Every autumn, one of the world’s largest freshwater lakes “belches.” That’s Harvey Bootsma’s colorful description of the weeklong discharge of carbon dioxide from Lake Michigan into the atmosphere.
“It happens when the CO2 concentration in the water is greater than the atmospheric concentration – a bit like a can of soda being popped open,” says Bootsma, an associate professor in the School of Freshwater Sciences. “We knew this degassing process happens in lakes in the fall, but we were surprised to find it occurring all at once in Lake Michigan.”
The event could offer new insight about maintaining a healthy food chain in Lake Michigan and also about the role large lakes play in climate change.
Like living organisms, lakes create and consume energy. CO2 figures prominently in those processes, which include food production and consumption, and the decomposition of waste and organic matter.
So Bootsma follows the carbon cycle for clues to the status of the lake’s food chain.
When the lake is “productive,” it is using lots of CO2 in photosynthesis, which is carried out by phytoplankton. These phytoplankton support the rest of the food chain, including zooplankton and fish, so the more CO2 that is used, the more fish the lake may be able to support.
Evidence of the curious CO2 expulsion event turned up while Bootsma was analyzing an unusually large data set he’s collected from a 10-year partnership with Milwaukee’s Lake Express ferry. The vessel carries equipment that takes continuous measurements during four to six daily crossings, May through October.
In 2013, to help complement the ferry data, Bootsma started taking Lake Michigan’s vitals during the winter months at a single location.
UWM scientists are among the first to study whether bodies of water as massive as Lake Michigan cycle carbon differently than smaller lakes. The research is needed to learn whether large lakes give off more CO2 than they take in, which determines whether they might contribute to climate change or help minimize it.
Studies have concluded that smaller lakes are usually producers of CO2. Now, halfway through analyzing his data set, Bootsma is seeing different results for Lake Michigan.
“So far, it suggests that Lake Michigan is close to being neutral,” he says. “It takes up as much CO2 as it releases.”
During warm months, Bootsma found the lake is a sink for atmospheric CO2 because food production is in high gear. Some of this food sinks to the deep water, where it decomposes and is converted back to CO2. During cold months, when winds whip up the water, this CO2-rich water returns to the surface, where it releases CO2. Most of this release occurs in about one week in November.
Although it’s too soon to definitively say if Lake Michigan is a sink or source for CO2, the data is revealing trends – and that unexpected belch – which Bootsma will further investigate. It also shows the importance of gathering meticulous data over a span of many years.
“Otherwise,” Bootsma says, “we would have missed this event, and our conclusions about CO2 in Lake Michigan would have been completely wrong.”