Long-term abundance patterns of Hexagenia (Ephemeroptera: Ephemeridae) in Southern Indian Lake, Manitoba: Responses to weather and hydroelectric development

In the mid 1970s, Manitoba Hydro impounded Southern Indian Lake (SIL) and diverted much of the flow of the Churchill River, which flowed through SIL, into the Nelson River Catchment for hydroelectric power generation. A dramatic decline in burrowing mayfly (Hexagenia ) populations following lake manipulation was originally attributed to the physical effects of impoundment and diversion. Further monitoring, however, showed a recovery of the population, despite the ongoing physical impacts, so other factors were investigated in an attempt to explain the post-impoundment decline in Hexagenia abundance. Patterns of population abundance in five of the lake regions were apparently due mainly to weather, whereas those in the remaining four regions resulted from impoundment and/or diversion effects as well as weather.