HISTORIC CHANGES IN POPULATIONS OF BURROWING MAYFLIES (HEXAGENIA-LIMBATA) FROM LAKE ERIE BASED ON SEDIMENT TUSK PROFILES

One of the most dramatic recorded changes in benthic invertebrate comm unity structure due to eutrophication was the elimination of the burro wing mayfly, Hexagenia limbata, from the western basin of Lake Erie. T his was a catastrophic event occurring as a result of two periods of a noxia in the summers of 1953 and 1955. From the examination of sedimen t cores, the changes in historic populations of Hexagenia have been re constructed from enumeration of mandibular tusks. The changes in the n umbers of tusks found in the sediment cores from the western basin of Lake Erie agree well with known historic events but suggest that numbe rs found in the earliest field samples represent a considerable increa se over historic populations. Cores taken from the central basin sugge st that Hexagenia was formerly only found in a band around the shore, and populations never occurred in the offshore waters. It is suggested that this is strong evidence for the occurrence of brief periods of a noxia in the central basin of Lake Erie prior to cultural eutrophicati on.