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Summary Alteration of flow and thermal regimes is a key consequence of human use of river and floodplain ecosystems, and these impacts result from a range of interacting ecological mechanisms. Environmental flow regimes are a management strategy to restore or maintain ecologically important aspects of river hydrology. However, inadequate understanding of the processes that determine the effects of flow on population dynamics of biota hinders the maximum benefits of environmental flows. Spawning and recruitment of riverine fish is mediated by temperature, access to nursery habitat and the timing and availability of resources. Differences in population sizes between regulated and unregulated rivers are often attributed to the effect of flow and thermal regime change on population dynamics. Dams and extraction of water have altered the flow and thermal regimes of rivers in the northern Murray–Darling Basin, Australia. Combining three studies, we (i) examined differences in the recruitment of fish and patterns of potential prey availability between a regulated and unregulated river, (ii) studied the relationships between patterns of flow and temperature on spawning of fish in two rivers with contrasting regulated flow regimes and (iii) experimentally tested the effects of environmental flow releases in a regulated river on fish spawning and recruitment by comparing patterns over time against two unregulated rivers. Comparisons between regulated and unregulated rivers indicated that the significantly lower recruitment of Macquaria ambigua in the regulated river was linked with large differences in available prey. Species‐specific differences in the abundance of fish larvae between two regulated rivers indicated that different reproductive strategies determine population‐level responses to flow regulation. Finally, the experimental flow releases in a regulated reach during late spring did not result in a change in spawning and recruitment of fish, although influences of antecedent flow and temperature on larval and juvenile assemblage composition were significant. These findings indicate that the responses of fish spawning and recruitment to flow regime change and restoration are dependent on the conditions that determine the success of these critical life‐history processes. Specifically, key effects of flow regulation are reduced summer water temperatures due to hypolimnetic discharge from dams, and the reduced frequency and duration of low flows. Reduced water temperatures limit opportunities for fish to spawn, and sustained base flows were associated with lower densities of potential prey sources in conjunction with potential flushing of larvae and juveniles, thereby influencing recruitment. The benefits of environmental flow programmes can be enhanced if flow regimes are modified to ensure that abiotic and biotic conditions that support persistence of biota are provided. In conjunction with flow pulses and floods, low flows are a natural and ecologically important component of flow regimes, particularly in drylan...

In dryland environments where rivers have been modified by water resource developments, the use of environmental water provides a management tool to restore or maintain riparian vegetation conditions. Relevant management actions include watering to promote the recruitment of highly valued woody species. However, for many riparian trees, little is known about the watering regimes needed to achieve successful recruitment. We conducted a glasshouse experiment to investigate the early growth of 5-week old seedlings of two Australian dryland riparian tree species, coolibah (Eucalyptus coolabah) and river red gum (Eucalyptus camaldulensis), in floodplain soil cores to provide insights into their requirements for successful establishment in dryland habitats. Both species exhibited similar growth patterns for their above-ground components. E. coolabah root penetration through soil cores was consistent over the 12-week experiment. In contrast, E. camaldulensis root penetration appeared to be considerably slower during the first 6 weeks, but then became more rapid over the following 6 weeks, with similar rooting depths after 12 weeks observed to those of E. coolabah. These findings suggest that coolibah seedlings are more likely to establish under rapidly drying conditions than river red gum seedlings. Such differences in early growth for these iconic species should be considered when determining the provision of environmental water for their management.

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