Scales and frequencies of disturbances: rock size, bed packing and variation among upland streams

Abstract

Summary 1. Disturbance is an important source of variability in species composition and diversity, but application of disturbance models is contingent upon a very good understanding of the spatial scales and frequencies of disturbance. Such information is particularly lacking from streams. In this study, we measured the disturbance levels of rocks (defined here as the proportion of the original sample disturbed after 6 months) of differing sizes and positions within the stream bed and looked at the variation between and within three upland streams. 2. Rocks were blazed with distinctive marks in situ and mapped using simple trigonometry and permanently marked points on the banks. Forty rocks were selected and marked completely at random, and a further sixty marked from random selections within three size classes (small, medium and large) crossed with two bed‐packing classes (on top of the bed or packed into the bed). This sampling design was used at each of two sites (an upper, order 3 location and a lower, order 4 location separated by ≈ 8–16 km) on each of three rivers and in two periods during the year (a dry summer period and a wetter winter period) for a sample size of 1200 rocks in all. During summer, on‐top rocks were removed from below the study sites and placed at random locations through the riffle after marking, to test whether human‐placed rocks can provide estimates of natural disturbance levels. 3. Rocks were relocated and classified as disturbed (moved or buried) or not disturbed (found at the same location) after ≈ 6 months. Log‐linear modelling revealed that human‐placed rocks moved half as often as on‐top rocks marked in situ. Overall, small rocks disappeared more frequently than medium ones, which disappeared more often than large ones, and rocks lying loosely on top of the bed were disturbed more often than those packed into the bed. There was no interaction between rock size and bed packing in their effects on disturbance and each of these factors affected disturbance levels in the same way at all six sites and in both seasons. During the summer, there were no differences between upper and lower sites, but disturbance was still relatively frequent and patchy in occurrence, with five of six sites showing significant spatial clumping of disturbed rocks. Disturbance levels were higher in the wetter, winter season than during the drier, summer season, but this was caused by a doubling of disturbance rates at all three lower sites, which also showed reduced (but, in two cases, still significant) levels of spatial aggregation. Disturbance levels at upper sites in winter were similar to summer rates, and the level of aggregation of disturbed rocks differed between sites. 4. The results obtained by this study suggest that disturbance levels should not be assessed using methods where rocks are placed in riffles. Disturbance models applied to rocky upland streams may need to heed differences seen at small scales (i.e. between individual rocks), as differences seen at these scales were a crucial source o...

Effects of outdoor education stream classes on substrate movement and macroinvertebrate colonization

“…The lack of difference (P > 0.05) in rock movement among rock size classes for all categories of movement was surprising. Other studies have shown differences in rock movement by size class in response to spates (Downes et al 1998), but we observed no significant difference in movement by rock size class even during spate weeks. Rock placement along the transect in each site in our study was determined randomly, but all rocks in the study were equally loose and unembedded as compared to naturally placed rocks, which may have been partially embedded.…”

“…The hydrologic forces within streams work primarily upon the substrate in a downstream direction and during spate conditions these forces are magnified by increased velocity and discharge. Spates are known to mobilize the streambed and move rocks of varying sizes downstream (Boulton et al 1992;Cobb et al 1992;Downes et al 1998;Death 2002;Death 2003;Gjerlov et al 2003). We also observed increased downstream movement of all rock sizes during spate flow conditions compared to baseflow conditions (Bossley 2016).…”

“…All of the large rocks at the class site were moved every week of the study (Table 5) making the large rock size class the least stable rock size class within the class site. In contrast, Downes et al (1998) observed a linear increase in stability with increasing rock size. Rock movement at the unused site was largely confined to the spate events during Weeks 2 and 4.…”

“…It has been referred to as the stage on which aquatic insects play out their ecological roles (Minshall 1984). Erman and Erman (1984) maintained that changes in substrate lead directly to changes in community composition, and the importance of quantifying substrate movement has since been acknowledged (Cobb et al 1992;Townsend et al 1997) and studied (Death and Winterbourn 1994;Downes et al 1998;Death 2002). Negative effects of substrate movement on aquatic macroinvertebrates have been documented for multiple disturbance types (Boulton et al 1992;Cobb et al 1992;Death 2002;Gjerlov et al 2003).…”

“…Spates have received the greatest attention as agents of substrate movement, and the magnitude of a spate has been shown to influence the biological response to a spate, although a consistent relationship between spate magnitude and biological response has not been demonstrated (Boulton et al 1992;. The patchy arrangement of substrate on the streambed and disturbance history of different patches yield areas CONTACT Jon P. Bossley bossley.2@osu.edu of streambed that are more prone to substrate disturbance than others (Cobb et al 1992;Downes et al 1998;. The amount of sunlight and the consequent reliance on algal productivity or availability of leaf litter in a stream may also influence effects of substrate disturbance on stream biota (Death 2003).…”