Evidence that Drift of Stream Benthos is Density Related

Abstract

The abundance of bottom fauna and of fauna in drift was studied in streams treated with DDT at various times in the past. Recovery of the bottom fauna was found to be rapid, but that of drift was delayed. The relationship of quantity of drift to the increasing bottom standing crop during recovery from spray effects was found to be curvilinear. This suggested that drift is a density—related process. Possible mechanisms to explain drift are discussed, and the conclusions of earlier reports on recovery of streams following insecticidal treatment are questioned.