TRAVERSE CITY, Mich. — It was the largest algae bloom in Lake Erie’s recorded history — a scummy, toxic blob that oozed across nearly one-fifth of the lake’s surface during the summer and fall of 2011.
It sucked oxygen from the water, clogged boat motors, and washed ashore in rotting masses that turned beachgoers’ stomachs.
It was probably an omen of things to come, experts said in a study released Monday.
The warming climate and modern farming practices are creating ideal conditions for gigantic algae formations on Lake Erie, which could be potentially disastrous to the surrounding area’s multibillion-dollar tourist economy.
The shallowest and southernmost of the Great Lakes, Erie contains just 2 percent of their combined waters but about half their fish.
According to the report, which was compiled by more than two dozen scientists, the 2011 runaway bloom was fueled by phosphorus-laden fertilizers that were swept from corn and soybean fields during heavy rainstorms. Weak currents and calm winds prevented churning and flushing that could have short-circuited its rampant growth.
The blend of natural and man-made circumstances “is unfortunately consistent with ongoing trends, which means that more huge algal blooms can be expected in the future unless a scientifically guided management plan is implemented for the region,” said the report’s lead author, Anna Michalak, of the Carnegie Institution for Science.
The United States and Canada limited the use of phosphate laundry detergents and cracked down on Great Lakes pollution from industry and municipal sewage systems four decades ago. Those policies led to a drastic algae drop-off in Lake Erie, which had been declared all but dead.
But algae began creeping back in the mid-1990s, and the blooms have gotten bigger.
They consist largely of blue-green strains that are poisonous and cause skin irritation. Measurements in 2011 found that concentrations of a liver toxin they produce were hundreds of times higher than levels approved by the World Health Organization for drinking and recreational waters.
The building blocks of algae blooms, particularly phosphorus, are well known. The new paper was compiled by experts from a range of disciplines to determine why the 2011 bloom got so huge and whether it’s a harbinger of things to come.
At its peak, that bloom covered 1,930 square miles, making it more than twice as big as the freshwater sea’s second-biggest bloom on record, which happened three years earlier.
Published in the online version of Proceedings of the National Academy of Sciences, the report said soil management practices in the region’s fields are partly to blame.
One such practice is no-till farming, in which seeds are planted in small holes and the ground is not plowed. While it helps the environment by preventing erosion, no-till farming keeps fertilizer in the upper soil.
Jeff Reutter, director of Ohio State University’s Stone Lab on Gibraltar Island, said at the recent Lake Erie Waterkeeper conference at Lourdes University that the practice of applying fertilizer and manure to agricultural fields in winter, when the ground is frozen, also contributes to the algae problem.
When the fertilizer and manure are hit with spring rains, the materials wash into ditches and streams and quickly make their way to the lake.
Mr. Reutter said the very dry 2012 essentially gave the lake a reprieve from the massive algal blooms, but that respite could be short-lived.
“We bought some time, due to the lack of major rain events in 2012, but if that trend changes again and we have another wet spring, we should expect to see the bloom return.”
Mr. Reutter said communication with the farming segment on the topic of phosphorus and nutrient loading has been positive, and he hopes to see conditions improve as farmers become more involved in limiting the amount of nutrient runoff that reaches the lake.
The trigger for the 2011 bloom came in the form of heavy spring storms.
The bloom formed that July around the mouth of the Maumee River. Under normal circumstances, choppy waters might have diluted the phosphorus and broken up the bloom. Instead, a calm spell enabled it to keep growing.
By October, it had zoomed past Cleveland — more than 100 miles to the east — and penetrated the lake’s central basin, where decomposed algae had already created an oxygen-deprived “dead zone” lethal to most fish and other aquatic organisms.
Scientists are studying how the algae outbreak might have affected fish populations but have reached no firm conclusions, said Jeff Tyson, Lake Erie program administrator with the Ohio Department of Natural Resources.
Numbers of the lake’s most prized sport and commercial species, walleye and yellow perch, have dipped recently in the fertile western basin. But because so many factors affect them, it’s uncertain what role — if any — the algae has played.
The lake’s algae cover was about 90 percent smaller during drought-stricken 2012. But the scientists analyzed computer models and concluded that as the planet warms over the next century, weather that fueled the 2011 mega-bloom may become “the new normal,” Ms. Michalak said.
Slowing climate change would require action on a global scale. But significant cuts in Lake Erie’s phosphorus levels could be achieved with different fertilizing techniques, scientists said.