Fluctuations in abundance of burrowing mayflies in Southern Indian Lake, Manitoba : mechanisms and implications for environmental monitoring

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Giberson, D. J.
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In the mid-1970's, Manitoba Hydro impounded Southern Indian Lake (SIL) and diverted much of the flow of the Churchill River into the Nelson river catchment for hydroelectric power generation. A subsequent dramatic decline in burrowing mayfly (Hexagenia) populations was originally attributed to the manipulation, until recovery of the population was observed despite ongoing physical impacts from impoundment and diversion. In 1986, an intensive study on Hexagenia in SIL was implemented to determine the actual mechanisms controlling population abundance in the lake to aid in the interpretation of the long-term monitoring data. The major factors affecting the population dynamics of Hexagenia in SIL during the study period were related to weather variables, primarily water temperature during development (and its relationship to air temperature during the open water period) and weather conditions (wind, air temperature) during the emergence period. The two species in the lake, H. limbata and H. rigida, approach their northern range limit at or near SIL. Their life cycles were similar in the study regions and closely related to thermal regime; requiring 3-yr in warmer areas of SIL and 4-yr in the coldest study region. Subimago emergence occurred from mid-July to mid-August in warm years and warm locations, but not until September in colder years or regions. September weather at SIL was often stormy and cold, and these conditions hindered emergence success. After emmergence, transformation to the adult (imago) stage was dependent on air temperatures; the final moult required ~24-48 h at 10-25oC and was unsuccessful at very low temperatures (near 0oC). Size of nymphs at maturity was related to thermal regime and the length of the life cycle. Mean body length of mature nymphs declined with decreasing thermal regime in the lake regions where 3-yr life cycles predominated. In the coldest region investigated, where the life cycle length increased from 3-yr to 4-yr, body size was similar to that from the warmest location. Body size in the female was strongly correlated to fecundity (number of eggs/female); therefore, mean fecundity showed the same relationship to temperature as body size. The link between nymphal development and temperature was confirmed in laboratory studies. Growth was noted at all temperatures evaluated (8o, 10o, 12.5o, 15o, and 20oC), and growth rates increased with increasing temperature for both species. Variations in growth rate affected both the time required for development and the size at maturity. Total development times (from egg hatch to first emergence) were 380 d at 15oC compared to only 183 d at 20oC, and individuals reared at 15oC were smaller and less fecund than those reared at 20oC. Growth and development was also affected by food availability: nymphs reared under limiting food conditions took longer to develop and adults were smaller and less fecund than those reared under non-limiting food conditions. Nymphal rearing density, at levels ranging from 215 to 1290 nymphs/m2, had no significant effect on growth. Egg hatch was related to temperature. Eggs hatched at temperatures from 8oC to 20oC (the highest temperature evaluated), but not at 4oC. Fewer eggs hatched and the time required for hatch was extended at the lower temperatures. Egg hatching success was improved by subjecting eggs to cold temperatures (4oC) during development. In addition, experiments indicated a bimodal hatching response in the eggs: some eggs hatched immediately if conditions were favourable, and the remainder apparently required exposure to cold to initiate hatch...