Blame It On The Rain: Study Ties Nutrient Loading in Lakes to Extreme Precipitation Events

While April showers might bring May flowers, they also contribute to toxic bacteria blooms, dead zones and declining water quality in U.S. lakes, reservoirs and coastal waters, says a new study published online by the journal Limnology and Oceanography.
In the Midwest, the problem is largely caused by phosphorus, a key element in fertilizers that is carried off of the land and into the water, where it grows algae as easily as it grows corn and soybeans. Previous research had found that our waterways receive the lion’s share of their annual phosphorus load in only a couple dozen events each year, says Steve Carpenter, lead author of the paper and director emeritus of the University of Wisconsin-Madison’s Center for Limnology. This new paper ties the phosphorus pulses “unequivocally” to extreme rain events, he says.

Steve Carpenter. Photo by: Jeff Miller, UW-Madison Communications

Carpenter and his colleagues used daily records of stream discharge from the U.S. Geological Survey to measure the amount of phosphorus running into Lake Mendota in Madison, Wisconsin from two of its main tributaries. The dataset spanned two decades, from the early 1990’s to 2015. They then looked at long-term weather data and found that big rainstorms were followed immediately by big pulses of phosphorus
To make matters worse, big rain events are predicted to become more common in most climate change scenarios, says Carpenter. In fact, the study found that this is already likely the case.
During the two decades they examined for stream phosphorus data, says Carpenter, “seven of the eleven largest rain storms since 1901 occurred.”
“This analysis shows that extreme rainfall is responsible for a large amount of the phosphorus that flows into inland waters,” says John Schade, a National Science Foundation (NSF) program director for long-term ecological research. And that phosphorus presents a big challenge for maintaining water quality. “We need to develop nutrient management strategies to meet the challenge,” Schade says. “Without long-term data like those presented here, the impact of these events would be difficult to assess.”
Carpenter agrees. “Without long term data, this study would have never happened,” he says. And, he adds, new strategies for managing nutrient runoff are undoubtedly needed.
David Garrison, chair of the National Science Foundation’s Long-Term Ecological Research working group agrees. “The study’s findings are important for maintaining water quality not only today, but into the future,” he says.
Aerial view of a boat wake in Lake Mendota, showing a trail of algae churned up to the surface. Photo: Carol Gillen

Maintaining that water quality starts on the land. Currently, farmers and conservation groups use all sorts of strategies to try to slow water down and capture some of the sediment and fertilizers it carries with it as it runs off of a field. But, “we’re not going to solve this problem with buffer strips or contour plowing or winter cover crops,” says Carpenter. Although those practices all do help, he says, “eventually a really big storm will overwhelm them.”
Since local land managers can’t control the weather, Carpenter says, the best available option for protecting our water quality is keeping excess phosphorus off the landscape in the first place. After all, a rainstorm can’t wash fertilizer or manure downstream if it isn’t there.
“I think there are some farmers who are having some success in decreasing their soil phosphorus and maybe we could learn from them,” he says, noting that, while there are countless acres in the Midwest that are completely over-saturated with phosphorus, there are also places that aren’t – suggesting that it is possible to keep loads low.
At the very least, he says, it’s an encouraging sign that, in the future, maybe not every spring shower will bring an inevitable summer algae bloom.

CONTACT: Steve Carpenter – steve (dot) carpenter (at) wisc (dot) edu – 608-262-3014

A full version of the report can be found here.

Top Photo Courtesy: Eric Booth, UW-Madison