Edwin T. Chester

Disturbance and the role of refuges in mediterranean climate streams

Refuges protect plant and animal populations from disturbance. Knowledge of refuges from disturbance in mediterranean climate rivers (med-rivers) has increased the last decade. We review disturbance processes and their relationship to refuges in streams in mediterranean climate regions (med-regions). Med-river fauna show high endemicity and their populations are often exposed to disturbance; hence the critical importance of refuges during (both seasonal and supraseasonal) disturbances. Disturbance pressures are increasing in med-regions, in particular from climatic change, salinisation, sedimentation, water extraction, hydropower generation, supraseasonal drought, and wildfire. Med-rivers show annual cycles of constrained precipitation and predictable seasonal drying, causing the biota to depend on seasonal refuges, in particular, those that are spatially predictable. This creates a spatial and temporal mosaic of inundation that determines habitat extent and refuge function. Refuges of sufficient size and duration to maintain populations, such as perennially flowing reaches, sustain biodiversity and may harbour relict populations, particularly during increasing aridification, where little other suitable habitat remains in landscapes. Therefore, disturbances that threaten perennial flows potentially cascade disproportionately to reduce regional scale biodiversity in med-regions. Conservation approaches for med-river systems need to conserve both refuges and refuge connectivity, reduce the impact of anthropogenic disturbances and sustain predictable, seasonal flow patterns.

Freshwater Invertebrate Life History Strategies for Surviving Desiccation

AbstractIn many regions, climate change is prolonging dry periods in rivers and wetlands, exposing freshwater invertebrates to increased periods of desiccation. Invertebrates show a range of strategies for surviving desiccation, but the effects of the degree of exposure to desiccation on the expression of particular traits is unknown. This review synthesizes existing information on the desiccation responses of freshwater invertebrates to examine the flexibility of these survival strategies and the relationship between strategies and the degree of desiccation to which individuals are exposed. It focuses on desiccation at the small spatial scales experienced by individuals and clarifies the terminology of resting stages present during desiccation. We provide a key to terminology used for different forms of dormancy, so that appropriate terms may be used. All invertebrate groups showed a range of strategies for surviving desiccation. Sometimes, different traits were expressed among different populations of a species; however, it is unclear how many species show multiple desiccation response strategies. Many crustacean taxa showed physiological dormancy responses to desiccation that enabled survival for long periods (years). Insects often rely on emigration from drying waterbodies as flying adults or on larvae occupying damp refuges on the benthos. Altered water regimes may alter the phenology of desiccation responses, potentially increasing local extinctions, even in species capable of prolonged dormancy because of constraints on life cycles. However, there is limited empirical evidence demonstrating the flexibility of, or limitations to, expression of these survival strategies and their potential fitness costs.

Aestivation provides flexible mechanisms for survival of stream drying in a larval trichopteran (Leptoceridae)

Some freshwater species aestivate to resist drying; however, little is known about factors affecting post-aestivation survival. Climate change prolongs drying and may make short bursts of flow more frequent in southern Australian streams, thereby affecting aestivation success. The tolerance of larval Lectrides varians (Mosley) to drying was tested by inducing aestivation in dry or moist sediment and then re-immersing larvae and measuring survival and activity. Survival did not differ between individuals that were continually immersed (78%) or aestivating on moist sediment (70.5%) after 16 weeks. Survival was significantly lower on dry sediment (29.3%). Furthermore, some larvae showed delayed responses to re-immersion; 65% of individuals showed activity within 4.5h, whereas over 30% of larvae did not become active until 72h after re-immersion. L. varians can survive extended periods (112 days) without surface water, showing a bimodal response to re-immersion that increases the likelihood of population persistence by enabling some larvae to remain aestivating during short-lived bursts of stream flow. L. varians populations will therefore be more robust to prolonged stream drying and short-lived flow events than are some other insect taxa, although as the duration of aestivation increases larval survivorship decreases, suggesting that there are limits to the flexibility of aestivation traits.

Constraints upon the response of fish and crayfish to environmental flow releases in a regulated headwater stream network

In dry climate zones, headwater streams are often regulated for water extraction causing intermittency in perennial streams and prolonged drying in intermittent streams. Regulation thereby reduces aquatic habitat downstream of weirs that also form barriers to migration by stream fauna. Environmental flow releases may restore streamflow in rivers, but are rarely applied to headwaters. We sampled fish and crayfish in four regulated headwater streams before and after the release of summer-autumn environmental flows, and in four nearby unregulated streams, to determine whether their abundances increased in response to flow releases. Historical data of fish and crayfish occurrence spanning a 30 year period was compared with contemporary data (electrofishing surveys, Victoria Range, Australia; summer 2008 to summer 2010) to assess the longer–term effects of regulation and drought. Although fish were recorded in regulated streams before 1996, they were not recorded in the present study upstream or downstream of weirs despite recent flow releases. Crayfish (Geocharax sp. nov. 1) remained in the regulated streams throughout the study, but did not become more abundant in response to flow releases. In contrast, native fish (Gadopsis marmoratus, Galaxias oliros, Galaxias maculatus) and crayfish remained present in unregulated streams, despite prolonged drought conditions during 2006–2010, and the assemblages of each of these streams remained essentially unchanged over the 30 year period. Flow release volumes may have been too small or have operated for an insufficient time to allow fish to recolonise regulated streams. Barriers to dispersal may also be preventing recolonisation. Indefinite continuation of annual flow releases, that prevent the unnatural cessation of flow caused by weirs, may eventually facilitate upstream movement of fish and crayfish in regulated channels; but other human–made dispersal barriers downstream need to be identified and ameliorated, to allow native fish to fulfil their life cycles in these headwater streams.

