The clearing of the water column is partly the result of the introduction and rapid spread of zebra mussels from Europe, which had the effect of covering the lake bottom, with each creature filtering a liter of fresh water each day, helping to restore the lake to a cleaner state. The 1972 Great Lakes Water Quality Agreement significantly reduced the dumping and runoff of phosphorus into the lake. The lake has since become clean enough to allow sunlight to infiltrate its water and produce algae and sea weed, but a dead zone persists. There have been instances of beach closings at Presque Isle because of unexplained E. Coli contaminations, possibly caused by sewer water overflows after heavy downpours.