The trend is likely to continue over the coming century, heightening the urgency to control runoff of dissolved phosphorus that promotes excessive algae growth, said Don Scavia, director of the University of Michigan's Graham Sustainability Institute.
"Weather really matters now," Scavia said during a conference in Cleveland for scientists, advocates and government officials from the eight states and two Canadian provinces in the Great Lakes region.
"Climate change is likely to make reducing phosphorous loads even more difficult in the future than it is now, which will likely lead to even more toxic algae blooms and larger dead zones unless more conservation is undertaken," he said.
The modern farming technique of planting crops without plowing the ground may also be playing a role, Scavia said. While helpful in preventing erosion, no-till planting leaves high concentrations of phosphorus from fertilizer in upper layers of soil, where it more easily flows into waterways during downpours.
Runaway algae degrades water quality. As it decomposes, oxygen levels can drop low enough to kill fish. "Dead zones" where almost nothing can survive have formed in Lake Erie.
When rotting algae mats wash ashore, they foul beaches and chase away tourists.
After the U.S. and Canada signed the initial Great Lakes Water Quality Agreement in 1972, many of the region's governments banned detergents containing phosphorus and the algae problem faded. But it has returned in the past decade.
A regionwide task force reported this week that phosphorus-fed algae blooms are showing up on all five Great Lakes. They are particularly heavy in Lake Erie, the shallowest and warmest. Other hot spots include Lake Huron's Saginaw Bay, Lake Michigan's Green Bay and near-shore areas of Lake Ontario.
The task force, which included academic and government experts, recommended more than 50 steps. Among them: providing funding and technical assistance for phosphorus reduction projects; giving state regulators authority to require pollution reduction measures in stressed watersheds; and working with farmers and equipment manufacturers to develop fertilizer application methods that avoid runoff.
Dave White, chief of the U.S. Agriculture Department's Natural Resources Conservation Service, said the agency has supported environmentally friendly fertilization methods and other anti-runoff measures such as buffer zones and planting cover crops.
"We can fix these things, and we can fix them in harmony with agricultural production," White said.
But 21st century storms may pack such a wallop that those steps might not be enough, Scavia said.
"We may need a whole new set of best management practices to control dissolved phosphorous runoff" as the planet heats up, he said.