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Understanding Urban Eutrophication

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When you think about the primary sources of water pollution, you probably imagine a factory pipe or perhaps massive livestock farms. But would you believe that your quiet neighborhood could be degrading water quality locally and downstream?

That was the topic of the season finale of Institute on the Environment's Frontiers in the Environment lecture series on Wednesday, May 12, on the University of Minnesota's St. Paul campus.
In "A Watershed Approach to Understanding Urban Eutrophication," Sarah Hobbie, an IonE resident fellow and professor of ecology, evolution and behavior in the College of Biological Sciences, discussed how nutrients from lawns, pets and boulevard trees contribute to excessive algal growth in urban water bodies.

"We get a lot of benefits from the lakes that we have here in the Twin Cities: recreation, aesthetics, lakes support biodiversity, among other services," Hobbie said. "But our lakes are widely impaired by a process that we call eutrophication, which is overabundant algal growth that occurs because of excessive inputs of nutrients, particularly nitrogen and phosphorus."

So what are the sources of those nutrients in urban watersheds? According to Hobbie, biological fixation of nitrogen, imported nutrients from pet waste or fertilizer and atmospheric deposition can all contribute to nutrients in residential neighborhoods.

During rain or snowmelt, the nutrients on these landscapes runoff into local water bodies. This problem is exceptionally bad in urban areas, given how the landscapes were developed.

"In an urban watershed, we've constructed the landscape in order to move water very quickly from land to downstream because we don't want the landscape to flood," she said. "We have a lot of impervious surface, so the landscape doesn't drain very well. So we want to get that water off of our impervious surface to prevent flooding and move it downstream. We've created a very dense drainage network in our urban landscapes."

Managing nutrient runoff in urban areas is challenging, but strides are being made by placing a restriction on phosphorus fertilizers and implementing street sweeping programs to remove nutrients from the landscape before they runoff. Because different tree species are influenced uniquely by seasonal variations in climate finding an optimal time for street sweeping can be challenging.

"One of the things that we've been doing is trying to gather more data on how trees vary in their phenology of both the flower drop in the spring, and seed drop and also in litterfall," Hobbie said. "So we've established a citizen science monitoring network and this was actually done with an IonE mini grant.

"So this is linked into the National Phenology Network if you're familiar with that but we call it the Minnesota Phenology Network and we have about 30 citizen scientists who monitor phenology of street trees and submit their data to this public database through their smartphones or their computers."

The Minnesota Phenology Network isn't the only way citizens can have an impact. The nutrient sources from urban landscapes are typically the result of individual decisions. While this makes the sources difficult to control, it also provides an opportunity for homeowners to improve water quality with their everyday actions.

"If we think of where nutrients are coming from that ultimately come into this landscape, they're coming from fertilizer, they're coming from pets, they're coming from atmospheric deposition which is arising from combustion of fossil fuels," she said. "And if you think about all of these sources of nutrients, they are controlled by individual household decisions. So if we want to try to manage nutrient inputs to these watersheds, we need to think about how we can influence household decisions."

Watch Hobbie's full presentation online.

John Sisser is a communications assistant with the Institute on the Environment.


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