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The vertical distribution within the substrate of three lotic mayflies (Rhithrogena sp., Paraleptophlebia sp. and Baetis tricaudatus Dodds) was examined in laboratory streams to determine if diel patterns of vertical movement across substrate surfaces existed and whether light intensity, temperature, substrate composition or food availability influenced the observed movement patterns. 2. Rhithrogena sp. and B. tricaudatus exhibited significant diel movement from lower substrate surfaces during the day to upper surfaces at night, although >64% of B. tricaudatus occupied the upper surfaces throughout the 24 h period. In contrast, Parakptophlebia sp. larvae did not change their vertical distribution over the 24 h period. 3. The proportion of mayflies occupying substrate surfaces was influenced by the amount of upper and lower surface area available in the substrate and, possibly, crevice sizes associated with lower surfaces. 4. The movement of Rhithrogena sp. from lower surfaces during the day to the upper surfaces during the night was influenced by changes in light intensity, but not by changes in food availability or water temperature. 5. The diel patterns of vertical movement exhibited by lotic mayflies in this investigation raise the possibility that at least some biotic interactions may vary over a diel cycle. These could include competitive interactions or temporal cycling of prey availability to predators. [ABSTRACT FROM AUTHOR]

The foraging and predator-avoidance behaviors of mayfly (Baetis tricaudatus) nymphs and predator--prey interactions were investigated under differing combinations of cadmium concentration and predation risk. Predators consisted of stonefly (Kogotus nonus) nymphs, juvenile brook trout (Salvelinus fontinalis), and longnose dace (Rhinichthys cataractae), either alone or in combination. Organisms were exposed to 0, 0.5, or 5.0 µg/L Cd for 7 d (invertebrates) or 30 d (vertebrates) prior to observation in artificial stream channels. Behavioral observations (20-min duration) were made both day and night over a 24-h period. Mayfly feeding rate was assessed by image analysis of grazed diatom mats cultured on ceramic tiles. Both predators and cadmium produced significant effects on the behavior of B. tricaudatus and K. nonus nymphs, with mayflies showing a hierarchical response in the presence of multiple predators. A significant increase in activity was observed in S. fontinalis exposed to 5.0 µg/L Cd, with significant decreases in capture efficiency of prey items apparent at 0.5 µg/L Cd. Conversely, the foraging behavior of R. cataractae was unaffected by cadmium, possibly as a result of species-specific cadmium tolerance. Such sublethal effects of a contaminant on the energy budgets of individuals and the interactions between species have implications for community structure in aquatic food webs [ABSTRACT FROM AUTHOR]