Scientists have concluded it's going to be more difficult than they initially thought to reduce phosphorus loads into Lake Erie by 40%.
That's the target set by Great Lakes states.
Phosphorus is a crop fertilizer that also encourages cyanobacterial blooms.
Rebecca Muenich worked on a study to evaluate ways to control the blooms.
Meunich is a former researcher at the University of Michigan's Graham Sustainability Institute, and now an assistant professor at Arizona State University.
Meunich says it's hard to identify which farms are most responsible for excess phosphorus. Another problem is so-called "legacy" phosphorus.
"Likely there is a lot of buildup of phosphorus in the soils, maybe in the stream banks and that's being released, that hasn't been applied this year," she says.
Meunich says there are methods known to be effective at reducing phosphorus from getting into streams. One is simply to apply less fertilizer, applying it only where it is needed.
Another is sub-surface application of fertilizer.
"There are ways to get that phosphorus into the soil so it's closer to where the crops need it," says Meunich. "And then it's not as vulnerable to runoff."
Another control measure that is effective is the planting of strips of vegetation in between farm fields and nearby waterways.
Meunich says she hopes the study encourages people to increase their efforts to push for better policies and initiatives to protect the Great Lakes.
Environmental groups say Great Lakes states should make the implementation of phosphorus control measures mandatory for owners of crop farms and livestock farms. Most programs currently are voluntary.