A graduate student's road trip around Lake Michigan revealed the effects of small streams on the Great Lakes.
Rob Mooney, a graduate student from the Center of Limnology at the University of Wisconsin- Madison, embarked on a trip around one of the Great Lakes in the United States. Starting in the summer of 2018, Mooney has already accomplished eight circuits of the lake since then, monitoring seasonal changes in the lake and its tributaries - small streams that "feed" the lake.
His latest effort, according to an article from the university, is to try and take water samples surrounding Lake Michigan as fast as possible - stopping at the lake's almost 300 tributaries. However, natural obstacles - road crossings and encounters with wildlife - made collection at all sites impossible. After a six-day journey back to the lake, Mooney collected samples from 235 tributaries. While the expedition "was just cool in itself," Mooney found a bonus in the patterns he found in the tributaries.
Understanding Tributary Impacts on Lakes
Mooney noted that while tributary streams and rivers provide "excessive nutrient inputs" contributing to coastal degradation around the world, its role in coastal dynamics remains unknown due to the lack of monitoring and regulatory efforts - limited only to major tributaries.
Similarly, small streams can have a big impact on the lake where they flow into. Researchers sampled about 40 tributaries a day, with each sample taken as close to the lake as possible. Mooney, together with UW-Madison alumnus Will Rosenthal would then find a crossing and toss a bucket held by a rope, and collect their samples. These were later filtered into smaller vials before keeping them in a cooler and heading to the next location.
The study from UW-Madison aimed to characterize how these streams and rivers, flowing into lakes, affected a phenomenon called nutrient loading - where components usually in agricultural fertilizers, such as phosphorus and nitrogen, are drained and taken downstream. While these fertilize the area and encourage algae growth, they can also lead to toxic algal blooms as well as dead zones with extremely low oxygen levels.
Finding Outsized Nutrient Contributions
"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 said in a statement.
Subsequently, the snapshot taken from the study illustrated that six of Lake Michigan's largest tributaries including the Kalamazoo River and the Fox River are responsible for about 70 percent of the nutrients entering the Great Lake. Additionally, researchers found other trends suggesting underappreciated impacts that minor tributaries also have on the lake. Smaller streams and rivers that empty into the lake were found to have nutrient loads that were considerably higher for their size. These bodies of water also contained a higher percentage of phosphorus, a readily available resource for aquatic plants and algae.
"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," Mooney said.
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