One of the more interesting natural phenomena along western Lake Erie is the appearance of vast swarms of mayflies (Ephemeroptera) each spring. Aquatic mayfly nymphs emerge from the mud this time of year, molt into adults, and spend several days mating and laying eggs. Their numbers can become so large that they appear on weather radar, as shown by this image from Toledo news station 13ABC:
Although thick congregations of mayflies on streets, sidewalks, and buildings can be a bit of a nuisance, their abundance can be an indicator of favorable water quality. Many species are sensitive to water pollution, and their presence (along with other insects like caddisflies and stoneflies) is often a sign of relatively low pollution levels. Although regulations on pollution have improved the health of Lake Erie since the 1960s and 1970s, livestock waste and plant fertilizer runoff from surrounding farms continue to be ongoing concerns.