A new study has found that, despite relatively similar climate conditions, there is a big difference in how lakes in northern and southern Wisconsin are responding to a warmer, wetter world.
Using data collected on eight Wisconsin lakes over the last forty years, a team of researchers built a computer model to better understand how the “annual life cycle” in each lake was both changing over time as well as predict those conditions in the future.
Specifically, the study documented how lakes’ oxygen supplies are depleted each year. While the northern lakes were usually “breathing” just fine later in the year, the “annual life cycle” of the southern lakes always ends with a “dead zone.”
The pattern for the southern lakes was “the same every year,” says Robert Ladwig, a postdoctoral researcher at the University of Wisconsin-Madison’s Center for Limnology. Since routine monitoring began in 1995, a pattern emerged of the lakes running out of oxygen earlier in the year and experiencing longer lasting dead zones. Ladwig was lead author of the report, released last month in the journal Limnology and Oceanography.
What their study documented, says Ladwig, is an irony of “life killing life.”
The culprit is an overabundance of nutrients, especially phosphorus, that wash into the lakes from the surrounding landscape. For the southern Wisconsin lakes, the land that drains into them is dominated by agriculture, which means that fertilizers used to grow corn and soybeans – as well as the manure produced by the state’s iconic dairy cows – often end up in nearby waterways after big rain events or spring snowmelt. And all those nutrients result in a whole lot of growth in the lake.
Specifically, high levels of nutrients in a lake can trigger big blooms of organisms like algae and tiny plants called phytoplankton. While these organisms do add oxygen into the lake while they’re alive, when they die they drift to the bottom of the lake and begin to decompose. Over the course of the year, the microscopic bacteria responsible for this decomposition use up a lot of oxygen in the deeper waters of a lake.
In the study’s four southern lakes, Ladwig says, so much life is growing and dying that the water near the bottom literally runs out of oxygen, creating zones where no oxygen-breathing organism can survive.
Scientists call lakes like this “eutrophic,” which means that they have such high amounts of nutrients that it results in excessive growth of algae and other plant life.
What surprised the researchers, Ladwig says, is that this eutrophic status seemed to be the only information needed to predict their annual oxygen levels.
“The southern lakes, even though they have different characteristics, they all have the same pattern of anoxia every year and there’s no change to the pattern,” he says. “The nutrients override things like individual lake characteristics and climate change in the region [when it comes to predicting their annual cycles]. In the end, eutrophic lakes are more or less the same in their dissolved oxygen and metabolism patterns.”
While the southern lakes are similar to each other, the northern Wisconsin lakes in the study behaved “more like individuals,” Ladwig says. These lakes are surrounded mostly by wetlands and forests and have far lower amounts of nutrients entering their water column. Without that “overriding” presence of excessive nutrients, northern lakes were more prone to annual variability.
“Maybe in the north there’s a slight increase in [summer] oxygen drawdown in some lakes,” Ladwig says. “It might be a trend, but it’s hard to see. Northern lakes in our study were less predictable on how climate conditions would impact them.”
These findings have big implications for lake conservation and management in our changing world, Ladwig says.
“Managers have to know their lake before making landscape level predictions [and developing management strategies],” he says. For the lakes in northern Wisconsin, more research is needed to better understand what a warmer world means for each lake individually.
For the southern lakes, Ladwig says, the message is already pretty clear – until we fix the nutrient pollution problem, any other attempts to address lake health or climate change aren’t going to do much to help them breathe any easier.