Benefits and costs ofLeptophlebia(Ephemeroptera) mayfly movements between river channels and floodplain wetlands

Abstract

Dramatic drift of nymphs of the mayflies Leptophlebia Westwood, 1840 and Siphlonurus Eaton, 1868 (47 600 individuals/trap per hour) was observed between a southeastern USA river and adjacent floodplain following natural flooding events. Active movements of these nymphs against the water flow were also detected. Large numbers of leptophlebid nymphs reside in floodplains during the winter and spring, but how and why they colonize and develop in temporary habitats is uncertain. We detected few environmental advantages (i.e., temperature, food abundance or quality) for mayflies in floodplains compared with river habitats. Despite this, experiments indicated that mayflies had 36% higher growth rates in the floodplain than in the river and demonstrated tolerance to short-term drying (up to 12 d). Migration entails costs of migrating against flows during daylight when vulnerability to predation is high, colonizing a habitat subject to drying, hypoxia, and supporting amphibian predators, with few apparent benefits. Dispersal into wetlands may be attributed to avoiding swift river flows or exploiting largely fishless habitats. However, dispersal could be an evolutionary relic of behavior beneficial in more northerly habitats; our study populations of Leptophlebia mayflies were at the southern extreme of their range and perhaps operating suboptimally. Our results emphasize the importance of river–floodplain linkages to the ecology of both habitats.