Agricultural land cover is a predictor of ecological impairment for streams. Photo: Catherine Riseng
Around a quarter of Michigan’s Great Lakes tributary streams and rivers are ecologically impaired, according to a recent study.
That means that those streams don’t support as much aquatic life as they ought to, based on their environmental profiles. In most cases of impairment, human activity like agriculture and urban development kept species diversity down.
While only around 5 percent of the state’s streams that drain into Lake Superior aren’t living up to their ecological potential, around 50 percent of those draining into Lake Erie are hampered by human stressors, according to the study.
“We think it’s a very reasonable picture of streams across the state,” said Catherine Riseng, study co-author and assistant research scientist at the University of Michigan’s School of Natural Resources and Environment. “Although in any one location, it may or may not be correct.”
The study’s trade-off – sacrificing pinpoint accuracy for a broad, statewide view – is part of the authors’ push for an ecosystem-wide perspective on Great Lakes restoration.
That means a perspective that accounts for the health of the entire Great Lakes watershed, not just the open lakes, or “the bowls”, as study co-author Jan Stevenson calls them.
“Our long term views of the Great Lakes and this region would be better served by not being stuck in the bowls, not being stuck in the land, but having a holistic view and understanding of the Great Lakes watershed as a whole,” said Stevenson, co-director of Michigan State University’s Center for Water Sciences.
Streams rated "poor" and "very poor" (orange and red) were considered impaired in the study. Click to enlarge.
Rating streams
To contribute to that watershed level of understanding, Riseng, Stevenson and their colleagues set about assessing the environmental health of every stream in the state by building one of modern science’s most thrilling tools: a statistical model.
The model assesses streams based what kind of fish and insect species swim there. And the kinds of fish and insects that ecologists would naturally expect to see in a stream is based on a bundle of questions, like: How big is the stream? How fast is it? How warm? Is it surrounded by forested wetlands or farms and cites?
The answers to these and other questions come from past stream studies across the state. The model uses the answers about a particular reach of stream to guess how many fish and insect species ought to live there.
If actual records show that a stream’s species diversity lives up to the model’s prediction, the stream gets a good score.
But if a stream’s fauna falls short – which typically occurs if the stream is surrounded by agriculture or urban development – the score gets worse and the stream is stamped “impaired.”
The problem with the model is that ecologists don’t have species records for many of Michigan streams. When you’re looking for a big picture of stream health across the state, that’s a big blank spot.
To fill in that blank, the researchers strung together another model based on the results of the first. This one can predict whether a stream’s species diversity is lacking based on other data about the stream and the land around it.
Between the two models, the study estimates that around 25 percent of Michigan’s 48,000 miles of Great Lakes tributaries are ecologically handicapped.
The watersheds of Lake Superior’s Sturgeon River and Lake Michigan’s Manistique and Menominee rivers scored the highest. Only around 2 percent of the streams in those systems were impaired.
Lake Huron’s Saginaw River, Lake St. Clair’s Pine River and Lake Erie’s Rouge River watersheds fared the worst. Those systems were between 70 and 90 percent impaired.
It's not all bad news: around 50 percent of Michigan's streams were ecologically acceptable or better. Photo: Riseng
The whole wide watershed
Scientists and environmental agencies are increasingly thinking about Great Lakes restoration in terms of the entire ecosystem, Stevenson said. They’re linking upstream causes to downstream effects.
But the programs that keep track of the Great Lakes ecosystem’s environmental health and restoration progress, like the binational State of the Great Lakes reports, are lagging behind.
“Almost all Great Lakes assessment to date remains on the open lakes and a relatively narrowly defined coastal zone,” the authors say in the study.
In other words, it’s “stuck in the bowls,” said Stevenson.
“To think about managing and restoring the Great Lakes is a bit different than managing and restoring the Great Lakes watershed,” he said.
One reason that the watershed deserves more attention is because that’s where people tend to use the natural resources for recreation and in other ways that “support our well being,” he said.
“I don’t have the statistics for this, but I’d say the average person uses streams and lakes of the Great Lakes watershed more than they actually use the bowls,” he said.
An ecosystem-based initiative for restoring the Great Lakes watershed wouldn’t take a brand new, billion-dollar plan, he said. But it will take better harmonization of the work that governments and scientists are already doing.
“Scientists tend to be up in the watersheds, or in the bowls. Policy makers tend to be up in the watersheds or in the bowls,” he said. “And we need to integrate across the shoreline, basically.”
Michael Wiley (of the University of Michigan’s school of natural resources and environment) and Paul Seelbach (of the U.S. Geological Survey’s Great Lakes Science Center) also contributed to the study.