The River Murray’s Lower Lakes and major tributary streams were sampled in February 2011 to assess their biological condition at a time when floodwaters from interstate had freshened the region.
Three sites on Lake Alexandrina and two sites on Lake Albert were sampled to assess potential variability across each lake. Another three sites were sampled on Currency Creek, Finniss River and the Goolwa (or Lower Murray) channel upstream from the Goolwa Barrage.
Most sites were given a Poor rating because each ecosystem showed evidence of major changes in animal communities and plant life, and moderate changes to the way the ecosystems functioned. The site on Lake Albert near Waltowa was given a Very Poor rating because the extremely low diversity and abundance of aquatic macroinvertebrates warranted a more severe assessment compared to the other sites that were assessed in 2011.
The biological responses to the freshening of each waterbody had not resulted in diverse aquatic macroinvertebrate or plant communities establishing at any site. Each waterbody showed evidence of a shift towards more of a freshwater ecosystem due to the collection of a wider range of freshwater species. However, abundances were very low and many types of invertebrates and plants that occurred in the region before the drought had not yet established in the lakes and tributary streams. It is likely that any recovery towards more of a pre-drought assemblage of species will take many months to years to occur.
The sites on Lake Alexandrina provided habitat for at least a few macroinvertebrates normally found in floodwater and freshwater wetland habitats of the River Murray in South Australia (e.g. chironomid Cladotanytarsus and freshwater shrimp Paratya australiensis). A range of 12-20 species of macroinvertebrates was collected from the three sites sampled from the lake but it still lacked many groups (e.g. bivalves, native snails, mites, mayflies and several types of flies) that occurred in the lake prior to the drought. The lake should, however, slowly recover towards more of a pre-drought state over the following months to years, if prolonged inflows of freshwater continue to push water through the river and into the lake, Goolwa channel and river mouth.
Lake Albert remained a saline lake with salinities ranging from 4,230–4,784 mg/L, compared with pre-drought conditions when salinity levels rarely exceeded 1,000 mg/L. The current salinity is too high for many freshwater species to tolerate, which has resulted in a sparse assemblage of 8–13 generalists, organic feeders and saline tolerant macroinvertebrate species occupying the lake in early 2011. The salinity would need to be lowered significantly for many months to years for a freshwater community of animals and plants to be able to re-establish in the lake; this will be difficult to achieve in a lake with only one connection to Lake Alexandrina for salt to be exported out of the lake. It seems likely that Lake Albert will continue to support a large plankton community, including saline tolerant zooplankton and large growths of green algae, and possibly cyanobacteria, but it will provide limited habitat for aquatic macroinvertebrates and plants. Only a few macroinvertebrates and plants that favour saline waters will be persist including a few species of flies, amphipod crustaceans, reeds and club-rush.
The Goolwa channel had a salinity of only 280 mg/L and provided habitat for a low diversity and abundance of freshwater macroinvertebrates when sampled in February 2011. This contrasted with similar work a year earlier when the salinity was 6,650 mg/L and a few saline tolerant species were collected. The Goolwa channel will probably always undergo major shifts in ecosystem condition in response to freshening floods and the intrusion of marine waters during low flow periods, and the time taken for different types of aquatic organisms to extend into and exploit the environmental conditions within the channel.
The lower sections of the tributary streams also continued to provide limited habitat for many animals and plants, despite significant freshening compared with February 2010 when salinities in the Finniss River and Currency Creek ranged from 14,000–33,000 mg/L. The Finniss River was moderately fresh with a salinity of 1,340 mg/L, supported many freshwater aquatic plants but had a low diversity and abundance of aquatic macroinvertebrates. Currency Creek was also moderately fresh with a salinity of 2,825 mg/L but supported fewer freshwater plants and the sparse macroinvertebrate community included a number of saline tolerant species.
All sites were enriched with nutrients (e.g. concentrations of nitrogen >1 mg/L and phosphorus >0.05 mg/L), and supported large growths of phytoplankton in the water. The characteristically opaque floodwaters from the River Murray were noted at all sites apart from Currency Creek, which was isolated from floodwaters by a regulating bank, and Lake Albert near Waltowa, where the floodwaters had not yet mixed with the water near the edge of the lake.
The sediments consisted of fine particles such as sand, silt and detritus. They were sulfidic and anaerobic below the surface at Lake Albert, Milang on Lake Alexandrina, Currency Creek and Goolwa channel; these sites may become aerobic in future months in response to prolonged flooding from the River Murray or from the upstream Currency Creek catchment. This is expected to create conditions that will favour a wider range of burrowing species compared to the low number of species that inhabit anaerobic sediments.
The sites sampled from the region are located within the Coorong and Lakes Alexandrina and Albert Wetland, which was listed as a Wetland of International Importance under the Ramsar Convention 1985 because it provided a wide range of wetland habitats for many nationally threatened bird and fish species.
The largely constant water levels and permanently freshwater conditions that persisted in the region since the Goolwa Barrages were constructed in the 1940s up until the mid to late 2000s, provided significant habitat for many types of threatened fish species in the lower reaches of Finniss River, Currency Creek and parts of Lake Alexandrina.
Many other significant species of plants and animals inhabited this region prior to the recent change from freshwater to more saline habitats, including frogs, birds, macroinvertebrates and plants. A large proportion of the freshwater species were lost or severely restricted due to the changes that occurred in the lower Murray from about 2007 to late 2010.
A series of studies have subsequently been carried out since 2010 to describe the environmental assets that have persisted or been mitigated as part of the Murray Futures Lower Lakes and Coorong Recovery program.
