Despite the annoyance that mayflies pose each June when they swarm across the region, Ohioans have generally held the broad-winged insects in high esteem because of what they have symbolized: Lake Erie’s recovery.

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Now, a decade after pollution dissipated enough for mayflies to make their comeback, some researchers believe they could be hurting the lake by helping algae grow.

The theory is that they stir up tiny particles of nutrient-laden sediment, a subtle action at the lake’s bottom that might even make the central basin’s infamous “dead zone” worse. Those tidbits were among the relatively new pieces of information emerging at yesterday’s annual Ohio Lake Erie Conference, which drew nearly 200 people to Lorain County Community College. The event was sponsored by the Toledo-based Ohio Lake Erie Office, an arm of the governor’s office, which reports to state agency directors.

Other items included a report about Ohio’s first attempt to predict beach bacteria for Cleveland-area swimmers, which could be the wave of the future.

The mayfly theory is just that: a theory. It was floated by Jeff Reutter, Ohio Sea Grant executive director and head of Ohio State University’s Stone Laboratory on Gibraltar Island.

But Mr. Reutter said it illustrates how deep researchers are going to seek answers about a toxic blue-green algae known as microcystis, which has re-emerged between Toledo and Sandusky almost every summer since 1995, after vanishing for about 20 years.

The algae is in full bloom in the lake’s western basin and is expected to stay there until lake temperatures cool late this month. Just two years ago, near South Bass Island, it was 60 times greater than what the World Health Organization deems acceptable, he said.

Phosphorus, a nutrient, feeds algae. Algae robs water of oxygen. Major sources of phosphorus are sewage overflows and runoff from farm fertilizers.

But David Baker, director emeritus of Heidelberg College’s National Center for Water Quality Research, said the lake’s higher phosphorus levels over the last 10 years go beyond inputs.

Zebra mussels excrete phosphorus they take in through the water, disrupting the lake’s balance between that nutrient and nitrogen. But phosphorus levels might be driven up by millions of mayflies as they burrow in sediment during their first two years as nymphs.

“It’s like a lot of things,” Ken Krieger, a Heidelberg mayfly researcher, said. “One mayfly doesn’t make a difference. A million might.”

Also yesterday, Ohio’s first try at predicting bacteria levels at a public beach got mixed reviews from the project’s lead researcher, Donna Francy of the U.S. Geological Survey.

Would-be visitors at Huntington Beach in Bay Village, Ohio, west of Cleveland, had the chance this summer to see the anticipated bacteria levels there at www.ohionowcast.info. The formula was based on factors including wave height, turbidity, and rainfall within 48 hours of a given date, Ms. Francy said.

The formula worked 80 percent of the time. But she said it was too conservative and underestimated the upcoming day’s bacteria on too many dates.

She said officials will tweak the formula for that beach next summer and may try it at three others in the Cleveland area. No plans have been made to try it in the Toledo area.