Patchy surface stone movement during disturbance in a New Zealand stream and its potential significance for the fauna

Abstract

A patchy movement of surface stones during disturbance in streams has been proposed from field observations, but few attempts have been made to quantify this phenomenon. If spates produce a mosaic of stable and disturbed areas, the former could serve as refugia for benthic invertebrates. We monitored the stability of surface stones at three geomorphically contrasting sites in a river with an unstable bed during three spates and one large flood. Stones were marked in situ by drilling holes in them or by scraping them with a chisel. For each stone, we determined visible surface area, embeddedness in the substratum, water depth, size of surrounding stones, and presence or absence of large stones upstream. During the first monitoring period, which covered one spate and the large flood, we marked 400 stones at each 20‐m‐long site in a systematic grid with 40 to 60 cm between stones. Stones were relocated after each disturbance, but each stone set was used to monitor both subsequent events. After the large flood, few marked stones were left. Therefore, additional sets of 200 stones were marked for a second monitoring period covering the remaining two spates; these stones were located at the same sites, but the distance between transects was doubled. Patchy bed movement occurred during all four disturbances, especially during the three smaller ones, which moved only 33 to 72% of marked stones. Stability of individual stones was mainly influenced by their size and embeddedness and sometimes by the water depth above the stone. Larger‐scale parameters (e.g., the position of the stone across the transect) were less important. Whole‐site stability differed little among sites. During the three smaller events, many stable surface stones were available as potential invertebrate refugia. In contrast, invertebrate refugia may have been restricted to the hyporheic zone and inundated flood‐plain gravels during the large flood. Because patchy stone movement was observed in a river with an unstable bed, it is likely to be a feature of most rivers. Therefore, small‐scale experiments may be able to simulate the effects of disturbance on the benthic community more effectively than previously thought.