Recurrence of Hexagenia Mayflies Demonstrates Improved Water Quality in Pool 2 and Lake Pepin, Upper Mississippi River

Abstract

Hexagenia mayflies are good indicators of general water quality because they have long life cycles and because their burrowing nymphs, which are unable to tolerate anaerobic conditions or swim long distances, live in sediments where toxins tend to accumulate. While chemical tests only describe water quality in terms of specific parameters and times, Hexagenia distribution indicates synergistic effects of many toxins, anoxia and other stresses throughout the year. Over 1,400 collections of imagoes and subimagoes along the Upper Mississippi River in 1957-1968,1976 showed that most of the 29 navigation pools supported large populations, as did impoundments upstream of Minneapolis-St.Paul. Populations were non-existant or meager in Pool 2 and Lake Pepin, however, due to METRO pollution. Collections made in 1986 showed that recent pollution abatement measures have enabled Hexagenia to attain nuisance levels in the two areas, thus establishing that mayfly distribution can be utilized to assess the well-being of a river which is so large that it is difficult to monitor effectively or economically by standard methods.