Ecological processes in large rivers are controlled by their flow variability. However, it is difficult to find measures of hydrological variability that characterize groups of rivers and can also be used to generate hypotheses about their ecology. Multivariate analyses of the hydrographs of 52 rivers worldwide revealed distinctive patterns of flow variability that were often correlated with climate. For example, there were groups of rivers that corresponded broadly with ‘tropical’ and ‘dryland’ climates. However, some rivers from continental climates occupy both extremes of this range, illustrating the limitations of simple classification. Individual rivers and groups of rivers may also have different hydrographic ‘signatures’, and attempts to combine measures of hydrological variability into indices mask biologically significant information. This paper identifies 11 relatively independent measures of hydrological variability that help categorize river types and are each associated with aspects of fish biology. Ways are suggested by which the Flood Pulse Concept can be expanded to encompass hydrological variability and accommodate differences among groups of rivers from different climatic regions. Such recognition of the complex role of hydrological variability enhances the value of the concept for river conservation, management and restoration.

Regulation of lowland rivers often alienates large areas of the floodplain, altering the natural regime of flooding frequency, duration, and magnitude. The extent to which such changes after the productivity of river-floodplain ecosystems and the contribution of aquatic invertebrates emerging from dry floodplain sediments is unknown. To examine this in a section of the River Murray in South Australia, fourteen replicate sods of dry sediment collected from four areas of the floodplain that had experienced different average flood recurrence frequencies (annually, 1 in 7, 1 in 11, and 1 in 22 years) were inundated in the laboratory, simulating flooding. Invertebrates emerging from the dry sediments were sampled with replacement 1, 2, 3, 7, 10, 14, 21, and 28 days after inundation, enabling comparisons of temporal changes in faunal composition and biomass. The greatest biomass and numbers of invertebrates emerged from annuallyflooded sods whereas sediments usually flooded once in 22 years yielded only protozoans. Large numbers of cladocerans and rotifers were recorded within two days of inundation whereas ostracods were not numerous until two weeks later. Heterogeneity in faunal composition and biomass among replicates probably reflected patchiness in microtopography and resting stage settlement, and variable hatching 'strategies'. Although experimental conditions rapidly diverged from the natural situation over time because of the unrealistic constraints of enclosure, results suggest a potentially significant contribution by emergent invertebrates to the newly-inundated floodplain foodweb. Reducing floodplain inundation frequency through regulation and flood mitigation probably severely reduces this reserve, removing a food resource for young fish and other predators.

High and often variable salinity is an obvious feature of salt lakes. Correspondingly, salinity is usually assumed to be an important ecological determinant in such lakes. An investigation of the macroinvertebrate fauna of 79 lakes (salinities from 0.3 to 343 g 1-') in the Western District of Victoria, Australia, examined this assumption. Over the total range of salinity, species richness and composition are highly correlated with salinity. However, these relationships become nonsignificant over intermediate ranges of salinity. Furthermore, many taxa have very broad tolerances to salinity at these intermediate ranges, implying that factors other than salinity may determine their distribution. An appreciation of scale (that is, the range of salinity over which observations are considered) resolves the paradox that, despite these broad tolerances by most taxa, species richness and composition strongly reflect salinity over the entire salinity range. 16.5, 20.0 2

1. Temporal changes in species richness, total abundance, and numbers of individuals in seven functional feeding groups were compared between pools and riffles at four sites on two intermittent streams in Victoria sampled during a drought year followed by a wetter year. 2. Species richness increased steadily over time, peaking in the riffles just before flow ceased. Maximum species richness occurred in the pools shortly after flow stopped, implying emigration from the drying riffles. There were two peaks in total numbers, first in autumn when flow commenced and again in late summer as flow diminished. Generalist feeders (collector-gatherers and collector-scrapers) were usually abundant throughout the year. Scraper densities rose in late spring-early summer in conjunction with enhanced periphyton growth, whereas predator numbers steadily increased during the year, peaking in all habitats just before they dried. 3. Several floods in spring signiflcantly reduced species richness and total numbers of individuals but recovery to pre-flood levels was generally complete within 2 weeks. Although floods had little effect upon succession, as indicated by trajectories in ordination space, the dry period interrupted otherwise cyclical sequences, consistently resulting in arch-shaped trajectories. Fauna! change appeared to be allogenic, driven by the seasonal cycle of environmental phases. 4. A group of early colonists or 'pioneer' species was replaced after several weeks by heterogeneous groups of 'mid-successional' taxa responsible for differences in faunal composition between riffles and pools. When flow diminished, these groups were replaced by 'summer' species, apparently capable of tolerating deteriorating environmental conditions, and potentially intensifying biotic interactions. 5. We suggest that the cyclical temporal changes in assemblage composition observed in this study represent 'seasonal periodicity', whereas recovery after unpredictable disturbances such as floods that temporarily disrupt the cycle fits the definition of 'succession'. Succession rate is a measure of resilience, recovering to the cyclical trajectory that would exist in the absence of disturbance. 'Lag effects' brought about by reduced recruitment during the drought were not evident until the following year, emphasizing the significance of historical events and the need for long-term studies to describe the ecology of intermittent streams adequately. 123

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