Mayflies have aquatic larvae that are sensitive to water pollution. Their absence can indicate poor water quality and high pollution. The Danube River near Tahitotfalu, Hungary was once a site of large mayfly swarms but the water was degraded by high levels of phosphates, nitrates and sewage. These pollutants can lead to conditions that deplete oxygen in the water. The mayflies cannot survive in low oxygen water and the mayflies disappeared from that stretch of the Danube.
Recent efforts have greatly reduced levels of phosphates and nitrates and upgrades to sewage treatment systems have left the water much cleaner. Now mayflies are able to grow and thrive in the river once more. This week, the mayflies put their stamp of approval on the cleanup efforts with the first swarm in 40 years. Millions of the mayflies emerged from the water covering the area with a blizzard of mayflies. Welcome back and here’s to good river health!
Mayflies have returned to the Danube River. Large swarms obscure a passing car from view.
Living With Insects Blog 
This is a very interesting blog post that gives insight into one aspect of the life of mayflies. Mayflies are only able to survive in freshwater and it is crucial to have access to this supply. In this article, I found it very interesting that the absence of mayflies can act as a warning sign to humans of poor water quality. This absence may occur in poverty stricken areas or those without a fresh supply of water. Although the mayflies spend much of their early lives in a water setting, they do not harm the water in any way. Mayflies are simply another example of an insect that has positive influences on humans.