters that release greenhouse gases, reservoirs and lakes are major emitters," explained IGB researcher Professor Hans-Peter Grossart. "This is because organic material from dead plants and animals sinks to the oxygen-poor water bottom of lakes and reservoir more than it sinks in flowing waters. This methane release is intensified by higher temperatures. In small gas bubbles, the methane then rises from the bottom to the water surface and thus enters the atmosphere."
For a long time, researchers assumed that methane is only formed in water bodies where there is no oxygen. "Recent studies show that this greenhouse gas is also produced in the oxygen-rich water column: for example, various phytoplankton species—cyanobacteria, diatoms and haptophytes—emit methane during their photosynthesis," said IGB researcher Dr. Mina Bizic, who compiled the knowledge on methane formation by phytoplankton in a scientific article.
Methane is also produced on dry sediment
Methane is even produced where there is no water at all: water bodies that are drying 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 drying 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 soils on slopes in all climate zones and in all aquatic systems except streams. Globally, dry inland waters emit 2.7 million tons of methane per year, according to the projections, and emissions are likely to increase."
The type of water body itself and the climate zone had no influence on the amount of methane released. The content of organic matter in the bottom of the water body in combination with the local temperature and the humidity were the significant influencing factors. Particularly high amounts of methane are produced at the beginning of the drying process and during the so-called ridge-flush—the moment when water comes back onto the drained area, for example, due to heavy rainfall.
Higher methane emissions as a consequence of climate change
Climate change processes could further drive methane emissions. For one thing, water bodies are getting warmer. In addition, the oxygen content in lakes is decreasing worldwide. Hans-Peter Grossart was involved in a Nature study that quantified oxygen depletion for 400 lakes in different climate zones: On average, the oxygen content of the water bodies studied fell by 5.5 percent at the surface and by 18.6 percent in the deep zone over the last 40 years.
"Phytoplankton will also emit more methane in the future, simply because more of it will be present in water bodies," predicted Mina Bizic. This is because increasing nutrient loads and warming of water bodies are considered the main causes of recent increases in phytoplankton blooms. In addition, phytoplankton blooms can increase the occurrence of oxygen-free, so-called dead zones. This, in turn, boosts classical methane formation under oxygen depletion.
"Methane releases from dried-up stretches will also increase as a result of more frequent extreme weather events—drought and heavy rainfall—because it is precisely during these changes that a particularly large amounts of greenhouse gases are emitted," added Hans-Peter Grossart.
What can be done?
To keep methane formation from water bodies in check despite climate change, measures to improve water quality help. "If fewer nutrients are introduced into water bodies, less organic material is formed. In addition, less phytoplankton blooms will occur," said Mina Bizic.
Measures that keep water in the landscape and stabilize groundwater are also helpful, because many lakes are fed by groundwater. So, water bodies are drying-up, not only because of increased evaporation, but often also because of falling groundwater levels. The creation of wetlands and peatlands ensures that both water deficiencies and water surplus will be compensated. Peatlands have another advantage: "An ecologically intact peatland acts as a long-term sink for carbon. If it dries out, greenhouse gases are released.
A drained bog releases an average of 15 tons of CO2 per hectare per year. In a semi-natural peatland, methane is definitely produced. However, the methane release from a drained peatland is usually higher—also due to the high methane release from the numerous drainage ditches. Peatland protection is therefore always climate protection," explained Dr. Dominik Zak, guest researcher at IGB and peatland researcher at Aarhus University in Denmark.