<P> In 1999, Doppler radar weather sensors detected millions of mayflies heading for Presque Isle in blue and green splotches on the radar in clouds measuring ten miles (16 km) long . These insects were a sign of Lake Erie's move back to health, since the mayflies require clean water to thrive . Biologist Masteller of Penn State Erie declared the bugs to be a "nice nuisance" since they signified the lake's return to health after forty years of absence . Each is an inch and a half long; the three main species of mayflies are Ephemera simulans, Hexagenia rigida and Hexagenia limbata . The insects mate over a 72 - hour period from June through September; they fly in masses up to the shore, mate in the air, then females lay up to 8,000 eggs each over the water; the eggs sink back down and the cycle repeats . Sometimes the clouds of mayflies have caused power outages as well as causing roads to become slippery with squashed insects . Since zebra mussels filter extra nutrients from the lake, it allows the mayfly larvae to thrive . </P> <P> There have been incidents of birds dying from botulism, in 2000, and in 2002 . Birds affected included grebes, common and red - breasted mergansers, loons, diving ducks, ring - billed gulls and herring gulls . One account suggests that bird populations are in trouble, notably the woodland warbler, which had population declines around 60 percent in 2008 . Possible causes for declines in bird populations are farming practices, loss of habitats, soil depletion and erosion, and toxic chemicals . In 2006, there were concerns of possible bird flu after two wild swans on the lake were found diseased, but it was learned that they did not contain the deadly H5N1 virus . There were sightings of a magnificent frigatebird, a tropical bird with a two - metre wingspan, over the lake in 2008 . </P> <P> Lake Erie infamously became very polluted in the 1960s and 1970s as a result of the quantity of heavy industry situated in cities on its shores, with reports of bacteria - laden beaches and fish contaminated by industrial waste . In the 1970s, patches of the lake were declared dead because of industrial waste as well as sewage from runoffs; as The New York Times reporter Denny Lee wrote in 2004, "The lake, after all, is where the Rust Belt meets the water ." </P> <P> The water quality deteriorated partially due to increasing levels of the nutrient phosphorus in both the water and lake bottom sediments . The resultant high nitrogen levels in the water caused eutrophication, which resulted in algal blooms and algae masses and fish kills increasingly fouled the shoreline during this period . There were incidents of the oily surfaces of tributary rivers emptying into Lake Erie catching fire: in 1969, Cleveland's Cuyahoga River erupted in flames, chronicled in a Time magazine article which lamented a tendency to use rivers flowing through major cities as "convenient, free sewers"; the Detroit River caught fire on another occasion . The outlook was gloomy: </P>

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