SUBSTRATUM CONDITIONING AND DIATOM COLONIZATION IN DIFFERENT CURRENT REGIMES 1

Short‐term (3 day) diatom colonization of clean ceramic tiles and tiles conditioned with a thin, nonalgal biofilm was examined in fast‐ and slow‐current outdoor experimental stream channels to assess effects of organic conditioning and current regime on diatom colonization. Colonization rates onto unconditioned tiles were 10 times lower in fast current that in slow. Substratum conditioning reduced the effects of current because organic conditioning of tiles significantly enhanced colonization in fast current, but not in slow current. Two diatom taxa (Nitzschia acicularis W. Sm. and Synedra radians Kütz) colonized unconditioned tiles more rapidly than conditioned tiles in slow current, suggesting the existence of negative interactions between these diatoms and bacteria and/or organics in conditioning films.