Monitoring of ecosystem responses to the delivery of environmental water in the Murrumbidgee system - Report 2
Commonwealth Environmental Water engaged Charles Sturt University to monitor the ecological response to the environmental water release that took place in the Murrumbidgee River in June 2011. This is the final report detailing the outcomes of the watering action.
In June 2011, nearly 110 gigalitres of Commonwealth environmental water were provided to a watering action managed by New South Wales, which totalled 161 gigalitres (including 23 gigalitres from the The Living Murray; 21 gigalitres from New South Wales Environmental Water Allowance and 8 gigalitres from private donations) targeting the mid-Murrumbidgee wetlands. The water was released from Burrinjuck and Blowering Dams with the environmental flow reaching a maximum daily discharge of 24,908 ML/day in the Murrumbidgee River downstream of Burrinjuck Dam on 17th June 2011 and 9,492 ML/day in the Tumut River downstream of Blowering Dam on 16th June 2011. From July 2011 to February 2012, natural and managed river flows further inundated a sub-set of wetlands in the mid-Murrumbidgee.
The monitoring program assessed two key components of the environmental release - the response of wetland flora, fauna and water quality after the water had filled key sites within the nationally important mid-Murrumbidgee wetlands (Environment Australia, 2001) and the response of biofilms and macroinvertebrates within the river channel (in-stream) as the water was released from Blowering and Burrunjuck Dams. These two components are covered separately within this report.
Summary of key outcomes from the June 2011 environmental release were:
- Improved water quality, including reduced dissolved organic carbon and stabilised dissolved oxygen levels.
- Increased native fish species diversity from 2005 survey levels (Gilligan 2005) and successful native fish recruitment.
- Successful recruitment for wetland frog species leading to an increased abundance of frogs within filled wetlands when compared to the control wetlands.
- Promoted nesting and created conditions that allowed for successful fledging of darters, great cormorants and little pied cormorants at key filled wetlands.
- Increased aquatic vegetation cover and species diversity within filled wetlands.
- Benefits to in-stream ecosystem due to reduced biomass of biofilms, increase in the relative proportion of early successional algal taxa (e.g. diatoms) and increased number of macroinvertebrate taxa.
- Scouring of biofilms also provided short-term benefit to the community by reducing the nuisance factor that occurs when the biofilms form mats and builds up to unacceptable levels.
Monitoring of the responses of wetland biota (vegetation, frogs, fish, waterbirds and freshwater turtles) organic carbon and water quality was undertaken at twelve wetlands in the mid-Murrumbidgee wetland complex between June 2011 and February 2012. Additional data was drawn from previous wetland monitoring surveys conducted between November 2010 and April 2011 by Wassens and Amos (2011). Three types of treatment were included in this study - filled (received environmental water during the release), control 1 (did not receive environmental water during the release) and control 2 (did not receive water during the environmental release, but received run-off from rainfall and drainage water from surrounding areas).
The key outcomes of the June 2011 environmental release combined with subsequent small scale natural river flow increases in late August/September and a small environmental release in December 2011 within the mid-Murrumbidgee wetlands are as follows:
- The June 2011 environmental release had a positive impact on dissolved, total and particulate organic carbon levels within wetlands. Very high levels of dissolved organic carbon can contribute to low dissolved oxygen events (commonly called black-water), the environmental flow had a dilution effect leading to an overall decrease in organic carbon levels within the filled wetlands and reducing the risks of future low dissolved oxygen events. In contrast dissolved organic carbon levels increased within the control wetlands which did not receive environmental water. Dissolved oxygen levels were similar between the filled and control wetlands and were within the normal range throughout the 2011-12 monitoring period.
- The June environmental releases assisted in the recovery of aquatic vegetation communities. That is, recovery of aquatic vegetation communities was greatest within the filled wetlands when compared to the control wetlands. In particular, the percent cover of aquatic vegetation within filled wetlands increased significantly over time, while the percent cover of aquatic vegetation within the control wetlands remained stable. The rate of recovery of aquatic vegetation was influenced by the length of time that the wetland had been dry prior to first re-filling in August 2010. Wetlands that had been dry for between three and five years had the greatest increase in aquatic vegetation cover after receiving the environmental flows, compared with those that had been dry for more than five years.
- Overall, the June 2011 environmental releases had a positive impact on frog breeding and abundance. Five frog species were recorded during the monitoring period. The abundance of barking marsh frogs and spotted marsh frogs increased significantly, compared to surveys in 2010-11 levels, in wetlands that received environmental water in 2011 but not in the control wetlands. Breeding activity in response to the environmental release commenced in August 2011 when ambient temperature started to increase, and tadpoles were recorded from October 2011 onwards. Further natural and managed river flows prompted calling by summer active species such as Peron’s tree frogs. Tadpoles of the barking marsh frog, spotted marsh frog and Peron’s tree frog were recorded in both filled and control 2 wetlands, but no tadpoles were recorded in the control 1 wetland.
