DISTURBANCE AND THE COMMUNITY STRUCTURE OF STREAM INVERTEBRATES - PATCH-SPECIFIC EFFECTS AND THE ROLE OF REFUGIA

1. We have previously shown that the impact of spates on stream invert ebrates may differ among patches separated by distances of metres or l ess. Here we analyse the species-specific flood responses of larval ch ironomids and adult and near mature copepods living in different patch types. Four patch types (with eight replicates of each) were compared : the sandy mid-channel, fine sediments around darns, coarse sediments around dams, and dam debris. Additionally, since some fine sediment p atches had been shown previously to act as flow refugia while others d id not, we also examined species specific responses in refugium vs. no n-refugium fine sediment patches. Detrended correspondence analysis wa s used to test for changes in assemblage structure (species compositio n and relative abundance). 2. Species richness was not altered in a pr edictable manner by floods; the least stable patch types (mid-channel and coarse patches) did not necessarily show reduced species richness during the spate. 3. As indicated by the spread of DCA ordination scor es, there was generally a high degree of overlap in the species compos ition among the four patch types. Nevertheless, copepod species compos ition and relative abundance were more similar among patch types durin g the spate than pre-spate. Spates may induce a re-distribution of cop epod species among the patch types. Chironomid species composition and relative abundance were no more similar among patch types during the spate than pre- or post-spate. 4. For both chironomids and copepods, s pecies composition and relative abundance (as assessed by DCA ordinati on scores) in refugium patches changed more in response to the spate t han in the non-refugium patches. An influx of individuals from just a few species for each group was responsible for the change in assemblag e structure. Thus, despite the fact that our past work has shown that refugia may confer enhanced resistance and resilience of copepod and c hironomid assemblages in terms of total faunal abundances, the present work suggests that resistance and resilience of the species compositi on of the community apparently are no greater in refugium patches than in non-refugium patches.