An assessment of short-term depletion of stream macroinvertebrate benthos by drift

A study was conducted to determine if emigration of drifting macroinvertebrates from a stream riffle which was blocked for one week from immigration by upstream colonists significantly reduced the abundance of drift collected from the tail of the riffle. The head of a 9 m long riffle of a 2nd order stream in Maryland (USA) was blocked from incoming drift by a 250 m mesh weir. Upstream immigration of invertebrates into the riffle was largely prevented by a partition placed at the tail of the riffle which held the drift nets. Benthos and drift samples were collected from the riffle prior to weir placement and following its removal, and drift was collected at dusk on each day. No difference in drift or in benthic abundance between the beginning and end of the study was observed. This is largely attributed to recruitment of immature insects (primarily hatching of eggs present at the outset), particularly of Dolophilodes distinctus and species of Tanytarsini, from within the riffle. Results suggest that recruitment of riffle species is of sufficient magnitude to more than compensate for short-term riffle depletion due to drift. Samples of drifting and non-drifting (benthic) animals were held without food for 12 h after collection and mortality within each group was determined. The mortality of drifting animals was three-fold that of benthic animals.