Lake Alexandrina, near Wellington
2011 Aquatic Ecosystem Condition Report
- Recent flooding freshened the lake but it continues to be turbid and nutrient enriched.
- Low diversity and abundance of macroinvertebrates and absence of rare or sensitive species.
- Composition of macroinvertebrate community starting to resemble a fresh, floodwater assemblage found in other wetlands in the Lower Murray.
- Low diversity of aquatic plants and only reeds are thriving around the edge of the lake.
- Biological condition remains poor and it will probably take months to years to recover from the recent prolonged drought.
- Any return to the saline, low water levels recorded in 2007–10 will produce catastrophic losses of freshwater species again.
About the location
Lake Alexandrina is located about 100 km southeast of Adelaide at the downstream end of the River Murray system where the river meets the sea. The largest freshwater lake in South Australia, it has a catchment area of about 360 km2 and has a maximum depth of about four metres.
The site selected for monitoring was located off the Wellington to Langhorne Creek Road, about 13 km west of Wellington, in the northeastern section of the lake.
The site was given a Poor rating because the ecosystem showed evidence of major changes in the animal community and plant life, and moderate changes to the way the ecosystem functions.
Prolonged drought and changes in water levels associated with limited River Murray flows in the mid to late 2000s had degraded the biological community of the site when in was assessed in February 2011. Until the recent prolonged drought, freshwater conditions had prevailed in the lake since the Goolwa Barrages were installed in the 1940s; salinity levels rarely exceeded 492 mg/L at a site monitored regularly near Boggy Lake during the 1990s.
During 2009–2010, salinities over 2,000 mg/L were recorded from this section of the lake and would have contributed to the very poor biological communities present in the lake. In 2011, some improvement in community structure has occurred but many groups are still missing, and those species that were collected were present in very low numbers.
The recent floodwaters had filled the lake in late 2010–early 2011 up to the historical well-vegetated shoreline. A sparse community of about 12 macroinvertebrate species was collected from the site in February 2011, including nematodes, worms, eusirid amphipods, freshwater shrimps, freshwater prawns, at least four chironomids, corixid waterbugs and two types of caddisflies. The most abundant species included a midge (Cladotanytarsus), shrimp (Paratya) and worms but the abundances of the other species were all very low.
The community shows some similarity to the sorts of species found in recently flooded freshwater wetlands from other parts of the River Murray floodplain in South Australia. The saline tolerant species that were recorded a year earlier were not detected in February 2011 and had been replaced by a simple freshwater assemblage of species. However, the low abundances and absence of many expected groups (e.g. molluscs, mites, odonates, beetles and mayflies) indicates that the lake is still in the early stages of recovering from the effects of the drought, prolonged low water levels and higher than normal salinity levels.
The water was fresh (salinity of 254 mg/L), well oxygenated (89% saturation), alkaline (pH 8.6), and very cloudy, or opaque. It also had high concentrations of nutrients such as nitrogen (2 mg/L) and phosphorus (0.25 mg/L).
Several submerged (Stuckenia or Ruppia) and emergent plants (Phragmites, Eleocharis and Juncus) were growing on the water’s edge and covered over 35% of the shoreline. Some green filamentous algae (including Cladophora) was recorded and a large amount of small, free-floating algae called phytoplankton was also present and probably contributed to the turbid appearance of the lake.
The riparian vegetation mostly consisted of introduced grasses and patches of Lignum (Muehlenbeckia cunninghamii). The surrounding landscape was introduced grassland and low shrubland consisting of sedges and saltbush.
Special environmental features
The site sampled lies within the Coorong and Lakes Alexandrina and Albert Wetland which is listed as a Ramsar Wetland of international importance due to the wide range of wetland habitats present, its importance for waders and waterfowl, and the presence of many nationally threatened species. No special features were noted at the site sampled in February 2011.
Pressures and management responses
|The extended drought conditions prior to 2010 caused severe salinity related impacts. The salinity has decreased but it has not returned to its normal level, despite freshwater inflows (reducing ecological integrity).||The SA government is negotiating through the Basin Planning process to secure water to achieve water quality and ecosystem health objectives for the region. South Australia has determined the environmental water requirements for the CLLMM site, this report can be obtained from the Goyder Institute. For further information follow the link to the DENR website on environmental water requirements.|
|Livestock have direct access to many creeks in the catchment, causing sediment erosion and adding excessive nutrients (which leads to habitat disturbance, algal growth and aquatic weeds).|
|Limited riparian vegetation throughout the catchment, which means there is minimal buffer protection from agricultural runoff carrying sediments and nutrients (causing habitat disturbance and algal growth).|