Feb. 23, 2016 (AP) — The U.S. and Canadian governments have called for deep cuts in phosphorus runoff from farms and other sources into Lake Erie, where an overload in recent years has fed harmful algae blooms that have fouled drinking water and killed fish.

The deal reached this week targets a 40% reduction of phosphorus for the lake’s central and western sections, a target previously endorsed by Ohio, Michigan and the Canadian province of Ontario. Those are the areas hardest hit by increasingly massive algae blooms. Last year’s was the biggest on record, while another in 2014 left more than 400,000 people in Toledo and southeastern Michigan unable to consume tap water for two days.

“The first step in our urgent work together to protect Lake Erie from toxic algae, harmful algal blooms and other effects of nutrient runoff is to establish these important phosphorus limits,” EPA Administrator Gina McCarthy said. “But establishing these targets is not the end of our work together. We are already taking action to meet them.”

Both nations agreed to develop plans for achieving the reductions within 2 years.

Scientists say fertilizers and livestock manure from the region’s farms are the primary generators of the type of phosphorus that feed harmful algae, although sewage from urban treatment plants and failed septic tanks also contributes. Although essential for aquatic food chains, algae can zoom out of control when waters contain excessive nutrients, including phosphorus.

A Great Lakes cleanup agreement in 1972 helped reduce algae levels in Lake Erie, but they began rebounding in the late 1990 and increasingly have consisted of bacteria that produce toxins. Levels of a toxin called microcystin in 2011 in Lake Erie reached levels 50 times above the World Health Organization limit for safe bodily contact.

Dozens of experts developed the reduction target using computer models that measured how particular changes in phosphorus levels would affect algae growth, officials said. They determined a roughly 40 percent decline would keep algae within acceptable levels for ecosystem health and prevent production of the toxins.

“A critical first step toward the restoration of Lake Erie,” said Mike Shriberg, regional executive director of the National Wildlife Federation.

But environmentalists said the biggest challenge would be crafting plans strong enough to achieve the cuts and making sure they are carried out.

“The U.S. EPA and Environment Canada must take a strong role in providing regional support and, when necessary, enforcement to ensure the states do what is needed to meet the targets released today,” said Molly Flanagan of the Alliance for the Great Lakes.