Emergence of Ephemeroptera from the Assiniboine River, Canada

Abstract

Emergence traps set on sand and on cobble substrates in a riffle in the Assiniboine River, Manitoba, Canada over the open water seasons of 1990, ’91, ’93 and ’94, collected mayflies over virtually all of the open water season. Individual traps produced 108 to 5,799 individuals, representing approximately 50 species, and average numbers/m2/year varied from 500–3800 depending on the year. Number of individuals over sand and cobble were not significantly different. The diverse and abundant mayfly fauna is probably a result of the rich carbon sources and higher than expected temperature of the River. The emergence patterns were very different in each of the four years probably because summer thunder storms created large discharge increases which inhibited emergence. Flood control structures on the river appear to have increased low, under-ice flows and decreased peak spring and summer flows probably improving mayfly emergence success. A proposed new water withdrawal scheme may modify the effects of the control structures at least during the low water seasons.