Scales of patchiness in the response of lotic macroinvertebrates to disturbance in a regulated river

Patchiness is hierarchically structured in stream ecosystems and transcends multiple spatial and temporal scales. Regulation of streams through channelization and flow stabilization often homogenizes in-stream habitats, thereby reducing this patchiness. We conducted a patch-disturbance experiment with individual brick substrata in 2 reaches (open vs closed canopy) and 2 seasons (summer vs winter) in a eutrophic regulated stream. Although the stream was regulated, we expected coarse-scale attributes (canopy cover, season) to influence the effects of disturbance at the fine scale of individual bricks. Brick substrata had 3 degrees of structural complexity (low = smooth, intermediate = techno, high = hetero). Replicates of each brick type were physically disturbed at frequencies of every 5 and 10 d or were left undisturbed during each 30-d experimental period. At the end of each experiment, individual bricks were collected, periphyton biomass was estimated, and macroinvertebrates were identified and counted. Natural densities, taxon richness, and assemblage composition of macroinvertebrates differed significantly between sites and seasons. Periphyton biomass was higher in the open-canopy than in the closed-canopy site in both seasons, especially summer. Periphyton biomass differed among brick types at each site and season, but disturbance had no effect on periphyton biomass within a brick type. At the closed-canopy site, undisturbed hetero bricks had higher macroinvertebrate density, richness, and biomass than the other brick types and disturbance treatments in both seasons. At the open-canopy site, no effect of disturbance was found for any brick type in summer, whereas undisturbed and 10-d hetero bricks had higher macroinvertebrate density, richness, and biomass than the other brick types and disturbance treatments in winter. Besides the dominant role of site and season in this study, our results also suggest that substrate patch complexity can be significant in the response of macroinvertebrates to physical disturbance in regulated streams. As in natural systems, coarse-scale attributes of stream reaches, e.g., degree of canopy cover and season, also provide the spatiotemporal hierarchical context in which disturbance effects are realized in regulated streams, thus having major implications for resource managers.