Lake Alexandrina, Poltalloch
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 during the drought will again produce catastrophic losses of freshwater species.
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 on the south-eastern margin of the lake at Poltalloch, about 10 km east of Narrung on the Narrung Road.
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 it 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 490 mg/L at a site monitored regularly near Poltalloch 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 refilling of the lake had extended the waterline back towards its historical shoreline when it was sampled in February 2011. A low diversity of about 16 macroinvertebrate species was found, including hydrozoans, nematodes, worms, eusirid amphipods, freshwater shrimps, springtails, biting midge larvae, at least four chironomids, three waterbugs, a damselfly and caddisfly. None were present in large numbers and no sensitive or rare species were detected.
A number of groups were missing from the site despite the presence of suitable habitats, including molluscs, mites, beetles, mayflies and a wider range of dipteran families. The most notable feature of the community sampled at Poltalloch was the presence of a chironomid (Cladotanytarsus) and freshwater shrimp (Paratya), species that are typically found in recently flooded freshwater wetlands of the Lower Murray.
The water was fresh (salinity of 200 mg/L), well oxygenated (89% saturation), alkaline (pH 8.7) and very cloudy or opaque. It also contained high concentrations of nutrients such as nitrogen (1.3 mg/L) and phosphorus (0.19 mg/L).
Small patches of green filamentous algae (including Cladophora) were recorded but a much larger growth of small, free-floating algae called phytoplankton was present in the water, and probably contributed to the turbid appearance of the lake.
Common Reed (Phragmites australis) and cumbungi (Typha) covered over 10% of the shoreline but other species were not evident within the newly flooded site. The riparian zone consisted of introduced grasses and reeds, and the surrounding landscape was largely cleared of its original vegetation and consisted of a few gums, introduced pines and Pepper Trees, and introduced grasses.
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).|