Aquatic Survey of Purda Billabong, Near Wentworth NSW

"April 2003".Project Number: Purda Billabong aquatic fauna survey - M/BUS/70.MDFRC item.Purda Billabong, otherwise known as 'Pink Lake', is an oxbow wetland located on 'Moorna Station' (Walsh family) approximately 30 km west of the township of Wentworth NSW. The wetland is approximately 174 hectares in size and is connected to the Murray River via a small feeder channel off Frenchman's Creek. Due to the influence of Lock 9, the majority of the surface area of the wetland is permanently inundated. The NSW Murray Wetlands Working Group, in conjunction with the Walsh family of 'Moorna Station', are investigating options to restore a more natural hydrological regime to the wetland by constructing a regulating device on the inlet channel that links the wetland to the Murray River. An aquatic fauna survey was conducted in November 2002 to obtain baseline information on the current diversity of biota within the wetland. The water quality of Purda Billabong was good with levels similar to those recorded in a nearby wetland known as Thegoa Lagoon (except for turbidity) and the Murray River downstream of Lock 8. Dissolved oxygen and pH levels were similar between sites and sampling occasions whereas turbidity, water temperature and electrical conductivity levels varied. Electrical conductivity was seen to increase from the inlet to the furthest site from the inlet due to evaporation and the diluting effect of water entering the wetland from the Murray River. Nutrient levels in Purda Billabong vaired little between sites and are were found to be substantially lower than levels measured in Thegoa Lagoon. Phytoplankton biomass (chlorophyll concentrations) varied little between sites within the wetland and were considered low in comparison to concentrations recorded in the Murray River upstream of the Darling River junction at a similar time of year. A total of 4584 individual microinvertebrates representing 10 taxa were collected in Purda Billabong, all of which were considered to be widespread in South Eastern Australia. The variability in taxa richness and abundance encounted between sites was considered typical of the spatial and temporal variability of open water microinvertebrate communities. Macroinvertebrates were sampled in the wetland with artificial substrates and a sweep net. A total of 34,930 macroinvertebrates representing 25 taxa were collected during the study. Of these 77% were captured in 5 artificial substrates (29,928 individuals and 23 taxa) compared to 23% (7,982 individuals and 14 families) in the single 10 m edge sweep sample. Artificial substrates were dominated by benthic taxa such as the dipterans (mostly chironomids) while the sweep net sample was dominated by more mobile pelagic taxa such as hemipterans (mostly corixids). SIGNAL (Stream Invertebrate Grade Number – Average Level) scores were used to calculate the average sensitivity of macroinvertebrate taxa at each site. SIGNAL scores ranged from 3 to 4 out of 10 suggesting that only relatively tolerant individuals were present in Purda Billabong. The low scores are not unexpected for lowland river environments where sensitive taxa are typically absent. A total of 1105 individual fish representing 11 taxa (4 alien and 7 native species) were collected from Purda Billabong. All of the species sampled were considered to be common in lowland environments. The Carp Gudgeon complex and Australian Smelt were the most abundant native species contribution to 48% and 29% of the total catch respectively. Carp were the most common alien species contribution to 10% of the total catch. The diversity and abundance of aquatic biota from Purda Billabong is similar to that of Thegoa Lagoon near Wentworth NSW. By reinstating the natural hydrological regime, the productivity of the Purda Billabong should increase in the short term and an improvement in the biodiversity values of the wetland in the long term. The following recommendations are based on results from the fauna survey. - A regulatory structure should be installed at the junction of the inlet channel with Frenchman's Creek to initiate a drying phase within the wetland. - The length of time, frequency and magnitude of the drying phase should be linked to the natural hydrological regime prior to river regulation. - Groundwater testing and monitoring be conducted to determine the potential for groundwater to affect the wetland. - The drying phase should be initiated from the current level of the billabong to minimize habitat loss for invertebrates and fish. - Stock access to the drying sediment should be restricted to maintain water quality and invertebrate egg masses, particularly at the shallow eastern end of the billabong where the majority of the wetland sediments will be exposed. - Continued monitoring of the aquatic macroinvertebrate community during drying and wetting cycles