1.We have previously shown that the impact of spates on stream invertebrates may differ among patches separated by distances of metres or less. Here we analyse the species-specific flood responses of larval chironomids 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 dams, coarse sediments around dams, and dam debris. Additionally, since some fine sediment patches had been shown previously to act as flow refugia while others did not, we also examined speciesspecific responses in refugium vs. non-refugium fine sediment patches. Detrended correspondence analysis was used to test for changes in assemblage structure (species composition and relative abundance). 2. Species richness was not altered in a predictable 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 scores, there was generally a high degree of overlap in the species composition among the four patch types. Nevertheless, copepod species composition and relative abundance were more similar among patch types during the spate than pre-spate. Spates may induce a re-distribution of copepod 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, species composition and relative abundance (as assessed by DCA ordination scores) in refugium patches changed more in response to the spate than in the non-refugium patches. An influx of individuals from just a few species for each group was responsible for the change in assemblage structure. Thus, despite the fact that our past work has shown that refugia may confer enhanced resistance and resilience of copepod and chironomid assemblages in terms of total faunal abundances, the present work suggests that resistance and resilience of the species composition of the community apparently are no greater in refugium patches than in nonrefugium patches.1995 BlackweU Science Ltd 343
1. A combination of field and laboratory experiments was used to assess the impact of chironomid grazers on taxonomic composition, abundance and dispersion of epiphytic algal assemblages. 2. In the laboratory, Psectrocladius sp. reduced the biovolume of algal spedes preferred as food and increased the degree of clumping of non-preferred spedes. Thienemanniella cf. fusca had both positive and negative effects (depending on the algal spedes) on the biovolumes of algal spedes preferred as food and increased the degree of clumping of non-preferred spedes. 3. In field exclosures, no effect of removal of chironomid larvae from the grazer ' assemblage could be detected in autumn or winter experiments. A third, longer removal experiment, conducted in summer, resulted in increased biovolumes of edible Cosmarium spp. and Aphanocapsa spp., preferred foods of chironomid larvae. Biovolumes of Lyngbya sp., Bulbochaete spp. and Oedogonium spp., filamentous taxa used extensively in larval case construction, also increased. Chironomid larvae had no effect on total algal biovolume or biovolume of large unicellular algae. 4. Chironomid larvae can influence epiphytic algal assemblages through selective • • grazing by redudng the biovolumes of preferred foods and through case-building activity by redudng the biovolumes of construction materials.
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