Invertebrate responses to stepwise and abrupt increases in non-scouring flow: The role of refugia

Aquatic invertebrates in many stream systems, particularly in gravel-bed streams, may avoid the force of the current during flow increases by entering the substratum. We tested the hypothesis that stepwise increases in non-scouring flow (i.e., 2.5-fold increase) in gravel-bed experimental streams would result in higher numbers of invertebrates moving into the substratum than would abrupt increases in flow. Potential refugia for invertebrates were identified on the basis of hydraulics (i.e., shear stress frequencies as discharge increased and invertebrate threshold shear stress values). Although mean numbers of invertebrates increased in the hyporheic zone of some streams during flow manipulations, statistically significant evidence that the fauna moved into the hyporheic zone during abrupt or stepwise increases in flow was not found. Percentage of invertebrate erosive patches in the base-flow channel increased considerably as flow increased, but their increase on an areal basis was lower than that of the newly wetted area; the latter experienced low shear stress and may have acted as refugia for invertebrates displaced by flow.