The concentration of methane – a potent greenhouse gas – in the atmosphere has more than doubled since the start of the industrial revolution. This is alarming, as methane contributes significantly to global warming. Coastal areas are important methane sources because rivers carry nutrients (artificial fertilizers) organic material, such as dead algae and untreated sewage, into them. As microorganisms break down this material, CO₂ and methane are released. Other microorganisms consume this methane before it escapes into the air; this natural process is known as the methane filter.
Wallenius’s research revealed that this process is particularly disrupted in Lake Grevelingen: ‘We were surprised by the amount of methane produced in the sediment,’ she said. ‘The methane cycle there is completely out of balance.’ Her studies showed that high levels of sulfide are produced in the lake’s polluted sediments: methane eaters produce this, but it’s also produced by breaking down organic matter. This sulfide is toxic to fish and other organisms and also prevents methane eaters from functioning properly: ‘Because of the high sulfide concentration, methane eaters can’t survive effectively, giving methane production free rein,’ Wallenius explains. ‘Due to the high organic matter, more methane is produced, so the starting point is already bad for methane eaters. The sulfide makes it even worse.’
Hope
Despite these troubling findings, Wallenius’s research also offers hope. In the Baltic Sea, she discovered alternative pathways for methane removal: ‘While methane eaters in Lake Grevelingen rely on sulphate, in the Baltic Sea they use iron and manganese,’ she says. ‘These metals can help methane eaters clean up methane more effectively.’
To study this, Wallenius collected sediment samples from the bottom of the lake using a Research Vessel equipped with a large drill, and analyzed each layer in the laboratory to determine its composition. ‘Now that we know iron and manganese can also support methane oxidation, we can use this knowledge to restore the health of methane filters and reduce emissions,’ Wallenius concludes. ‘We must act – if we continue as we do now, our coastal ecosystems will be destroyed.’