Life histories, microdistribution and drift of two mayfly (Ephemeroptera) species in the Pembina River, Alberta, Canada

Ephemerella inermis Eaton is a univoltine species that emerges in July. Eggs hatch in August and larvae grow rapidly in autumn and spring, but not during winter. Baetis tricaudatus Dodds produces three cohorts per year. Emergence periods occur in early June, July and early September. Only one cohort overwinters as larvae.
Microdistribution of both species is controlled largely by the discharge pattern of the river. During 1978, a year of frequent floods, animals were equally distributed among slow and fast water portions of the river. Nocturnal drift densities of both species were much greater in slow than in fast water areas. During 1979, a year of relatively stable flow, benthic larval densities were significantly greater in fast water than in slow water.
Early instar E. inermis larvae are most abundant in nearshore areas, but move to deeper water in late autumn. Baetis tricaudatus larvae are seldom numerous in near-shore areas.
Densities of both species were positively associated with concentrations of detritus and of other invertebrates, but only when relationships were considered in areas of similar current velocity independently of season.