Greenhouse Gas ‘Feedback Loop’ Discovered in Freshwater Lakes

Lake near Sudbury, Ontario, in the Canadian Boreal Shield, with aquatic plants in the foreground providing fuel for methane production. (Image: via Andrew Tanentzap)

Latest research finds plant debris in lake sediment affects the methane greenhouse gas emissions. The flourishing reed beds created by changing climates could threaten to double the already significant methane production of the world’s northern lakes.

A new study of chemical reactions that occur when organic matter decomposes in freshwater lakes has revealed that the debris from trees suppresses production of methane — while debris from reed beds actually promotes this harmful greenhouse gas.

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As vegetation in and around bodies of water continues to change, with forest cover being lost while global warming causes wetland plants to thrive, the many lakes of the northern hemisphere — already a major source of methane — could almost double their emissions in the next 50 years.

The researchers say that the findings suggest the discovery of yet another “feedback loop” in which environmental disruption and climate change trigger the release of ever more greenhouse gas that further warms the planet, similar to the concerns over the methane released by fast-melting arctic permafrost.

The study’s senior author, Dr. Andrew Tanentzap, from the University of Cambridge’s Department of Plant Sciences, said:

Methane greenhouse gas emissions

The researchers point out that the current methane emissions of freshwater ecosystems alone offsets around a quarter of all the carbon soaked up by land plants and soil — the natural “carbon sink” that drains and stores CO2 from the atmosphere.

Up to 77 percent of the methane emissions from an individual lake are the result of the organic matter shed primarily by plants that grow in or near the water.

This matter gets buried in the sediment found toward the edge of lakes, where it is consumed by communities of microbes. Methane gets generated as a byproduct, which then bubbles up to the surface.

Working with colleagues from Canada and Germany, Tanentzap’s group found that the levels of methane produced in lakes varies enormously depending on the type of plants contributing their organic matter to the lake sediment.

The study, funded by the UK’s Natural Environment Research Council, is published today in the journal Nature Communications.

To test how organic matter affects methane emissions, the scientists took lake sediment and added three common types of plant debris: deciduous trees that shed leaves annually, evergreen pine-shedding coniferous trees, and cattails (often known in the UK as “bulrushes”) — a common aquatic plant that grows in the shallows of freshwater lakes.

These sediments were incubated in the lab for 150 days, during which time the scientists siphoned off and measured the methane produced.

They found that the bulrush sediment produced over 400 times the amount of the greenhouse gas methane as the coniferous sediment, and almost 2,800 times the methane than that of the deciduous.

Unlike the cattail debris, the chemical makeup of the organic matter from trees appears to trap large quantities of carbon within the lake sediment — carbon that would otherwise combine with hydrogen and get released as methane into the atmosphere.

To confirm their findings, the researchers also “spiked” the three samples with the microbes that produce methane to gauge the chemical reaction.

While the forest-derived sediment remained unchanged, the sample containing the bulrush organic matter doubled its methane production. Dr. Erik Emilson, first author of the study, said:

Using species distribution models for the Boreal Shield, an area that covers central and eastern Canada and “houses more forests and lakes than just about anywhere on Earth,” the researchers calculated that the number of lakes colonized by just the common cattail (Typha latifolia) could double in the next 50 years — causing current levels of lake-produced methane to increase by at least 73 percent in this part of the world alone. Tanentzap added:

Provided by: University of Cambridge [Note: Materials may be edited for content and length.]

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