Commonwealth Environmental Water Office Long Term Intervention Monitoring Project Gwydir River system Selected Area – 2017-18 Annual Evaluation Report

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Frazier, Paul
Ryder, Darren
Southwell, Mark
Wing, Ying Tsoi
Butler, Gavin
Carpenter-Bundhoo, Luke
Hill, Ronnie
Burch, Linden
Henderson, Tim
Mace, Nathalie
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2018
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Abstract

The Gwydir catchment, located in the northern Murray Darling Basin, extends from the Great Dividing Range west to the Barwon River. Downstream of Moree, the system fans out into a broad alluvial nearterminal floodplain. Numerous anabranches and distributary channels characterise the lower half of the Gwydir catchment, with the Mehi River and Moomin Creek to the south, and the lower Gwydir River, Gingham watercourse and Carole Creek to the north. These channels support wetland and floodplain assets including the lower Gwydir, Gingham and Mallowa wetlands. Commonwealth environmental watering targets channel, wetland and floodplain assets with expected environmental outcomes downstream (west) of Tareelaroi Weir on the Gwydir River. The 2017-18 year saw below average rainfall in July, August, September, December, January, April, May and June with above average temperatures in every month except November. Rainfall within the Selected Area for the last three months of the water year (April-May) totalled 22.4 mm. Commonwealth environmental water was delivered to the channels of the lower Gwydir system and wetlands via a number of watering actions throughout the water year. These were a combination of both Commonwealth and State managed water. This report considers the combined influence of both Commonwealth and State managed environmental water. These flows aimed to protect and maintain wetland vegetation conditions, maintain habitat for waterbirds and fish and support fundamental ecosystem function processes within the Gwydir River System Selected Area (Selected Area). Two early season environmental flows were delivered to the channels of the Selected Area to provide conditions conducive to fish spawning and recruitment. Environmental water was used to offset the component of consumptive extraction taken during a supplementary flow event to provide inflows into the Gingham and lower Gwydir wetlands. Environmental water was also delivered through the Mehi, Moomin and Carole channels as part of the Northern Connectivity Event, later in the water year. Key Outcomes Ecosystem functioning • Environmental water contributed to connectivity in the Gwydir River, lower Gwydir River, Mehi River, Moomin Creek and Gingham watercourse during 2017-18. • 119 ha of the lower Gwydir wetlands was inundated because of environmental water. Maximum inundation within the Gingham Watercourse (364 ha) occurred early in the year due to residual water from the previous water year. • Semi-permanent wetland vegetation species such as water couch, spike-rush, tussock rush, lignum and river cooba were inundated during the 2016-17 water year. Water Quality • Environmental water deliveries improved water quality through the dilution of variables such as pH and conductivity, and water nutrients such as nitrogen and phosphorus. • Delivery of environmental water during the Northern Connectivity Event significantly increased dissolved oxygen concentrations in the Mehi and Gwydir Rivers. • As in 2016-17, water column primary production during 2017-18 appeared to be driven by temperature rather than high nutrient concentrations. Biodiversity • Environmental water influenced seven of the ten ecosystem types monitored in the LTIM project in the 2017-18 water year, including five riverine types, one floodplain type and one lacustrine type. • Environmental flows maintained water levels in the lower Gwydir wetlands, which continued to support the most diverse and abundant macroinvertebrate community in the Selected Area. • Environmental water deliveries were correlated with a spike in microinvertebrate diversity and richness within the river channels of the Selected Area. • The largest movements of Murray cod and freshwater catfish were observed during the early season stimulus flow, where individuals moved within and between the Gwydir and Mehi Rivers. • The generally lower flow conditions across much of the Gwydir in 2017-18 were less conducive to a large-scale breeding event for common carp or goldfish compared to 2016-17. • The cover and species richness of wetland and floodplain vegetation communities within survey plots declined in 2017-18, due to a relatively small area of inundation and generally dry conditions. While the cover of native water couch appears to have remained relatively stable, the cover of the weed species lippia has increased. • Waterbird numbers were lower in 2017-18 than in 2016-17, but higher than in the previous two drier years (2014-16). Higher diversity and abundance was noted at inundated sites. The lower Gwydir supported both threatened and migratory species during 2017-18. Resilience • The fish community in the Selected Area is under extreme stress, with low numbers of adults and barriers to movement hampering recruitment success. Fish movement monitoring suggest that fish move during environmental water deliveries, which may increase the chances of important recruitment events in between larger flooding events. • Consistent with the dry conditions, waterbird numbers and vegetation cover and richness decreased in 2017-18. Small volumes of environmental water inundated a range of vegetation communities and maintained macroinvertebrate communities which provided a food source for higher consumers during this dry year. Implications for Commonwealth environmental water management • The findings from the 2017-18 water year suggest that the current practice of using environmental water based on natural flow cues is working in the lower Gwydir River system, and more broadly that the long-term environmental watering strategy being employed in the Gwydir river system continues to be effective for maintaining ecological communities within the Selected Area. • Flow events delivered earlier in the water year (winter/spring) improve water quality, stimulate fish to move through the system and encourage the development of diverse invertebrate communities. Primary and secondary production during flows at this time of year are limited by colder water temperatures. • Flows delivered over the summer/autumn period tend to improve water quality, and promote primary and secondary production, thus supporting animals further up the food chain such as fish, frogs and waterbirds. • Providing flows to the wetlands has been shown to promote invertebrate production, waterbird populations and vegetation condition. • The fish population in the Gwydir River system remains under stress, with many native species in low abundance. This may reflect the carrying capacity of the system in its current state. While some species appear to be breeding and recruiting, others, especially some of the more iconic species such as golden perch, freshwater catfish and Murray cod, are not recruiting sufficiently to improve their populations. Along with providing environmental flows, other options such as habitat rehabilitation, restocking and barrier remediation should be considered to improve the fish communities of the Selected Area.

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© Copyright Commonwealth of Australia, 2018. Commonwealth Environmental Water Office Long Term Intervention Monitoring Project Gwydir River system Selected Area is licensed by the Commonwealth of Australia for use under a Creative Commons By Attribution 3.0 Australia licence with the exception of the Coat of Arms of the Commonwealth of Australia, the logo of the agency responsible for publishing the report, content supplied by third parties, and any images depicting people. For licence conditions see: http://creativecommons.org/licenses/by/3.0/au/

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Frazier, P; Ryder, D; Southwell, M; Wing, YT; Butler, G; Carpenter-Bundhoo, L; Hill, R; Burch, L; Henderson, T; Mace, N, Commonwealth Environmental Water Office Long Term Intervention Monitoring Project Gwydir River system Selected Area – 2017-18 Annual Evaluation Report, 2018, pp. 1-22

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