Rehabilitating the snowy river : the influence of environmental flow releases on dissolved organic carbon supply and utilisation

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Environmental flows are the quantity, timing and quality of water flow required to sustain and protect ecosystem and social values. Environmental flows delivered as managed water releases from large reservoirs often form the basis of rehabilitation programs in regulated rivers, and may benefit the aquatic food web by mobilising basal food resources, including dissolved organic carbon (DOC). However, the linkages between managed water releases, organic carbon delivery and microbial metabolic responses remain poorly understood. This thesis aimed to examine and compare the influence of dam and tributary water releases on DOC transport and microbial utilisation using field monitoring and manipulative mesocosm studies in the Snowy River in south-east Australia. Field monitoring revealed positive, linear relationships between DOC concentration and discharge in the unregulated tributary that were absent directly downstream of the dam, and re-emerged below the tributary confluence. Variability in tributary DOC concentration was dampened downstream of a weir facilitating diversions of tributary water. These water diversions prevented approximately 80% of annual tributary DOC export from reaching the main stem. Tributary water releases supplied rapid pulses of terrestrial DOC to the regulated main stem, whereas dam releases produced low, constant DOC concentrations, and mobilised a mixture of terrestrial and microbial DOC. The mesocosm studies showed that benthic microbial communities can respond rapidly to hydrologically-driven variations in DOC and nutrient regimes, providing a potential mechanism through which environmental flows may trigger increased rates of microbial processing. In the field mesocosm study, rapid, short duration increases in benthic metabolic respiration occurred following exposure to high-flow dam release waters. A manipulative laboratory study simulating different DOC regimes on benthic substrates found that a faster terrestrial DOC input rate facilitates stronger effects on microbial enzyme expression and bacterial taxonomic structure relative to press and control treatments. This thesis contributes to a more comprehensive understanding of organic carbon supply and utilisation in regulated rivers, as well as the ecological mechanisms linking resource supply regime and biotic processes. These results reveal the considerable scope for dissolved organic matter in river flows to be actively managed through environmental water delivery. In particular, these results support the wider implementation of tributary environmental water releases in river rehabilitation programs.
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