The influence of flow on benthic assemblages in the Severn River, New South Wales, Australia

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NO FULL TEXT AVAILABLE. This thesis contains 3rd party copyright material. ----- The effects of flow regulation and to a lesser extent flow restoration on stream ecosystems are increasingly well documented and studied internationally. Australian stream ecosystems have changed in response to flow alteration, but little is known how they might change should flow volumes and variability increase through the introduction of environmental flows. The Severn River is an upland river in northern NSW, Australia. It is a tributary of the Border Rivers sub-catchment, a part of the Murray–Darling River System. Pindari Dam on the Severn River, which can store 312 GL, regulates its flow. Typically, high flows to meet irrigation demands (up to 1,000 ML d⁻¹) occur in warmer months (October-March). Unlike many other regulated rivers that experience higher flows than pre-regulated conditions in summer, the seasonality of high flows in the Severn River has not been reversed. However, natural flow variation is lost as releases of ~10 ML d⁻¹ are discharged at other times. A Water Sharing Plan (WSP) governs management of flows in the Severn River including an environmental flow release from Pindari Dam. Seasonal water chemistry, benthic algal and macroinvertebrate assemblage data were collected at sites on the regulated Severn and unregulated Mole and Dumaresq Rivers. Sites differed in the level of hydrological variation. Flows in the Severn River never ceased and were relatively stable, compared to unregulated sites on the Mole and Dumaresq Rivers that experienced high flows after rainfall and no flow periods when base flows ceased. Severn River sites had a higher abundance of benthic algae (including filamentous algae) but lower algal diversity than that at sites of the Mole and Dumaresq Rivers. Macroinvertebrate densities in all rivers were comparable; however, regulated sites were typically characterised by a higher density of filter feeding macroinvertebrates. Two environmental flow scenarios designed to reduce the density of benthic algae were trialled in the Severn River at Glenora Bridge and Ashford Bridge. Flows that peaked at ~1,000 ML d⁻¹ (velocity of ~0.9 m s⁻¹) increased the density of benthic algae particularly filamentous algae such as Stigeoclonium and Leptolyngbya. A third flow event from an unregulated tributary with its confluence between sites, increased flows to 2,180 ML d⁻¹ (velocity of ~1.2 m s⁻¹), which reduced chlorophyll a and filamentous algal abundance. The similarity in flow velocities suggests that suspended sediments in the unregulated tributary flow may be important in reducing of algal biomass. Benthic algal biomass accrual (measured as chlorophyll a concentration) and community changes were determined after artificial scouring of cobbles. Algal biomass and diversity on scoured cobbles took two weeks to return to levels similar to that on the reference cobbles in summer and took five weeks in winter. Chlorophyll a concentrations on cobbles in summer were initially 0.24 ± 0.06 mg m⁻², but by day 16 had increased to 9.74 ± 1.97 mg m⁻² and were no longer significantly different (P >0.05) from that on reference cobbles. Chlorophyll a concentrations in winter were initially 0.47 ± 0.13 mg m⁻², but by day 37 increased to 44.7 ± 10.9 mg m⁻² and were no longer significantly different (P >0.05) from that on reference cobbles. Peak chlorophyll a concentration accrual during summer and winter was 1.64 and 2.63 mg m d⁻¹, respectively. Diatoms such as Cocconeis, Synedra, and Fragilaria dominated the early succession assemblages, while an abundance of the filamentous green alga Stigeoclonium indicated a late succession assemblage. The Severn River does not experience natural flow variations and periods of drying due to dam operation, although naturally they would have occurred. Summer and winter experiments to determine the response of algal assemblages to drying for 1, 2, 4 or 9 d and re-immersion periods of 13 or 28 d were performed to determine if drying could be used to reduce biomass and re-set algae to early stage assemblages. In winter, drying reduced the abundance of green algae, including Stigeoclonium and other filamentous algal taxa. Chlorophyll a concentrations declined from 22.92 ± 4.71 mg m⁻² to 4.85 ± 0.42 mg m⁻². In summer, drying promoted the growth of Cocconeis and chlorophyll a concentrations were stable at 2.33 ± 0.36 mg m⁻². A grazer-exclusion experiment was conducted to determine macroinvertebrate grazing preferences for early and late succession algal assemblages. Grazers did not prefer the early succession algal assemblages as expected; instead, more grazers were found on late succession cobbles. Treatments with grazing increased chlorophyll a concentrations (from 9.69 ± 0.69 mg m⁻² to 13.15 ± 2.46 mg m⁻²) more than grazer excluded cobbles. Similarly, mean overall abundance (as cells cm⁻²) was greater on grazed cobbles (1030.4 ± 38.7) than cobbles without grazing (881.9 ± 19.2). The filamentous green alga Oedogonium and the chain-forming diatom Fragilaria decreased in abundance in the presence of grazing. This study answered important research questions in the Severn River, focussed on flow management and the effectiveness of proposed environmental flows from Pindari Dam as gazetted in the Border Rivers WSP. Current environmental flow volumes will not be effective in reducing algal growths and will probably increase growths. Environmental flow velocities >1.2 m s⁻¹ should be more effective at reducing algal growths however regrowth to original conditions in warmer times may occur within several weeks. Drying periods could be used to better simulate natural conditions. After drying, benthic algal assemblages had less regrowth of filamentous algae taxa. Because of the work carried out in the study, it is possible that the findings and collected knowledge regarding the effects of flow on benthic algal assemblages contained herein will be used in other regulated streams locally, nationally and internationally. Particularly when increases to flow downstream of a water storage are proposed.
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