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"June 2010".Project Number: Understanding blackwater events and managed flows in the Wakool River system: a scoping study - M/BUS/319.MDFRC item.1 of 2 reports associated with project see (Current understanding of blackwater events relating to the Edward-Wakool River System).The objective of this study was to determine the drivers of low dissolved oxygen blackwater events in the Edward-Wakool river system. Blackwater events are characterized by high levels of dissolved organic carbon (DOC) in the water column. They are a natural and important part of the ecology of lowland rivers. However, if the blackwater event occurs at the wrong time, the dissolved oxygen (DO) in the water column can decline to the point where aquatic organisms die. This was the case for the fish kill that occurred in the Colligen Creek during the summer of 2008/09. A combination of field and laboratory-based approaches were used to examine drivers of low DO blackwater events. Field observations took place at locations within the Edward-Wakool system that had previously experienced blackwater and related fish deaths in summer 2008/9 and included: observations of thermal stratification within pools; measurement of dissolved oxygen and dissolved organic carbon in waterholes; quantification of standing stocks of different carbon types in river channels, including River Red Gum (Eucalytus camaldulensis) saplings, aquatic macrophytes and leaf litter, and estimates of litterfall input.Major findings: Stratification of waterholes can contribute to episodic periods of low oxygen. During this study, low DO was recorded in the hypolimnion (bottom waters) of a waterhole on the Niemur River downstream of the Werai State Forest; the period of low oxygen was co-incidental with the flooding of the forest. Periodic breakdown of stratification, caused by increased flow and/or storm events, led to periods of very low oxygen concentration in the surface water; Litter in dry river channels is the main contributor to water column DOC and to DO depletion upon inundation. Laboratory-based experiments were used to quantify rates of carbon leaching from different sources of carbon and the subsequent impact on dissolved oxygen levels. Our laboratory experiments revealed that litter loadings equal to or greater than 370 g m-2 are sufficient to cause the DO in a shallow water column at 20 °C to fall to zero within two days. This loading is approximately half of that found in the dry channels in the Edward-Wakool system – thus, initial inundation of these channels at a low flow rate is highly likely to trigger a blackwater event; One month of peak litterfall is sufficient to deliver enough carbon to dry river channels to create a blackwater event in a shallow water column upon inundation. Peak litterfall was found to occur between December and January at most locations along the monitored sites. It is hypothesized that the fish kill in Colligen Creek in the summer of 2008/09 was directly linked to peak litterfall. An earlier flow in the system in late spring 2008 would have mobilised organic C present in the creek bed. However, unlike the previous summer, there was no flow in the creek during the period of peak litterfall and therefore a large store of carbon would have accumulated in residual pools or on the dry channel bed during the peak litterfall period. The flow pulse put down the creek in late January and early February 2009 mobilised all of this carbon, creating the low dissolved oxygen blackwater event.Recommendations: Maintain flows in the system during the warmer months of the year. River flow in the Edward-Wakool River system has changed substantially since river regulation. Maintaining flows in the Edward-Wakool system, particularly during the warmer months, would substantially decrease the likelihood of low dissolved oxygen events occurring. Continuous flows would prevent prolonged stratification and the subsequent onset of anoxia (dissolved oxygen depletion ≈ O mg L-1) in shallow waterholes (generally less than 3 m in depth) and more importantly, would prevent the build-up of organic matter in dry channels - particularly during periods of peak litterfall. The timing of flows is critical in avoiding blackwater events since peak litterfall contributes significant quantities of organic matter to the system within a very short period of time. If flows commence prior to peak litterfall, the carbon will be diluted and hence there is less likelihood of low dissolved oxygen in the water column; Assess litter loads prior to flooding dry channels, especially if inundation coincides with warm weather. Monitoring of leaf litterfall is simple and inexpensive. It would allow an estimate of litter build up in the inter-flooding period and hence the potential for low dissolved oxygen blackwater events occurring. This can be complemented by measurement of the leaf litter loads actually in the channel. In this project, we developed a mathematical model which translates litter load and fall into DOC concentrations along river and creek channels which will aid the interpretation of the litter data; Assess the timing and magnitude of environmental flows in red gum forests in the Edward-Wakool system with regard to their impact on receiving waters. An extended period of anoxia in the hypolimnion of a deeper pool in the Colligen Creek downstream of the Werai State Forest coincided with the release of environmental flow water from the forest. Periodic breakdown of stratification led to periods of low dissolved oxygen in the surface water. There would have been less of a likelihood of this occurring if release of water from the forest had occurred during cooler months. The impact of floodplain flood return water on the ecology and water quality in the receiving waters should be assessed as a matter of course to inform the adaptive management of these forests; Monitor stratification and subsequent effects on water quality in waterholes. Stratification in waterholes can lead to periods of poor water quality. A program that monitors the effect of stratification (both temperature and salt driven) on water quality in key waterholes in the Edward-Wakool system could help to inform the flow requirements to maintain the viability of drought refugia in the river system. |
