In 1989 blooms of the river benthic diatom Didymosphenia geminata (didymo) first appeared and rapidly spread among rivers on central Vancouver Island, covering the bottoms with thick, woolly‐looking mats. Although didymo is native to North America, extensive field surveys of rivers on Vancouver Island and other data indicate that didymo blooms are new. No known environmental changes were associated with the onset of didymo blooms. However the pattern of didymo spread among rivers on Vancouver Island correlates with the activity of fishermen and the commercial introduction and widespread use of felt‐soled waders in the late 1980s. Since 1994 nuisance blooms of didymo have appeared in numerous other places in the Northern Hemisphere and South Island, New Zealand, all areas frequented by fishermen. Actions by government agencies to educate the public and restrict the use of felt‐soled waders have been undertaken in some jurisdictions and at least one commercial manufacturer of waders will discontinue production of felt‐soled models in the near future.

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1. Changes in species composition of the periphyton on introduced substrates were determined in an oiigotrophic mountain stream subject to long-term heavy metal contamination.2. At the upstream control site, the numerically most abundant taxa were Bacillariophyta {Achnanthes minutissima, Achnanthes microcephala and Achnanthes linearis) as well as, in summer, the Chlorophyta (Mougeotia spp. and Ulothrix subtilissima).3. At the downstream contaminated site the periphyton community was totally dominated by Bacillariophyta throughout the sampling period. A. minutissima and A. microcephala were co-dominants during spring. Seasonal succession patterns did not parallel those at the upstream site. Chlorophyta were virtually absent and A. minutissima comprised 94% of the community during summer.4. Species diversity, species evenness and dissimilarity index were utilized to delect differences in species composition, abundance and number. Slight differences were found in spring samples while summer samples indicated major differences between sampling sites.

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