by Adam Hinterthuer
Back in July of 2018, Center for Limnology graduate student, Rob Mooney, set out on an epic road trip around Lake Michigan. He’d already completed eight circuits of the lake over the previous two years for a study on seasonal changes in tributaries flowing into the lake.
This time, however, Mooney’s plan was even more ambitious. He would take water samples at as many of the nearly 300 tributaries that flow into the lake as he could in as little time as possible.
While obstacles like dangerous road crossings and encounters with aggressive dogs made stopping at every stream impossible, at the end of six days Mooney had a snapshot of 235 tributaries that empty into Lake Michigan. That, says Mooney, “was just cool in itself,” but “a bonus was that we actually started to see patterns.”
In an article on his adventure published this October in the Proceedings of the National Academy of Sciences, Mooney says these patterns revealed that small streams play an outsized role in feeding algae blooms and impacting coastal waters.
CFL professor Emily Stanley says Mooney’s study “is unique in quantifying the amount and type of nutrients being delivered to a lake at such an epic scale.” She likes to joke that Mooney’s massive sampling effort was a like “NASCAR science, except he went clockwise!”
Fueled by weak motel coffee and a glovebox full of sugary snacks, Mooney and recent UW–Madison graduate Will Rosenthal sampled nearly 40 streams a day. For each tributary, they would find a road crossing as close to the lake as possible, then walk out to the middle of the bridge or culvert over the stream and toss a bucket tied to a rope into the water. After filtering these water samples into small vials and tucking them into a well-iced cooler, they headed along the coast to the next stream.
“Within the Great Lakes there are hundreds and hundreds of small tributaries that are flowing in but, for the most part, they haven’t been considered by previous nutrient-loading studies because they’re so small compared to the big ones,” Mooney says.
That focus, he says, overlooks some important impacts. For example, smaller tributaries often have nutrient loads that are high for their size and contain higher percentages of soluble reactive phosphorus, a compound that is readily available for nutrient-starved algae or aquatic plants to snap up and use to feed their growth.
They also don’t have high stream flows, which is important, Mooney says, because “they aren’t blasting water out into the lake but getting pushed back against the shore so all of the nutrients they have stay available along those coastlines.”
And that fact may point to the most important takeaway of Mooney’s study.
“It’s hard to think of a Great Lake, like Lake Michigan, as a singular lake. It is just so massive and built up of all of these smaller segments of coastline that have different tributaries running in,” he says.
In other words, no one experiences the lake at a whole-lake scale.
“One of the reasons why I wanted to stop at every tributary is because, on my earlier sampling trips, I would stop and get lunch somewhere and talk to the people who lived and owned businesses and rented out hotels along the coast,” Mooney says. “If any stream is going to affect their daily livelihood, it’s going to be the smaller ones that will have a direct impact on the shoreline they live along.”
Mooney hopes his study can help resource managers target nutrient-reduction efforts to have the biggest impact for nearshore ecosystems and local communities. “There is still a lot to learn about how these smaller tributaries influence coastal water quality throughout the Great Lakes,” he says.
But that doesn’t mean he’s feeling the itch for a tenth lap around the lake. While he may miss the open road and front-row seats to every mile of the Lake Michigan coastline, he laughs, “for me, sampling forty streams a day and driving hundreds of miles for six days straight is definitely not the most sustainable research model.”