An underappreciated source of methane discovered in shallow coastal waters

Coastal waters: underestimated methane source coverage. Credit: Tim de Groot
Shallow coastal waters are hotspots for methane emissions, releasing large amounts of this powerful greenhouse gas into the atmosphere and contributing to global warming. New research reveals how tides, seasons, and ocean currents have a huge impact on methane emissions, and how tiny microorganisms called methanotrophs can help reduce that impact. These findings are part of a paper by NIOZ Ph.D. Candidate Tim de Groot is scheduled to give his defense at Utrecht University on January 31, 2025.
While anthropogenic sources of methane are well studied, natural sources such as coastal waters are less understood. These shallow, dynamic ecosystems are rich in methane, and because the water is not very deep, methane-eating microorganisms (methanotrophs) have little time to break it down before it escapes into the atmosphere.
The study investigated three regions: the Doggerbank upwelling area in the North Sea, the Wadden Sea in the Netherlands, and coastal waters near Svalbard in the Arctic. The findings revealed that methane emissions are strongly influenced by natural factors such as tidal and seasonal changes, which also affect the activity of methane-eating microorganisms.
Insights from the Wadden Sea, North Sea and Arctic
In the Wadden Sea, methane levels and emissions increased during warmer months when microbial activity increases. However, methane concentrations remained high during the cold season, and strong wind conditions contributed to large amounts of atmospheric emissions. Tidal currents carried the methane to nearby oceans, where it can also leak into the atmosphere, highlighting the far-reaching effects of coastal methane dynamics.
In the Dogger Bank seep region, the falling tide triggered a burst of methane emissions, while also stimulating microbial activity in the deep sea. However, during the cool autumn months, the mixing of water reduces microbial activity, allowing more methane to escape into the atmosphere than in the summer.
In the Arctic near Svalbard, methane concentrations were highest near the ocean floor, where a diverse and abundant microbial community exists. Ocean currents played a key role in the diffusion of methane and microorganisms, limiting the microorganisms’ ability to completely break down the gas before it reached the atmosphere.
Adaptability of microorganisms
In addition to fieldwork, laboratory experiments revealed that methanotrophic microorganisms are remarkably adaptable. They thrive in a variety of environmental conditions, including changes in temperature, salinity, and methane levels.
“As ecosystems change, methane-eating microorganisms also adapt. When one group struggles, another takes over, keeping natural methane filters running in a warming world,” says Tim de Groot. he says.
“Coastal regions may cover only a small portion of the ocean, but they are hotspots for methane emissions. How methane emissions will evolve as climate change reshapes these systems, and It is becoming increasingly urgent to understand how they can be mitigated.”
Further information: Tim de Groot. Environmental control of methane oxidation by microorganisms in coastal oceans (paper),
Provided by the Royal Netherlands Institute of Marine Research
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