The South Australian Murray-Darling Basin Natural Resources Management Board (SA MDB NRM Board) is considering blocking river connections to a number of wetlands along the South Australian River Murray, to prevent evaporative losses, as an emergency drought response. This survey was commissioned to gather some of the ecological data required to inform a risk assessment for 18 of these wetlands, and to contribute information to develop future wetland management plans and drought response strategies. The wetlands sampled included four from the Riverland region (NSW/VIC/SA border to Overland Corner: Nelwood II, Horseshoe and Nelwart Swamps; and Tanyaca Creek) and 14 from the Murray Gorge region (Overland Corner to Mannum: Jaeschke Lagoon; Ross Lagoon; Donald Flat Lagoon; McBean Pound North; Reedy Island; Wombat’s Rest Backwater; Big Bend; Bowhill; Craignook; Henley Park; Maidment Lagoon; Mark’s Landing; Punyelroo; and Teal Flat). Each wetland was surveyed once over 24 hours for fish and water quality and to evaluate the risk of acidification associated with potential wetland drying. A further nine wetlands, for which Baseline ecological data had been collected previously (i.e. for fish, vegetation, macroinvertebrates, water quality, frogs and birds), were also evaluated for potential risk of acidification. These included Woolenook Bend, Gurra Gurra Lake, Lake Bonney, Yatco Lagoon, Pyap, Murbko South, Devon Downs North, North Purnong and Saltbush Flat (South Purnong). Surveys occurred during 15 December 2006 to 15 January 2007. In total 33,506 fish from 15 species, including 10 native species and five invasive species were captured. Native species represented 85% of the total catch. The most abundant and widely distributed native fish taxa were carp gudgeons, unspecked hardyheads and bony herring (together comprising 74% of the total catch). Common carp and eastern gambusia were the most abundant and widespread invasive species, comprising 15% of the total catch. Two of the native species captured (silver perch and freshwater catfish) are currently protected under state (South Australian Fisheries Act 1982) or federal (EPBC Act 1999) legislation, and they were only sampled from one site (Tanyaca Creek). The majority of wetlands surveyed had permanent connections with the River Murray, facilitating sufficient water exchange to maintain desirable water quality characteristics for fish: conductivity < 1000 μs cm-1; pH > 7; dissolved oxygen > 6 mg L-1 (B. Smith, Unpub. Data). The highest mean conductivity reading (5993 μs cm-1) was taken at Ross Lagoon, where the single shallow inlet has become choked with emergent reeds. The least turbid wetland was Maidment Lagoon (maximum recorded Secchi disc depth was 60 cm), which had abundant submerged and emergent aquatic vegetation. Laboratory analyses of water samples highlighted that 70% (n = 19 of 27) of the wetlands surveyed may contain sulfidic sediments that are toxic to aquatic organisms and can produce noxious odours. Oxidation of reduced sulfidic sediments (e.g. through exposure to the air when the wetland is dried out) can also cause problems on re-wetting such as anoxia in the overlying water column (as oxygen is consumed to oxidise sediments), potential toxicity to aquatic plants and animals caused by the mobilisation of metals from the sediments and increased levels of acid. However, these wetlands also had soil pH levels greater than ~ 6, indicating a high buffering (or acid-neutralising) capacity, which would potentially preclude the majority of these wetlands from becoming acidic if they were dried (there is still a risk of sulphate toxicity and possible anoxia, however). Further comprehensive analyses are recommended (see Discussion in PART B). Finally, the combined survey results from all of the River Murray Wetlands Baseline Surveys and from fish-habitat studies in the Chowilla region (B. Zampatti, Unpub. Data) indicate that native fishes, particularly the less abundant large bodied species, prefer habitat with flowing water, whereas invasive fishes prefer still waters. Thus, consideration should be given to the preservation of all wetlands with good connections with the river that facilitate relatively high velocity current flows within their inlet channels (in the range 0.2-0.7 m s-1). This is especially relevant to wetlands in the region between the barrages in the Lower Lakes and Lock and Weir 1 at Blanchetown, where the prevailing north-south winds drive significant changes to river/wetland water levels (thus, high current velocities within the inlet channels). The importance of river flow and naturally-driven current flows (or even flows from irrigation pumps) to native aquatic fauna deserves more attention, particularly in heavily regulated systems such as the Murray-Darling Basin.
MDFRC funding agency: South Australian Murray-Darling Basin Natural Resources Management Board
MDFRC client: South Australian Research and Development Institute, Aquatic Sciences
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Copyright (2007) Murray-Darling Freshwater Research Centre.