When we think of the consequences of rising sea levels, we often imagine coastal erosion, flooding, and displacement of millions of people.
But there is another, less visible, but potentially more devastating impact that is lurking beneath the surface: the release of large amounts of methane from wetlands.
Methane is a powerful greenhouse gas that traps heat in the atmosphere and contributes to global warming.
According to the Intergovernmental Panel on Climate Change (IPCC), methane has a global warming potential 28 times higher than carbon dioxide over a 100-year period.
Wetlands are natural sources of methane, as they provide anaerobic conditions for microbes that produce the gas from organic matter. Wetlands account for about 30% of the global methane emissions from natural sources.
However, not all wetlands emit the same amount of methane. The salinity of the water, which affects the availability of sulfate, plays a key role in regulating the methane production and consumption by different types of microbes.
Sulfate is an alternative electron acceptor for some bacteria that can outcompete the methane-producing microbes in saltwater environments.
Therefore, it was assumed that tidal wetlands, which are influenced by seawater, would emit less methane than freshwater wetlands.
But a recent study by biologists at Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley challenged this assumption and revealed a surprising finding: a wetland region exposed to a slight amount of seawater was emitting more methane than any of the freshwater sites.
The researchers examined the microbial, chemical, and geological features of 11 wetland zones in the San Francisco Bay Area, ranging from freshwater to full seawater salinities.
They used high-throughput sequencing to analyze the DNA of the organisms found in the soil samples, including bacteria, viruses, and fungi. They also measured the methane fluxes and the concentrations of various compounds in the soil and water.
They found that across most of the sites, the amount of methane emitted was inversely related to the amount of salt water that was flowing in and mingling with the river water.
This was consistent with the expected effect of sulfate on methane cycling.
However, at one site, which had been restored in 2010 from a seasonal grassy pasture for livestock grazing back to its original wetland habitat, the team saw high methane emissions despite the moderate amount of salt water.
This site had the highest methane flux of all the sites, even higher than the freshwater sites.
The researchers were puzzled by this result, as they could not explain it by the abundance or diversity of the methane-producing or methane-consuming microbes. They also ruled out other factors, such as soil pH, temperature, and organic carbon content, that could affect the methane production.
According to their speculations, the high methane emissions at this site could be due to the presence of iron oxides in the soil, which could act as electron donors for the methane-producing microbes.
Iron oxides are common in wetland soils, and they can be reduced by microbes under anaerobic conditions.
The researchers also suggested that the restoration of the site could have triggered a pulse of methane release, as the decomposition of the grassy vegetation could have provided a large amount of organic matter for the microbes.
However, they noted that more research is needed to confirm these hypotheses and to understand the mechanisms behind the methane spike.
The findings of the study have important implications for climate change mitigation strategies, as they indicate that the factors governing how much greenhouse gas is stored or emitted in natural landscapes are more complex and difficult to predict than previously thought.
The researchers emphasized that wetlands are valuable ecosystems that provide many benefits, such as water purification, flood control, biodiversity conservation, and carbon sequestration.
They also pointed out that wetlands can help buffer the effects of sea level rise by accreting sediment and organic matter.
However, they also warned that wetlands could become significant sources of methane in the future, as sea level rise and land use change alter their hydrology and biogeochemistry.
More comprehensive and long-term studies should be pushed on methane emissions from tidal wetlands, especially in regions where wetland restoration projects are underway or planned.
They also suggested that wetland management practices could be optimized to reduce methane emissions, such as by controlling the water level, salinity, and vegetation.
For example, planting salt-tolerant plants could help increase the sulfate input and decrease the methane output in wetlands.
The study highlights the need for a holistic and adaptive approach to wetland conservation and restoration, one that considers not only the ecological and social benefits, but also the potential climate impacts of these vital habitats.
