An underestimated source of methane found in shallow coastal waters

Shallow coastal waters are hotspots for methane emissions, releasing significant amounts of this potent greenhouse gas into the atmosphere and contributing to global warming. New research highlights how tides, seasons, and ocean currents strongly influence methane emissions and how tiny microorganisms, called methanotrophs, help reduce their impact. These findings are part of a dissertation by NIOZ Ph.D. candidate Tim de Groot, which he will defend on January 31, 2025 at Utrecht University.

While human-made sources of methane are well-studied, natural sources like coastal waters remain less understood. These shallow, dynamic ecosystems are rich in methane, and because the water is not very deep, methane-eating microbes (methanotrophs) have little time to break it down before it escapes into the atmosphere.

The study investigated three regions: the Doggerbank seep area in the North Sea, the Dutch Wadden Sea, and coastal waters near Svalbard in the Arctic. Findings revealed that methane emissions are highly influenced by natural factors like tides and seasonal changes, which also affect the activity of methane-eating microbes.

Insights from the Wadden Sea, North Sea and Arctic

In the Wadden Sea, methane levels and emissions were higher during warmer seasons when microbial activity was stronger. However, even in colder seasons, methane concentrations remained high, with windy conditions contributing to significant atmospheric releases. Tidal currents transported methane into neighboring waters, where it could still escape into the atmosphere, highlighting the broader impact of coastal methane dynamics.

At the Doggerbank seep area, falling tides triggered bursts of methane release while also stimulating microbial activity in deeper waters. However, during cooler autumn months, when water mixed, microbial activity decreased, leading to more methane escaping into the atmosphere compared to summer.

In the Arctic near Svalbard, methane concentrations were highest near the seafloor, where diverse and abundant microbial communities were present. Ocean currents played a key role in spreading methane and microbes, limiting their ability to fully break down the gas before it reached the atmosphere.

Adaptability of microbes

In addition to fieldwork, laboratory experiments revealed that methanotrophic microbes are remarkably adaptable. They thrive in a range of environmental conditions, including shifts in temperature, salinity, and methane levels.

“As ecosystems change, methane-eating microbes adapt. When one group struggles, another takes over, keeping nature’s methane filter running even in a warming world,” says Tim de Groot.

“Coastal areas may cover only a small part of the ocean, but they are hotspots for methane emissions. As climate change reshapes these systems, understanding how methane emissions will evolve—and how we can mitigate them—becomes increasingly urgent.”