Though it can be a bit unsettling to see, the mayfly hatch is an annual summer rite that the Mississippi River has long missed. So much so that the recent return of mayflies should be a welcome sign of a healthy river and improved water quality for communities along the Mississippi River.
From the 1960s to the 1970s little or no treatment of sewage water meant that cities were flushing their toilets down the Mississippi River. As Minnesota’s population increased, so did the amount of sewage. In fact, there were often reports of "cakes of fecal matter" floating on the river’s waters.
For the delicate mayfly — which is sensitive to chemical pollutants, increases in sediment and decreases in oxygen levels in the water — pollution ensured the collapse of their populations. By the 1980s, mayfly hatches had disappeared from rivers and streams in Minnesota. The collapse of mayflies from the aquatic food chain also meant the disappearance of stoneflies, caddis flies and even some species of fish from the Mississippi River.
Adult mayflies spend 99 percent of their lives as nymphs on the water, being fed upon by other invertebrates, amphibians, reptiles, fish, birds and mammals. MPCA water quality expert Will Bouchard explains, “The larvae or nymphs spend a year burrowed in the sediments of the river and during this relatively long larval cycle they can be exposed to toxic chemicals in the sediment or low levels of dissolved oxygen. As a result, this mayfly can be a good indicator of water quality because these forms of pollution can kill the larvae.”
Today, modern sewage treatment facilities and chemical disposal regulations have brought back mayfly populations. “The large swarms of mayflies emerging from the Mississippi River are an indication that the river has recovered considerably since the days when it was essentially an open sewer,” says Bouchard.
Mayflies often emerge in synchronized hatches, usually in small clouds, but once in a while they arrive as frenzied masses. Mayflies go as quickly as they come, mating and laying their eggs in less than a month.
Maintaining healthy population of mayflies and the fish that feed on them is important. You could be polluting our waters — without even knowing it — by sweeping grass clippings into the street, not cleaning up pet waste, overusing fertilizers, and more. The challenges mayflies face today are the chemicals that often come from homes and yards and head downstream into major waterways. Flushed pharmaceuticals and medicine also pose a threat to the fish that feed on mayflies.
So next time you see a cloud of mayflies, remember that even though it may look like a scene out of a horror movie, their healthy population means improved water quality for our state.