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Few studies have examined the effects of low flows on stream biota—a paradoxical situation considering the mounting pressure on streams as a result of increased human abstraction. We present a conceptual model describing processes operating in rivers during low flows and argue that such processes reflect both direct effects of reduced flow on benthic invertebrates and indirect effects of enhanced plant growth occurring during such periods. Our model suggests that the longer the duration of low flow, the more the plant community will change and, in turn, the more habitat quality will change, with consequences to benthic invertebrate communities and higher trophic levels. A fundamental part of the model is the recognition of different stream types, based on a habitat-template matrix of resource supply, interflood velocity, and substrate stability during flood events. This habitat template results in 12 stream types, each of which supports specific plant communities that set the antecedent plant conditions prior to a low-flow period. During low flows, hydraulic factors interact with the antecedent plant communities, which undergo specific responses. For example, in low-nutrient streams with gravel substrates and dominated by diatoms, low-flow conditions will cause little or no change to the antecedent plant community because this plant community is structured by top-down grazing pressure. Consequently, benthic invertebrate composition will not change. In contrast, cover and biomass of filamentous green algae in higher-nutrient gravel-bed streams will increase during low-flow events, and this increase will affect habitat suitability for invertebrates over and above those caused solely by hydraulic changes. The greatest changes to invertebrate communities are expected in macrophyte-dominated streams because these plants change instream habitat conditions the most during extended periods of low flow. Therefore, current flow-management techniques must consider the type of plant communities that exist prior to low flows and be cognizant of that fact that these plant communities can have dramatic influences on benthic communities during low-flow periods.

Landscape classifications, such as ecoregions, are widely promoted as spatial frameworks for aquatic conservation and environmental management. The River Environment Classification (REC) is an alternative approach to landscape-scale river classification. The REC classifies individual sections of river networks according to the climate, topography, geology, and land cover of the unique upstream watershed of each section. The classification is hierarchical and is based on the assumption that climate, topography, geology, and land cover are dominant causes of spatial variation in physical and biological characteristics of rivers at 4 spatial scales. The ability of the REC to explain variation in invertebrate samples collected from 237 sites in New Zealand was compared to that of region-based classifications using classification strength tests. Classifications with differing numbers of classes were compared using relative classification strengths calculated by expressing absolute classification stren...

Substrate stability in streams is fundamental to benthic communities, many of which decline in biomass and taxon richness with decreased stability. Although of obvious importance in describing a stream's disturbance regime, substrate movement is often difficult to quantify accurately. Traditional techniques rely either on subjective scoring systems (e.g., Pfankuch stability score), or require repeated site visits to follow downstream displacement of painted stones. Newbury's Instability Index measures substrate movement based on calculations of a stream's tractive force at bankfull discharge, but the assumptions on which this method is based limit its use to lowland streams. We have developed a new method to assess substrate movement suitable for shallow, steep mountain streams with a high bed roughness, based on an extensive review of existing hydraulic literature. We calculated the % of the streambed in motion at bankfull discharge (P BF ) in 48 New Zealand headwater streams using our method, t...

Effects of 3 disturbance parameters (% of the streambed in motion at bankfull discharge [P BF ], catchment-specific discharge [SPECQ BF ], and frequency of bankfull floods [N BF ] on aquatic bryophyte richness and community composition were assessed. Relationships between taxonomic richness and P BF and SPECQ BF were strongly curvilinear, such that richness was low in both stable and highly unstable streams, and in streams of both low and high SPECQ BF . Richness was highest in streams with moderate P BF and SPECQ BF . Interspecific competition may have reduced species richness at stable sites. In contrast, only a few bryophyte taxa were able to grow at unstable sites. Low richness at sites with a low SPECQ BF most likely reflects the intolerance of some species to desiccation. Low richness at sites with high SPECQ BF may reflect differences in the resistance of different taxa to high discharges. Community compo...