Using fractal geometry to make rapid field measurements of riverbed topography at ecologically useful spatial scales

A method is described for making rapid in situ field measurements of riverbed topography over spatial scales of ≅1–10 m. This method uses rolling balls to make quick, accurate measurements of river-bed roughness at several spatial scales. Random sampling and replication generate multiple estimates of the fractal dimension (d) that can be used to test for significant differences in the complexity of riverbed architecture between habitat types and spatial scales.

The use of aluminium sulphate to control algal blooms and chironomids in Jackadder Lake, Western Australia

The use of aluminium sulphate to control algal blooms and chironomids in Jackadder Lake, Western Australia M. A. Lund & E. T. Chester To cite this article: M. A. Lund & E. T. Chester (1991) The use of aluminium sulphate to control algal blooms and chironomids in Jackadder Lake, Western Australia, SIL Proceedings, 1922-2010, 24:2, 1129-1133, DOI: 10.1080/03680770.1989.11898929 To link to this article: https://doi.org/10.1080/03680770.1989.11898929

Does sampling method or microhabitat type determine patterns of macroinvertebrate assemblage structure detected across spatial scales in rivers

Patterns in macroinvertebrate assemblage structure detected across spatial scales in rivers vary among studies, and it is not clear whether methodological differences in sampling methods or differences between rivers are responsible. We aimed to separate the roles of microhabitats and sampling methods in determining patterns in macroinvertebrate assemblages across spatial scales in the Johanna River, south-eastern Australia. We hypothesised that less sample-to-sample variability in the structure of macroinvertebrate assemblages would be detected using a Surber sampler than by scrubbing stones. However, Surber samples were more variable than individual stones, although invertebrate abundance did not differ. To examine the effect of habitat and substratum, we hypothesised that taxa richness, invertebrate abundance and sample variability would differ among leaf packs and stones. Variation among samples in taxa richness and abundance was higher among leaf packs than among cobbles, whereas variability in taxon composition was much higher among stones. Leaf packs in runs had four times as many taxa as did stones in riffles, and more individuals. Leaf packs may therefore provide a more representative sample of the assemblage than do stones. Surber samplers may randomly subsample riffle assemblages, inflating the sample-to-sample variability detected.

Resistance, Resilience, and Community Recovery in Intermittent Rivers and Ephemeral Streams

Intermittent rivers and ephemeral streams (IRES) are temporally and spatially dynamic ecosystems, experiencing alternating wet and dry phases and supporting both aquatic and terrestrial habitats. For aquatic species to persist in these variable habitats, they must be resistant or resilient to disturbances such as flow cessation and drying. Resistance mechanisms include tolerance of extreme physicochemical conditions and possession of desiccation-resistant dormant stages, whereas resilience mechanisms require the ability to disperse instream or overland. Similarly, terrestrial species inhabiting IRES during dry phases must be resistant or resilient to periodic inundation of their habitat. Resistance and resilience processes interact to facilitate community recovery after unfavorable periods (e.g., drying for aquatic taxa, flooding for terrestrial taxa). Human disturbances such as flow diversions and streambed gravel mining alter recovery pathways and affect the long-term persistence of aquatic and terrestrial species in IRES.

Do recolonisation processes in intermittent streams have sustained effects on benthic algal density and assemblage composition

When intermittent streams flow, benthic algae develop from both colonising propagules and regrowing dried biofilm. We aimed to determine whether colonisation processes influence algal densities and taxonomic composition beyond the period immediately following commencement of winter flows, and whether regulation modifies those processes, in the Victoria Range, Australia. Stones were placed in two unregulated streams, and upstream and downstream of weirs in three regulated streams, after dry biofilm was removed. Epilithic algae on treatment and control stones were collected after winter flows (12 weeks). Treatment effects were still apparent in one (unregulated) stream, but not in the other streams. Algal assemblages and densities upstream and downstream of weirs differed, but there was no systematic pattern among streams. In intermittent headwater streams, recolonisation processes may influence algal assemblages until spring; but in most streams, the duration of influence will be shorter, depending on the assemblage composition in regrowth and refuges, which is also shaped by conditions during the previous flow season. If the effects of regulation depend on how idiosyncratic flow regimes and assemblage compositions influence recolonisation, they may be difficult to predict. Similarly, recovery trajectories for stream communities after drought will differ among streams, depending on whether biofilm can develop during potentially short seasonal flows.

Fringing trees may provide a refuge from prolonged drying for urban wetland invertebrates

Climate change is causing prolonged drying in many seasonal wetlands, including urban wetlands, potentially affecting aquatic invertebrates that take refuge in wetland sediment during dry periods and thereby threatening wetland biodiversity. We collected sediment from two habitats: open water (OW) and fringing trees (FT), in eight urban wetlands after seasonal inundation had ended. Both habitats are inundated during winter–spring and dry in summer–autumn. Each sediment sample was divided into subsamples. One set of subsamples were inundated in the laboratory to test the hypothesis that emerging invertebrate assemblages would differ between OW and FT sediments. Another set of subsamples was dried, stored for a year, and inundated to test the hypothesis that prolonged drying would reduce the abundance and taxa richness of emerging invertebrates. The composition of emerging invertebrate assemblages differed between habitats, with more amphibious species found in FT sediment. Invertebrate responses to prolonged drying and storage varied among species: for some, effects depended on habitat type, while others delayed emergence or showed no response. Microcrustacean abundance was unaffected by drying, suggesting that their productivity during refilling may resist drier water regimes. Surface temperatures of dry sediment are cooler beneath FT, and this sediment has higher organic matter, holds more water and is less dense than OW sediment; and FT sediment remained cooler than OW sediment in the laboratory, despite the absence of shading. Fringing trees may therefore provide a refuge for some freshwater invertebrates relying on dormant stages in the sediment to survive drying in urban wetlands.