Spring runoff from farm fields can wash lots of phosphorus and nitrogen into nearby waterways. When these nutrients arrive in slower waters and the weather warms up, it can cause harmful algae blooms that make water dangerous for recreation and drinking.
Wisconsin Public Radio reports:
Steve Carpenter, director emeritus of the University of Wisconsin-Madison’s Center for Limnology, said blooms of toxic cyanobacteria come from high levels of phosphorus pollution, which often comes from manure spread on farm fields. These blooms are more likely to occur after floods.
“We’re having perfect storm conditions for runoff right now,” Carpenter said. “There’s snow melting, ice melting and we’ve had some big rains on it. That large amount of water can move a lot of phosphorus into streams and lakes.”– Spring Flooding Could Mean Summer Algae Blooms, March 20, 2019, WPR
The Lower St. Croix River is listed as impaired by the Environmental Protection Agency for too much phosphorus in its waters. Scientists from the St. Croix Watershed Research Station have determined that harmful types of algae have been increasing since the 1960s.
“Blooms thrive in shallow, warm, non-moving bodies of water,” the Saint Croix National Scenic Riverway says. “High levels of nutrients like nitrogen and phosphorous, warm water temperatures and high light levels – or a combination of all three factors – may stimulate the rapid reproduction of algae until it dominates the local aquatic ecosystem, forming an algal bloom.”
Significant efforts are being made by local, state, and federal government agencies, as well as farmers and other property-owners, to reduce nutrients flowing into the river.
“It’s always a bad idea to spread manure during high-risk runoff times, and we strongly advise against it,” says Richard Castelnuovo, chief of resource management with the Wisconsin Department of Agriculture, Trade and Consumer Protection.
To reduce nutrient runoff, farmers are encouraged to use Wisconsin’s Runoff Risk Advisory Forecast tool before spreading manure on their fields.