Diel epibenthic activity of mayfly nymphs, and its nonconcordance with behavioral drift l

Diel changes in the numbers and activity of mayfly (Ephemeroptera) nymphs on the substrate surface in a stony-bottom stream were determined by direct observations, to investigate the relationship between benthic activity and drift. Individuals were viewed through a glass box (0.1 m2 area) at 2-h intervals for 24 h; dim red light was used during darkness. Baetis was relatively unaffected by the presence of the observer, whereas Cinygmula was very easily disturbed. The number of individuals in view, activity per individual, and total activity all were greatest by day and least during the night for both species. Each of these measures was significantly correlated with temperature, which varied 6”-8°C over the diel cycle. Drift activity, in contrast, was strongly nocturnal and generally peaked immediately after nightfall. Because of the clear lack of correspondence between drift and the activity of mayfly nymphs on stone tops, behavioral drift in this system cannot be explained as the passive consequence of foraging. The downstream transport of organisms in rivers, termed drift, has been the subject of many studies (e.g. Waters 1969, 1972; Miiller 1974). Particular attention has been focused on the pronounced diel rhythm in the drift activity of crustaceans and immature insects, which typically exhibit low daytime activity, a sharp increase in numbers drifting just after dark, and a high but variable level of drift during the hours of darkness. After the independent discovery in about 1960 of the diel periodicity of drift (Tanaka 1960; Waters 1962; Miiller 1963), several investigators attempted to determine whether drifting individuals represented accidental, passive entry into the water or an active and purposeful entry. The passive explanation emphasizes the risks of dislodgement experienced by organisms living in running, often torrential waters, whereas the active model emphasizes purposeful movements between habitats. By about 1970, accumulating evidence seemed to strongly favor the passive model. Numbers drifting appeared to correlate well with changes in discharge but poorly with changes in benthic density, suggesting that physical risk of dislodgement mattered more than crowding (Elliott 1967). Laboratory studies of mayflies (Ephemeroptera), abundant in the drift, revealed a negative phototaxis ’ Supported by NSF grant BSR 82-14487. and an endogenous rhythm of higher nocturnal activity (Elliott 1968). Thus, the tentative hypothesis emerged that insect larvae hid under stones by day, probably inactive but possibly feeding (e.g. Chapman and Demory 1963), and moved to stone tops to feed on attached algae, transported particles, etc. after dark. The observed nocturnal increase in numbers drifting is explained as the consequence of a nocturnal increase in densities on the substrate surface, together with accidental dislodgement (e.g. Elliott 1967,1968; Bishop 1969; Bishop and Hynes 1969; Chaston 1972). This argument was supported by evidence from insect gut analysis, which indicated that insects fed more by night than by day (Chapman and Demory 1963; Meier and Bartholomae 1980; Ploskey and Brown 1980). Evidence that individuals that drift by day suffered greater risk of predation than individuals that drift by night was provided by Allan (1978) and Newman and Waters (1984). Thus, it seemed convincing that activity (i.e. feeding) on stone tops was restricted to hours of darkness to reduce the risk of accidental dislodgement during daytime when predation risk potentially would be much greater. Although the passive model has some convincing support and appears to prevail in most reports, there are doubts as to the completeness of this explanation. Elliott (1968) pointed out that the nocturnal pat-