Drought Contingency Monitoring Back Creek and Tumudgery - M/BUS/250
MDFRC Publication 08/2010
1 of 4 reports associated with project see (Assessment of sulfidic sediments at Tumudgery Creek, NSW), (Assessment of sulfidic sediments at Back Creek, NSW) and (Drought Contingency Monitoring at Back Creek, NSW).
Tumudgery Creek is a wetland of 136 ha on the Edward-Wakool River system located 40 km north-west of Deniliquin, NSW. In 2007, it was selected for disconnection from the Edward River to achieve water savings as part of drought contingency planning. The Murray-Darling Basin Commission (now the Murray-Darling Basin Authority) engaged the Murray-Darling Freshwater Research Centre (MDFRC) to monitor changes in the environmental condition of the wetland to detect and respond to risks of environmental damage. This report deals only with the outcomes and implications of this monitoring and does not present the broader details of the program. Ecological indicators monitored at Tumudgery Creek included presence of sulfidic sediments, water levels and quality, blue-green algae, fish richness and abundance, River Red Gum condition, vegetation community, birds (waterbirds and terrestrial) and groundwater levels. Ten tri-monthly surveys were undertaken during the monitoring period of 6 September 2007 to 18 December 2009. Monitoring results were assessed against pre-determined 'trigger values’ for management intervention based on Wallace et al. (2007) and DWE (2007). These trigger values provided a point at which managers could assess if a management intervention was required. Water levels and quality Tumudgery Creek received four significant inflows during the sampling period. One inflow event occurred before and three occurred after the refurbishment of the Tumudgery Creek regulator in June 2008, with the largest inflow event occurring between September and December 2009. The wetland contained surface water for the duration of the study period, although sections of the creek bed dried between sites T1 and T3 during periods of low water levels. Cross-sectional measurements of the creek bed taken in the single pool between sites T3 and T5 showed water depths of 1.0 to 2.4 m in October 2008 when water levels were at their lowest. The electrical conductivity of surface water ranged from 46 to 154 µS.cm-1 @ 25 °C and was therefore below the 5,000 µS.cm-1 trigger value of DWE (2007) and the 1,000 mg.L-1 (≈ 1,500 µS.cm-1) trigger value of Wallace et al. (2007) at all times. Surface water pH mostly remained within the acceptable pH 6-9 range of DWE (2007) and Wallace et al. (2007), but was outside this range at a single site in August 2008 (pH 9.10) and November 2008 (pH 5.95). Mean dissolved oxygen concentrations at the surface (0.2 m depth) ranged from 4.74 to 10.29 mg.L-1 and were above the 5 mg.L-1 threshold of DWE (2007) on 26 of 28 sampling occasions, and were always above the 2 mg.L-1 threshold of Wallace et al. (2007). The lowest individual site recording for dissolved oxygen was 1.88 mg.L-1. Surface water temperatures ranged from 8.8 °C to 25.1 °C, and were within the DWE (2007) acceptable range of 6–30°C at all sites and monitoring events. Turbidity levels ranged from 45 to 937 NTU, with most readings being <250 NTU. Blue-green algae Twenty two blue-green algae taxa were sampled over the study period. Counts exceeded the trigger value of 15,000 cells.mL-1 of Wallace et al. (2007) for 15 of the 30 samples. Two potential toxin producers, Anabaena circinalis and Microcystis aeruginosa were sampled at concentrations above the 15,000 cells.mL-1 threshold at one site in March 2008. The remaining 14 samples exceeding 15,000 cells.mL-1 were of taxa not known as toxin producers. Fish In total, 12,391 fish comprising nine native and five exotic species were sampled. 