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Effects of flow regulation on disturbance frequencies and in‐channel bryophytes and macroalgae in some upland streams

River regulation can cause various downstream changes to the physical environment of stream channels. Here, we examined whether disturbance of the stream bed and/or degree of emergence of large substrata affect the diversity and abundances of bryophytes and macroalgae in three regulated and three unregulated upland streams. We marked and mapped randomly selected rocks in situ for each stream and measured the rates at which these substrata disappeared. We recorded percentage covers of bryophytes and macroalgae in each stream on rocks of differing sizes (‘small’ <10 cm, ‘medium’ = 10–20 cm, ‘large’ >20 cm maximum top dimension) and lying either loosely on top of the bed or packed into it; we also recorded when rocks were emergent. We found strong positive associations between plant cover (mostly bryophytes) and substrate size, consistent with the hypothesis that substratum stability primarily drives bryophyte abundance. Nevertheless, highest covers of bryophytes in unregulated streams were found on emergent rocks, which tended to be large, meaning that disturbance and emergence effects were difficult to discriminate. Regulated streams did not have lower disturbance frequencies than unregulated systems. Percentage covers of plants, primarily bryophytes, were lower in regulated systems because of reduced cover on large substrata, but not small or medium ones. Together, these two pieces of evidence suggest that effects of river regulation on bryophytes were not caused by altered disturbance frequencies. A more likely explanation is that regulated streams have little of the daily or weekly rises and falls in discharge that occur in unregulated streams because of small rainfall events. Consequently, large rocks in regulated streams have only narrow zones that are subject to a variety of wetted conditions, which may be more suitable for bryophyte growth and colonization than constant submergence. Distinguishing between disturbance and emergence effects is important for setting environmental flows: alleviating the former requires more flushing flows whilst the latter requires greater temporal variability in non‐flood flows. Copyright © 2003 John Wiley & Sons, Ltd.