We conducted two experiments to assess drift and benthic invertebrate responses to stepwise and abrupt changes in non-scouring flow in gravel-bed experimental streams. Intuitively, a stepwise flow increase should allow aquatic invertebrates more time to seek refuges than would an abrupt increase. We hypothesized that abrupt flow increases would result in larger increases in taxon richness and in the number of invertebrates in the drift, and a larger decrease in benthic density than would stepwise flow increases. Two kinds of drift response to flow increases were observed in the stepwise experiment: (1) no response (e.g. Caenis sp. [Ephemeroptera] and Sphaeriidae [Pelecypoda]); (2) threshold response of some aquatic (e.g. Crangonyx pseudogracilis [Amphipoda]) and semiaquatic (e.g. Ormosia sp. [Diptera]) taxa. Drift richness and drift density in both experiments declined after reaching a peak. The peak was reached almost immediately in the abrupt treatment and later in the stepwise treatment. Maximum richness of taxa and taxon composition in the drift were similar in both experiments. Despite significant increases in drift, stepwise and abrupt increases in flow did not have a significant effect on benthic density. However, relative to reference streams, the percentage of total benthic invertebrates in the drift increased 10 × in the stepwise experiment and 33 × in the abrupt experiment. These non-scouring increases in flow were non-trivial. Our results suggest that several high flow events of the same magnitude (i.e. 2.5-3.0 fold increases) may cause considerable losses of benthic populations to the drift. The rate of increases in flow appears to be important: abrupt increases in flow had a stronger effect on invertebrate drift than did stepwise increases.
Bibliographical noteFunding Information:
We thank Mr Tom Henry, Dr Michael Swift, John Foley, Lisa Allen, Rick Goenner, Dr Atilano Contreras Ramos, Dr Chris Holm and all the staff at the Monti-cello Ecological Research Station for their assistance during this project. Dr Ray Newman and Dr Michael Swift made very useful comments and suggestions during the project. Drs Frank Martin and Sanford Weisberg (School of Statistics, University of Minnesota) provided statistical expertise. Research was funded by the Basque Government, and the U.S. Environmental Protection Agency under Cooperative Agreement No. EPA/CR-816726. This paper has not been subjected to US EPA manuscript review procedures.