Lake Alexandrina, Milang
2011 Aquatic Ecosystem Condition Report
- Recent flooding has increased water levels and 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.
- 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 many months to years to see any significant improvement due to the time needed for freshwater species to re-establish in the lake.
- Any return to the low water levels of the mid to late 2000s will again produce catastrophic losses of freshwater species in the lake.
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 western outskirts of Milang along the normal shoreline for the lake, about 300 metres southeast of the Milang Clayton 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 flow during the mid to late 2000s had dropped the level of the lake below sea level and dramatically increased salinity levels beyond the tolerance of most freshwater animal species. A range of species which occurred prior to the drought were subsequently unable to survive in a lake that had receded a long way from its historical shoreline, and created a beach-like environment along the water’s edge with little habitat complexity. The salinity had also increased to well over the maximum concentration recorded in the lake since the barrages isolated the lake from the sea during the 1940s.
The recent floodwaters that entered the lower lakes in late 2010–early 2011 has refilled the lakes and returned Lake Alexandrina back into a freshwater lake. Despite this, biological recovery has not yet occurred and it will probably take many months to years for a wider range of freshwater plants and animals to be able to colonise and again thrive in the lake.
The site selected for monitoring was located along the historical shoreline among dense growths of reeds and other aquatic plants when it was sampled in February 2011. This contrasted with the bare beach that was sampled in March 2010, when the shoreline had receded several hundred metres into the lake.
A sparse community of about 20 macroinvertebrate species was collected. It was dominated by worms and the other species were only present in very low numbers, despite the presence of a wide range of habitat types. The community included nematodes, snails, isopod crustaceans, shrimps, soldierflies, several chironomids, two waterbugs, two odonates and a caddisfly. However, the site lacked any mites, amphipod crustaceans, freshwater crabs, beetles and mayflies, and the only mollusc recorded was an introduced snail (Physa). No sensitive or rare species were collected.
The water was fresh (salinity of 416 mg/L), well oxygenated (61% saturation), alkaline (pH 8) and very cloudy, or opaque, due to the presence of a large amount of suspended clay particles in the floodwaters and phytoplankton in the lake. The water contained high concentrations of nutrients such as nitrogen (1.61 mg/L) and phosphorus (0.19 mg/L).
Both floating (Azolla) and emergent (Phragmites and Schoenoplectus) plants were growing in the water and covered over 65% of the shoreline. A large bloom of small, free-floating algae called phytoplankton was present and contributed to the turbid appearance of water in the lake. A small amount of one green filamentous alga (Cladophora) was also recorded growing among the reeds and sedges in the shallow water surrounding the lake.
The riparian vegetation consisted of an extensive growth of Common Reed (Phragmites australis), patches of Lignum (Muehlenbeckia cunninghami) and a few introduced poplar saplings. The surrounding terrestrial vegetation was introduced grassland and cropping land.
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).