While falling leaves blanket the University of Minnesota campus with maroon and gold, they may also be polluting metro-area lakes and rivers.

On rainy days, leaves wash into storm drains and their nutrients can build up, polluting urban lakes. University researchers found leaves account for about 60 percent of damage from too many nutrients in urban lakes. Excess nutrients increase algae growth — a common threat to lakes in the metro area.

When leaves fall in forests, they don’t cause problems, because the soil and surrounding plants recycle their nutrients.

“In the city you have a really different situation,” said Sarah Hobbie, an ecology, evolution and behavior professor. “A lot of the leaf litter actually falls into the street. And as those nutrients are released in decomposition, nothing can hold them into place.”

Forest resources junior scientist Christopher Buyarski, who conducted most of the experiments, said they identified the problem by filling bags with leaf litter, driving over them once a week to simulate traffic and testing the material once a month for carbon, nitrogen and
phosphorus.

At first, Buyarski said, the team expected leaves to break down slowly since the street isolated them from other plants and organic matter, but the opposite was actually true. The collected leaves shared space with dirt, grass and worms, and they decayed faster.

“You don’t normally think of streets being that productive,” he said.

Since excess nutrients can’t be recycled, they wash into storm drains. When the leaves progress into surrounding lakes, Hobbie said, they reduce species diversity and make lakes “smelly” and
unpleasant.

City street sweeping

Most cities purify water with rain gardens or stormwater holding ponds to dilute the leaf nutrients, but the methods are expensive and require space that some cities lack.

EEB assistant professor Jacques Finlay said the researchers are working on ways to demonstrate the benefits of street sweeping to city officials.

The researchers are also beginning to partner with some cities on their clean-up effort. Bioproducts and biosystems engineering research professor Lawrence Baker said excess nutrients have damaged about 140 lakes in the metro area.

Baker has worked closely with Prior Lake, Minn., and said sweeping streets three or four times during the fall removed more nutrients and cost less than other methods.

Most cities sweep streets twice a year — once in the late fall and once in the early spring, Baker said. Since leaves fall at different times throughout the season, this needs to happen more often to reduce
pollution.

The research team is developing workshops to show public officials the benefits of sweeping, Baker said. St. Paul and Minneapolis have both expressed interest.

Even if city officials decide to increase their street sweeping, it will take a few years to implement the new policies, Finlay said.

“I think we’re getting there,” he said. “It’s just hard to change the way things are done.”

Neighborhoods take charge

Some Minneapolis residents are tackling the algae problems in their neighborhoods.

In 2009, Janna Caywood  organized a neighborhood network to address the algae pollution in Como Lake.

“We saw all the algae out there, and it looked so sad,” she said. “We had a vague notion the city was working on a project. … We thought, ‘There has to be something we can do.’”

The group organized an annual project called the “Como Curb Cleanup,” which invites neighborhood residents to clean up and bag excess leaf material that accumulates in gutters, driveways and streets.

Caywood said they also inform neighbors how leaves — “public enemy No. 1” — pollute their lake.

“We think of ourselves as partnering with government agencies working hard on issues,” she said, “They can’t do it by
themselves.”