Excessive nutrient inputs from tributary streams and rivers contribute to harmful algal blooms and coastal ecosystem degradation worldwide. However, the role that small tributaries play in coastal nutrient dynamics remains unknown because most monitoring and regulatory efforts have paid little attention to small tributaries of large water bodies, despite their ubiquity and potential local importance.

To begin to address this issue, two scientists from the University of Wisconsin–Madison took a road trip in 2018 around Lake Michigan—the world’s fifth largest freshwater lake by volume—to take water samples at as many of the nearly 300 tributaries that flow into the lake as they could. At the end of six days, Rob Mooney, a graduate student at the UW–Madison’s Center for Limnology, and recent UW–Madison graduate Will Rosenthal, had a snapshot of 235 tributaries that empty into Lake Michigan and were beginning to see some very interesting patterns in these smaller tributaries.

MSU landscape hydrologist Anthony Kendall teamed up with colleagues at the University of Wisconsin–Madison, to investigate nutrient inputs from Lake Michigan’s nearly 300 tributaries. Kendall’s role in the UW–Madison–led study was to calculate stream flow at all of the sampling locations. Photo: MSU Hydrogeology Lab

These patterns are discussed in an article published October 26 in the Proceedings of the National Academy of Sciences and reveal that small streams can have a big water quality impact where they empty into the lake. The UW–Madison–led study also included collaborators from several institutions, including Michigan State University landscape hydrologist Anthony Kendall, whose role in the study was to calculate stream flow at all of Mooney’s sampling locations.

“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,” said Mooney, the study’s lead author.

The study’s primary goal was to understand how small tributaries contributed to what scientists call nutrient loading, where elements such as the nitrogen and phosphorus found in agricultural fertilizers run off the land and into downstream waters. These nutrients are also good at fertilizing algal growth and lead, among other things, to potentially toxic algal blooms and oxygen-starved dead zones.

And, indeed, the snapshot from the study showed that the six largest tributaries—rivers such as the Kalamazoo River in southern Michigan and the Fox River that runs through Green Bay—account for 70 percent of the nutrients entering Lake Michigan. But other trends emerged that pointed at unappreciated impacts that smaller streams have on the lake.

“Many if not most of the big streams entering Lake Michigan are monitored around-the-clock, but small streams have been mostly ignored because they don’t seem to offer the same bang-for-the-buck,” said Kendall, a research assistant professor in the Department of Earth and Environmental Sciences in the MSU College of Natural Science and study co-author. “After all, smaller rivers have smaller watersheds, and don’t seem to offer the same big view of water quality.”

Smaller tributaries often had nutrient loads that were high for their size and, more important to water-quality concerns, contained 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.

“But what we show in this study is that small streams probably have out-sized effects on the Lake Michigan coastline, Kendall explained. “If we want to have a holistic view of Great Lakes waters, we need to spread some of those dollars for measurements, observations, and study to more of the smaller streams as well.”

Not only do smaller tributaries tend to bring these more bioavailable nutrients to the lake, Mooney said, they also don’t have high stream flows.

“They aren’t blasting water out into the lake,” he noted, “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 the study.

“It’s hard to think of a Great Lake, such as Lake Michigan, as a singular lake,” he said. “It is just so massive and built up of all of these smaller segments of coastline that have different tributaries running in.”

In other words, no one experiences the lake on a whole-lake scale.

Mooney hopes the study can help resource managers target nutrient-reduction efforts on tributaries that will have the biggest impact for nearshore ecosystems or local communities.

“There is still a lot to learn about how these smaller tributaries influence coastal water quality throughout the Great Lakes”, he said.

“The work that we did here at MSU to help the lead scientists at UW–Madison highlights the collaborative nature of water sciences around the Great Lakes,” said Kendall. “We are all invested in learning more about what makes this incredible resource and ecosystem work, and how we might contribute to its preservation and restoration.”

Other study co-authors are Emily Stanley, UW–Madison Center for Limnology; Peter C. Esselman, Great Lakes Science Center, U.S. Geological Survey, Ann Arbor, Mich.; and Peter B. McIntyre, UW–Madison Center for Limnology and Department of Natural Resources, Cornell University, Ithaca, N.Y.

Via College of Natural Science 

Banner image: Pictured above is the Jordan River in northern lower Michigan, one of Lake Michigan’s nearly 300 tributaries. Photo: MSU Hydrogeology Lab