Influence of current velocity, density, and detritus on drift of two mayfly species (Ephemeroptera)

Abstract

Drift responses of Baetis tricaudatus Dodds and Ephemerella inermis Eaton larvae to manipulations of benthic density, sterile detritus, and current velocity were monitored in laboratory streams. The proportion of larvae departing from the substrate was independent of benthic density, amount of detritus, and interactions between these two variables. In spring, departure of larvae of both species was a positive linear function of current velocity during daylight hours. In darkness, departure of B. tricaudatus larvae became a negative linear function of current velocity. Nocturnal drift of E. inermis larvae was minimal at a mean current velocity of 25 cm∙s⁻¹ and greater at both higher and lower velocities. None of the factors tested influenced departure of B. tricaudatus larvae in autumn experiments. Dead larvae were eroded from the substrate in much smaller proportions than were their live counterparts. This suggests that appearance in the water column of live animals is the direct or indirect consequence of individuals' behaviour. Departure of animals during daytime is largely passive in nature, whereas a substantial proportion of nocturnal drift probably results from active desertion of the substrate.