Hexagenia Density and Distribution in the Detroit River
Larval Distribution, 1999
Few changes in either the distribution or abundance of mayfly nymphs were seen between the 1980 survey, a 1983 investigation (Hudson et al. 1986), and a study done in 1991 (Farara and Burt 1993). In 1991, Hexagenia mayflies were found at about 60% of locations sampled, at densities of between 8 and 100 nymphs per square meter (
Figure 4). However, more of the river supported densities slightly less than the 20 per square meter criterion suggested to indicate impairment (Ciborowski 2003b) than had been observed in 1980. Worms and midges remained the most common invertebrates along the United States shoreline of the river downstream from Zug Island.
Little benthic sampling was conducted in the Detroit River through most of the 1990s, so information on health of the zoobenthic community during this time is scarce. However, the flying adult stages of Hexagenia and other aquatic insects became more numerous along both the Canadian and United States sides of the river and along the shores of Lake Erie (Ciborowski and Corkum 1988; Kovats 1990; Kovats et al. 1996; Corkum et al. 1997) suggesting that some improvements in river condition had been occurring.
The Detroit River was next intensively studied in 1999. Samples were collected from almost 150 locations (Wood 2004;
Figure 5). Densities exceeded the 20 nymph per square meter impairment threshold at the head of the river and on the Canadian side of the lower reaches. Few nymphs were collected in the midreaches, however.
Figure 5. Density of
Hexagenia in the Detroit River in 1999 interpolated from data compiled from Wood (2004). Areas containing fewer than 20 nymphs per square meter indicate either degraded benthic conditions (if sediments are soft) or unsuitable habitat for mayflies (if sediments are hard). Map by Anita Kirkpatrick.
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