Of all the heat-trapping gases in the Earth’s atmosphere, none has drawn the attention of the public and policy makers like carbon dioxide. But new research enabled by advanced greenhouse gas detectors suggests that the scientific community should keep a closer eye on methane released from a seemingly unlikely source — clean oligotrophic lakes.
It’s no secret among scientists that methane is a critical greenhouse gas. Although its concentration in the atmosphere is much lower than carbon dioxide, methane is 25 times more potent than its counterpart. Using an ultraportable greenhouse gas detector, researchers from Leibniz-Institute of Freshwater Ecology and Inland Fisheries in Germany conducted a study to see just how much methane oligotrophic lakes emit.
According to Daniel McGinnis, author of the new paper and now an assistant professor at University of Geneva, aquatic systems in general have been excluded from many models that attempt to quantify methane release into the atmosphere.
“They’re very important components of the terrestrial ecosystem because they receive a lot of organic carbon,” he said. “It’s important whether this is buried or sequestered in the sediment, or gets washed out to sea, or if it’s broken down into methane or CO2, which would then be emitted back into the atmosphere.”
McGinnis hypothesized that microbubbles form due to gas oversaturation caused by algal photosynthesis, or are entrained from the atmosphere, or both, particularly during periods of lake turnover. Although his hypothesis was deemed controversial by some of his peers, he planned a 36-hour experiment to quantify methane flux at a lake’s surface.
Choosing a site at Lake Stechlin, just north of Berlin, McGinnis and his team constructed a small, floating test chamber. Tubes connected the chamber to greenhouse gas detectors onboard a small boat. Dissolved methane and carbon dioxide sensors hung off the side of the boat to monitor ambient surface levels of the gases.
Over the course of the experiment, the researchers went out onto the middle of the lake every hour. They took readings from the drifting test chamber in 10-20 minute increments, then returned to shore to download the data. During the experiment, the researchers also installed turbulence monitoring instruments to contribute to the lake’s routine monitoring.
The researchers to measure gaseous flux between the lake surface and the atmosphere, and in turn, the mass transfer rate of the gases. This, McGinnis said, led to a surprising finding.
“What’s interesting about that is you can standardize these rates and they should be the same for every sparingly soluble gas,” he said. “But what we found was that the mass transfer rate for methane was significantly elevated than that of CO2.”
This means that not only is methane far more potent a greenhouse gas than carbon dioxide, but it’s also capable of entering the atmosphere from the water at a faster rate. McGinnis ran calculations to explain the results, and found that 145 liters of gas per square meter of lake surface area would have to exchange daily to verify his observations — “and that’s a lot,” he said.
“It definitely shows that these lakes could play a bigger role in determining future levels of methane, or at least their part of the budget right now is not really known,” McGinnis said. “These clean, oligotrophic lakes aren’t really studied so often in this context.”
When they are studied, he said, it’s usually during the warmer parts of the year, or during the day. Excluding nighttime and winter studies fails to address critical periods in a lake’s gas exchange cycle.
McGinnis and his team hope to install gas flux instrumentation on research platform on Lake Geneva and establish a yearly monitoring program there. They also hope to develop an imaging system that accurately detects the microbubbles that McGinnis hypothesizes are significant for methane exchange in lakes.
“Once we have a better understanding of what exactly is driving it, we can have a better idea of when and where this might be significant,” McGinnis said.
Top image: The testing chamber floats on the lake’s surface and measures gas exchange between the water and atmosphere. (Credit: Sabine Flury)