TOLEDO, Ohio -- Scientists and farmers agree that phosphorus from agriculture runoff is feeding the blue-green algae blooms on Lake Erie linked to a toxin found in the drinking water of 400,000 people in Ohio and southeastern Michigan last week.
Ohio's political leaders are calling for more studies to find out why the blooms are increasing and how to control them. Several environmental groups say it's time for strict regulations on the agriculture industry.
But how much of a role do the farms play?
Scientists say climate change has brought on heavy spring rains washing fertilizers off farm fields and lawns and causing sewer overflows in cities. That dumps more phosphorus in rivers and streams that flow into the lake.
Also, scientists believe invasive zebra mussels in Lake Erie have disrupted the food chain and helped the algae flourish.
Then there is the question of where all the phosphorus is coming from. It's found in farm fertilizers, livestock manure and raw sewage.
The Ohio Phosphorus Task Force -- a group in Ohio representing the agriculture industry, environmental researchers and state regulators -- concluded nearly two years ago that agriculture was the leading source of the phosphorus.
What's much more difficult to pin down is exactly where those sources are and why it's happening.
There's an assumption farmers are simply overfertilizing their fields.
But industry sales figures also show farmers are using much less fertilizer because of advanced technology.
Another assumption is a main source of phosphorus is manure produced by large livestock or dairy operations, which have increased dramatically.
But there's not enough monitoring to know if that's the culprit, researchers say.