Scientists with the National Oceanic and Atmospheric Administration expect this year's bloom to rank among the top five since it began measuring their severity in 2002, according to their annual algae forecast for the lake.
What's not known is how toxic it might be or whether it will pose a threat for cities in Ohio and Michigan that draw their drinking water from the lake.
Algae blooms that produce toxins and can sicken swimmers and harm fish are becoming an increasing concern nationwide, causing water warnings this year from Vermont to the Gulf Coast.
Environmental officials in New Jersey have been telling people to stay out of the state's largest lake since late June and beaches in Mississippi have been closed in recent weeks by a bloom fueled by polluted Midwest floodwaters pouring from the Mississippi River.
Lake Erie, the shallowest and warmest of the Great Lakes, has been hit particularly hard over the past decade. It was five years ago when a toxic bloom caused a two-day shutdown of drinking water in Toledo.
Researchers, using a scale for rating the severity of the bloom, expect it to be a 7.5 this year - short of the most severe outbreaks this past decade but still a significant one.
Studies have shown that much of the phosphorus that fuels the algae comes from fertilizer runoff from farm fields along with sewage treatment plants and other sources.
Unusually heavy rains this spring washed large amounts of nutrients into the lake, but the rain also prevented many farmers from planting corn and applying phosphorus-rich fertilizers on their fields.
That may be why the amount of phosphorus found in the lake's largest tributary, the Maumee River, was much lower than what would be expected, said Laura Johnson, director of the National Center for Water Quality Research at Heidelberg University.
The forecasters say one thing they're unsure about is how the record-setting water levels on the lake this year will impact the algae.
"Each bloom is different in how they behave," said Rick Stumpf, of the National Oceanic and Atmospheric Administration.
Where the bloom will go on the lake's western end along Ohio and Michigan will vary based on wind direction, but researchers point out that most of the lake will be fine for boaters and swimmers.
Other factors that will determine the size and location of this year's bloom in the coming weeks include rain amounts and the lake's water temperature.
Although some areas along the Ohio shoreline already have algae starting to form and drift into the lake, researchers don't expect to see a large outbreak until the end of July.
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