Comparison of the drift of live and dead Ba�tis nymphs in a weakening water current

Abstract

The hypothesis thatBaetis nymphs in the drift would show the phenomenon of size grading in a weakening water current if they behaved as passive sediment particles was tested. Laboratory studies with dead animals confirmed that larger animals sank faster than smaller ones. Similar tests with live animals in daylight gave four behavioural reactions to sinking, three of which would result in eventual leaving of the drift and one, commonest in the smaller instars, which would result in maintenance in the water-column. In the field, three treatments of nymphs were sampled by a series of nets down the length of a pool (‘depositing habitat’) and their changes in average size found. The baseline treatment was of dead animals, which gave the decline in size down the pool of passively drifting particles. The natural, night drift in the pool, and live animals introduced in daylight, were compared to this. The former showed no decline in size, indicating active drift, while the latter, with a steeper decline in size than the dead animals, indicated active leaving of the drift.