The Effects of substrate stability/discharge relationships on the aquatic insects of Wilson Creek, Manitoba

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Cobb, D. G.
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The aquatic insects were studied in three reaches of Wilson Creek, a small headwater escarpment stream in West-central Manitoba, to examine the effects of discharge/substrate stability relationships on the abundance, diversity and distribution of the insect community. Substrate stability ranged from a maximum of 80% to a minimum af 20% substrate at incipient motion at bankfull discharge. Immature insects were sampled monthly and following spates from May 1986 to May 1988 using a modified Hess sampler. Adults were collected three times per week from May to September in 1986 using 1m2 box emergence traps. A total of 11 mayfly, 15 caddisfly, and 10 stonefly taxa was collected during the study. Insect density and diversity were negatively correlated to discharge and substrate stability at all stations. A substrate stability model could be used to explain more variation in the data for the whole stream than discharge. When station effect was introduced into the model, the regression was improved for discharge, but not for substrate stability. The effects of a stable riffle constructed in the most unstable reach had a mitigating effect on the community of insects. Insect density declined up to 94% following spates in the unstable reach, but not at the introduced stable riffle. The stable riffle supported a larger proportion of filter feeding caddisflies than the unstable reach during low flows. Life history adaptations were examined for 22 mayfly, stonefly and caddisfly species in the most unstable reach in relation to frequency, intensity and duration of disturbance, as indicated by substrate movement. Forty-one per cent of species examined were univoltine, with winter growth, and emerged prior to spring floods. Another 41% of species had complex life cycles, with multivoltinism or multiple cohorts. One species was semivoltine, but was collected infrequently. Life history adaptations of most species are suited for existence in an unstable, frequently disturbed habitat. It is concluded that substrate stability is an important determinant of diversity and abundance of aquatic insects in Wilson Creek, and should be considered when characterizing disturbance and its role in controlling aquatic insect communities.