The forecast for western Lake Erie for later this summer into fall?: Green and mucky.

The algae blooms that have plagued the lake in recent years are expected to be worse than normal this year, well above the size at which they can potentially become harmful to aquatic life and even humans, National Oceanic and Atmospheric Administration forecasters announced Thursday.

This year's algae bloom, which usually begins in late July and can continue into October, is expected to measure 7.5 on a severity index developed by NOAA and other researchers, but could range between 6.5 and 9.5. The largest blooms since the problem returned to Lake Erie in the late 1990s, in 2011 and 2015, were 10 and 10.5, respectively, on the severity index.

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The prediction is based on five modeling efforts conducted by NOAA and other researchers, including the University of Michigan, looking at factors including rainfall that pushes phosphorus off farmers' fields into rivers and other tributaries that feed the lake. That fertilizer, combined with sunlight and the relatively shallow western Erie basin, has in recent years led to widespread algal bloom outbreaks.

In addition to making swimming, boating, fishing and other lake-related tourism a potentially mucky mess, impacts from the blooms include blocked sunlight affecting aquatic organisms and fish choking with clogged gills. And when large algae blooms die, their decomposing rot can eat up much of the oxygen in the water, creating hypoxic dead zones where few living organisms can survive.

In western Lake Erie, blooms of blue-green algae, or cyanobacteria, can produce a toxin called microcystin, which can be harmful or even fatal to humans and animals. And boiling water doesn't help with microcystin; it makes it more concentrated.

It was alarming levels of this toxin that caused a do-not-drink advisory that shut down the water supply for almost 500,000 people in Toledo and southeast Michigan for a weekend in August 2014.

A large algae bloom doesn't necessarily mean the microcystin water problem is again imminent, said University of Michigan aquatic ecologist Don Scavia, a member of the forecast team, in a release. Local weather conditions, including wind direction and water temperature, play a role, he said.

"Even so, we cannot continue to cross our fingers and hope that seasonal fluctuations in weather will keep us safe," he said.

The bloom is shaping up as much larger than last year's, which only ranked at 3.2 on the severity index. And the 2016 bloom was preceded the previous year by the largest in recent record. The difference year by year? The amount of rain, and when it occurs.

"We've had quite a lot of rain — six major storm events from March 1 to July 12," said Laura Johnson, a research scientist with the National Center for Water Quality Research at Heidelberg University in Tiffin, Ohio.

"It seems like it's water volume and rain that drives all of the patterns we're seeing for phosphorus loading."

While it was the 2014 bloom when the Toledo water scare occurred, a microcystin threat didn't materialize the following year, during the largest algae bloom in western Lake Erie to date. The reason, researchers say, is there are many different types of algae and cyanobacteria, and while the size of a bloom can be predicted, scientists can't yet forecast a bloom's toxicity.

"One of the problems (in Toledo in 2014) was it was quite a toxic bloom, and it started quite early as a toxic bloom," said Rick Stumpf, an oceanographer with NOAA's National Center for Coastal Ocean Science.

How deep an algae bloom is present in the water column is another factor in how and whether water supplies — with their deeper intake pipes in the lake — are affected, he said.

"Generally, if you see a bloom well, it's a boater's problem; if you don't see it well, it's a water treatment plant's problem," Stumpf said.

But those enjoying western Lake Erie shouldn't guess as to whether an algae bloom can harm them; they should assume it can, he said.

"I can't emphasize enough: If there's gunk, stay out of the water," Stumpf said. "And if you have a dog, keep it out of the water. Every year in the United States, dogs die from microcystin toxin."

The bloom's location varies from year to year, driven by weather patterns, but the central and eastern portions of Lake Erie are typically spared.

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Some of the phosphorus driving the algae blooms comes from so-called point sources, like the huge Detroit Wastewater Treatment Plant. But the bigger culprits are nonpoint sources, the thousands of farms where runoff from fields reaches rivers or other tributaries feeding western Lake Erie. The biggest culprit, researchers say, is the Maumee River, running 137 miles from Ft. Wayne, Ind., to Toledo, lined with farms throughout the route. An estimated 85% of the phosphorus entering Lake Erie from the Maumee comes from agricultural sources.

To combat the algae blooms, Michigan and other Great Lakes states, as well as Ontario, have set a goal of reducing phosphorus loads into western Lake Erie by 40% less than 2008 levels, by 2025. Officials cite improvements in wastewater treatment and reform of farming practices, such as avoiding applying manure on frozen or saturated fields. Some, however, believe the goals will be unattainable unless the changes to farming practices move from voluntary to mandatory.

"What we've learned is, it's the load from the tributaries, the load from the Maumee that matters," Stumpf said. "If we get that under control, we will reduce the loads of phosphorus" in Lake Erie.

Contact Keith Matheny: 313-222-5021 or kmatheny@freepress.com. Follow on Twitter @keithmatheny.