- Five native and five introduced fish species were recorded during this study. The environmental release appeared to favour native over exotic species, and native fish were more abundant in filled wetlands than introduced species. Juveniles of all five native species were recorded in filled wetlands, with juveniles of carp gudgeon, bony bream and un-specked hardyhead making up 50% or more of the total catch for these species by February 2012.
- Two species of freshwater turtle were recorded (Macquarie River and long-necked turtles) within the filled wetlands. In addition, Macquarie River turtle hatchlings were detected in two of the filled wetlands, Yarrada and Molleys lagoons, during the December 2011 and February 2012 surveys.
- Waterbird communities were diverse in the mid-Murrumbidgee wetlands, with 36 species recorded during ground surveys undertaken from June 2011 – February 2012. Colonial waterbird breeding was recorded following the June 2011 environmental release with a small number of nests recorded in Gooragool, Yarrada and McKennas lagoons. Surveys to date have indicated that the mid-Murrumbidgee wetlands are in a recovery stage, with vegetation communities of the wetlands re-establishing following their filling in 2011. Most waterbirds prefer wetland habitats with aquatic vegetation, either as foraging habitat or to provide shelter from weather and predators. We expect the wetlands to become more attractive to waterbirds as wetland vegetation is further established.
These findings suggest that while the mid-Murrumbidgee wetlands are still in a recovery phase following an extended dry period, they still support high levels of diversity in terms of aquatic vegetation, frogs, native fish, waterbirds and freshwater turtles. The 2011 environmental releases had a positive impact on fauna, flora and water quality within the filled wetlands and created conditions suitable for frog, native fish, turtle and waterbird breeding. By creating conditions for successful recruitment, the environmental release is likely to have contributed to the longer-term recovery of the fauna and flora communities within the mid-Murrumbidgee wetland area. Small top-up flows in spring and summer extended wetland hydroperiod, in many cases increased recruitment outcomes for native fish, frogs and freshwater turtles, and helped to maintain waterbird diversity through summer months. Small spring and summer top-up flows were therefore crucial to the overall success of this watering event, and the provision of small top-up flows should be incorporated into future watering plans, especially in situations where the bulk of the environmental release occurs in autumn or winter.
The in-stream ecosystem responses to the environmental flow in June 2011 were as follows:
- The environmental flow had a positive effect by significantly reducing the biomass of biofilm at several sites, most likely due to scouring of biofilms from increased water velocity. Scouring of biofilm provides benefit by contributing nutrients and food into the water column, thus providing an important resource for downstream communities. It also provides benefit to the community by reducing the nuisance factor that occurs when the biofilms form mats and builds up to unacceptable levels. However, approximately five weeks after the recession of the environmental flow following a return to normal regulated dam operations, the biofilm biomass had increased to levels higher than observed prior to the environmental flow, particularly at the most upstream sites. In contrast, biofilm biomass remained consistently low throughout the study in the unregulated Goobarragandra River.
- The biofilm was comprised of 58 algal taxa including red algae, green algae, blue-green algae and diatoms. In the Murrumbidgee River, the environmental flow provided benefit by reducing the relative proportion of red, green and blue-green algae and increasing the proportion of diatoms, which are common in unregulated river systems. In the upstream reaches of the Tumut River, the environmental flow provided only short-term benefits because dam operations returned to normal regulated practices soon after the environmental flow. The environmental flow provided a longer-term benefit at Tumut sites 3, and 4, downstream of the confluence with the unregulated Goobarragandra River, because the unregulated flows from the Goobarragandra River contributes to the health of these downstream reaches.
- The environmental flow had a short-term benefit on the in-stream ecosystem by increasing in the number of macroinvertebrate taxa in the Tumut River immediately after the environmental flow. However, the total number of taxa reduced after dam operations returned to normal regulated practices. For the majority of the dominant macroinvertebrate taxa there was also a short term increase in abundances immediately after the environmental flow.
These findings suggests that while the environmental flow provided a short-term benefit to the in-stream ecosystem through scouring of biofilms, the benefits were not sustained when the dam operations returned to normal regulated practices. The in-stream benefits of the environmental release were far greater in the Murrumbidgee River than in the Tumut River. This occurred because the environmental release from Blowering Dam to the Tumut River had a similar river height and discharge to normal delivery for consumptive use. This highlights the importance of understanding the normal operational flow regimes that are present within a regulated river system and understanding of the key flow components that reflect the variability of a more natural flow hydrograph. Environmental releases should specifically aim to restore key flow components that have been lost through river regulation and avoid delivering flows that would exacerbate negative impacts of regulated operations. By using a careful release strategy, environmental flows can be delivered to optimise benefits for both wetland and in-stream ecosystems.
- Monitoring of ecosystem responses to the delivery of environmental water in the Murrumbidgee system - Report 1