922 fish were caught with electrofishing (5 surveys) and 11,469 fish were caught in fyke nets (10 surveys). Silver perch Bidyanus bidyanus (n=3), listed as Vulnerable under the NSW Fisheries Management Act 1994 and Murray cod Maccullochella peelii peelii (n=10), listed as Vulnerable under the Commonwealth Environment Protection and Biodiversity Conservation Act 1999, were sampled in low abundance and by electrofishing only. River Red Gum The majority of River Red Gum Eucalyptus camaldulensis had a crown condition rating of 'moderate cover’ on each sampling occasion. Trees at site T2 had a lower crown condition than those at sites T3 and T4. At site T2, three trees met the Wallace et al. (2007) trigger point for management notification when their crown condition deteriorated from category 4 (moderate foliage cover) to category 0 (sparse crown of dead leaves). It was difficult to relate wetland disconnection to the decrease in tree condition given the wetland contained water throughout the monitoring period. Vegetation community Thirty-five plant taxa, including five introduced species, from eight functional groups were identified from nine surveys of vegetation community. The most abundant species sampled was Common Rush Juncus usitatus, followed by River Red Gum Eucalyptus camaldulensis and Common Sneezeweed Centipedia cunninghamii. No rare or threatened plant species were recorded. The noxious weed Arrowhead Sagittaria platyphylla was observed in March 2009 at site T1 and managers were notified of its occurrence. Birds A total of 180 waterbirds from 11 species were recorded at Tumudgery Creek from nine surveys. The richness of waterbird species was low, ranging from 2 to 5 species for each sampling event. The White-bellied Sea-eagle was recorded at Tumudgery Creek on two surveys. This species is listed on the China-Australia Migratory Bird Agreement and as a migratory bird under the Commonwealth Environmental Protection and Biodiversity Conservation Act 1999. Forty-eight terrestrial bird species were recorded during the bird surveys. Species richness ranged from 12 to 25 for each survey. No species of conservation concern were recorded, although five terrestrial bird species are considered to be declining in abundance in the region. Groundwater The influence of groundwater on Tumudgery Creek was based on interpretation of regional data. The closest monitoring site with recent groundwater level information was bore GW502020 located 4.5 km to the south-east of Tumudgery Creek. Regional groundwater levels declined during the drought period and were not expected to have any salinity impacts on Tumudgery Creek. Groundwater salinity in the region was 10,000 to 20,000 µS.cm-1. The resulting low electrical conductivity of surface water when the wetland was disconnected to achieve water savings supported the prediction that Tumudgery Creek would not intercept saline water. Achievement of the study objective The monitoring program was successful in achieving its objective of assessing the ecological condition of Tumudgery Creek following its disconnection from the Edward River. There was no evidence for a decrease in the ecological condition of Tumudgery Creek over the study period with the exception of some River Red Gums at site T2 and high blue-green algae counts from half of the samples. The general maintenance of ecological condition is primarily due to the wetland containing surface water throughout the monitoring period from the four separate inflow events. Monitoring results were evaluated against pre-determined trigger values and communicated regularly to the MDBA and NSW government agencies. This communication was important to ensure managers were able to make timely decisions, if required, based on recent and appropriate environmental data. Future management and recommendations The regular connection of Tumudgery Creek with the Edward River is important ecologically, as it allows for the exchange of water, biota and nutrients between Tumudgery Creek, the wetland complex within the Werai State Forest and the Edward River. Given the relatively low commence-to-flow of this wetland, this exchange can occur at low river flows if the regulator is open. Based on the findings that Tumudgery Creek supports a significant fish community, that the authors believe the maintenance of ecological condition during the disconnection was primarily due to the creek not drying out, and the wetland has a natural water regime of 'permanent’ (NSW MWWG 2008), the authors recommend that: Tumudgery Creek is managed to allow the two-way exchange of water between the wetland and the Edward River and to maintain sufficient aquatic habitat within the creek to support any isolated fish community. A management plan is developed for Tumudgery Creek to maximise the ecological values of the wetland, to develop operating rules around the closing of the regulator and to scope other issues important for the effective management of the site. The management plan should develop an optimal water regime for the wetland that accommodates the first recommendation.