Why does methane bubble up in lakes and rivers (and is that a problem)?

11 September 2024 Research news item

Our surface waters, such as lakes and rivers, emit methane. Methane is a natural gas that is not harmful to our health in low concentrations, but it is a very potent greenhouse gas.
Methane in the atmosphere forms a kind of blanket around the earth, causing the earth to warm up. For this reason, it is important to investigate why and how much methane is emitted from our surface waters. When we understand the underlying mechanisms we may also be able to reduce the methane emissions, perhaps even with natural solutions! Ecologist Sarian Kosten is researching this.

How is methane formed?

When biological material (composed of carbon) is broken down by microorganisms in lakes, rivers and ponds, it releases CO2 and methane. The microorganisms are crucial because, without them, dead plants and animals would accumulate over time. This process primarily occurs in the shallow bottoms of lakes and rivers, within the sediment. When a significant amount of biological material is broken down there, it often exceeds what other microorganisms can consume and the excess methane is released into the air.

Bubbles

As mentioned earlier, methane contributes to global warming. To mitigate this, it's essential to reduce the amount of methane entering the atmosphere. Therefore, understanding the conditions under which more (or less) methane is released from surface waters is crucial. Methane bubbles, which can sometimes be seen rising up in the water column, and sometimes get stuck in the ice where you may have seen them, significantly contribute to methane levels in the atmosphere. Sarian Kosten and her colleagues have investigated and discovered several aspects of these methane bubbles in the Netherlands and Brazil:

Methane emissions from water are challenging to measure accurately because the emissions vary a lot spatially: you could measure almost nothing at one location while some meters away a lot of bubbles are emitted.
Methane can be 'released' from the sediment by disturbances such as boats, fish, or low-pressure areas above the water.
Higher temperatures cause more methane bubbles to be released from the sediment.
Increased nutrients in the water, such as fertilizers, lead to higher methane emissions.
Emissions from polluted waters such as eutrophic ponds used for fish farming tend to be particularly high.

Did you know that the researchers had a special device built at the TechnoCentre of Radboud University to better measure methane bubbles?

How can we reduce methane emissions?

Sarian Kosten and her colleagues have discovered that certain plants can help reduce methane emissions. Plants in the water perform various functions: they can trap methane on the surface with their leaves, causing the methane to dissolve back into the water. They can also oxidise methane: a microbiological process during which methane is converted into something else. It turns out that underwater plants, with roots in the soil, are most effective at limiting methane emissions.
Additionally, other discoveries have been made: fish can sometimes increase oxygen levels in the sediment by stirring up the bottom, which leads to the formation of fewer methane bubbles. Certain worms in the soil have a similar effect. Moreover, it is crucial to prevent water pollution from substances like agricultural fertilizers, as this pollution significantly increases greenhouse gas emissions from water.

Pollution reduction

Sarian's conclusion is that there are indeed situations and methods to limit methane emissions. This is necessary because more methane bubbles are released as our climate warms. However, further research is needed to determine the extent of methane emissions and the best solution for each situation. Reducing the amount of fertilisers entering the water would be a positive step for water quality, aquatic ecology and the climate.

This blog is based on the Huygens colloquium of 4 December 2023, where Prof. Dr. Sarian Kosten delivered a lecture titled "Bubbles on the rise: Greenhouse Gas Emissions from Natural and Human-made Water Systems."

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