Goolwa (or Lower Murray) channel, Hindmarsh Island
2011 Aquatic Ecosystem Condition Report
- Fresh, turbid floodwaters from upstream states reached Lower Murray and was flowing out to sea through the channel in February 2011.
- High nutrient concentrations and large amount of phytoplankton but no waterplants detected due to high water level.
- Sparse macroinvertebrate community consisting of a few generalist and tolerant species.
- Expect more diverse freshwater community to establish if the channel remains permanently fresh.
About the location
The Goolwa channel flows from the western edge of Lake Alexandrina, near Clayton to the Goolwa Barrage. Also known as the Lower Murray channel, it should not be confused with the channel which starts at the barrage and flows to the Murray Mouth. Located about three kilometres southeast of Goolwa, the barrage is one of five structures which separate the sea from the normally freshwater environments of the Lower Lakes and River Murray. The barrage extends about 650 metres across the channel from the southwestern edge of Hindmarsh Island to Sir Richard Peninsula.
Prior to the barrages being installed in the 1940s, seawater and freshwater would have regularly mixed in the Goolwa channel and parts of Lake Alexandrina to create highly variable environments. These environments probably supported animals and plants with short lifecycles capable of colonising suitable habitats rapidly, or very tolerant of changing salinity levels. Since the barrages have been installed, waterbodies upstream have been maintained as permanent freshwater environments, which is likely to have reduced the ability of resident plants and animals to adapt to the rapid increases in salinity that occurred in the Lower Lakes region from 2007 to late 2010.
The site selected for monitoring was located about 1.5 km upstream from the Goolwa Barrage, on the western shore of the Hindmarsh Island Marina.
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.
The Goolwa channel represents a transitional environment undergoing major changes in response to the recent, large inflows of very fresh, turbid floodwaters into what was a saline waterbody. The floodwaters that originated in the upstream states of Victoria and New South Wales in mid-2010 reached the Lower Murray a few months later and only started to significantly lower the salinity of the Goolwa channel and Lake Alexandrina from about December 2010 onwards.
At the time of sampling in February 2011, the channel was a freshwater environment, compared with the last biological assessment of the site in February 2010 when it was saline. The channel remained enriched with nutrients and continued to support a large growth of phytoplankton algae in the water, despite the flushing flows that had passed through the channel and out to sea for about 2-3 months. The limited diversity of macroinvertebrates and plants recorded in February 2011 showed that a complex freshwater aquatic ecosystem has not yet had the time to establish in the channel and that it may take several months or longer for a wider suite of species to reach and colonise the most downstream part of the basin.
At the time of sampling, the channel extended about 900 metres from Hindmarsh Island to Sir Richard Peninsula, and the habitats sampled along the edge consisted of shallow, sandy beaches with areas of rocky outcrops. Only 11 macroinvertebrate species were found, including a flatworm, snail, worm, two types of amphipod crustaceans, freshwater crab, an immature caridean crustacean, hypogastrurid springtail, chironomid from the Family Orthocladiinae, corixid waterbug and a leptocerid caddisfly. The community consisted of a range of generalist freshwater species that were all found in very low numbers. No sensitive or rare species were collected.
The recent flooding of the channel with fresh, turbid water presumably contributed to the poor diversity and abundance, and the general lack of many major macroinvertebrate groups that what would be expected to occur in the channel (eg additional molluscs, mites, dipterans, beetles, more waterbugs, mayflies and odonates) given the diversity of habitat types available. A wider range of species would, however, be expected to colonise the channel in the future if sustained freshwater flows continue. Otherwise the more saline tolerant species, recorded a year earlier, are likely to re-establish once the effects from the freshwater inflow abate in future months and salinity levels start to increase again.
The water was fresh (salinity of 280 mg/L), alkaline (pH 8.1), well oxygenated (89% saturation) and very turbid or opaque. It contained high concentrations of nutrients such as nitrogen (2.1 mg/L) and phosphorus (0.24 mg/L).
The sediment was mostly detritus, pebble, gravel and sand; it was blackened, sulfidic and anaerobic below the surface layer, which means that the sediments were a harsh place for burrowing species to be able to tolerate.
A large growth of small, free-floating algae called phytoplankton was present and contributed to the very turbid appearance of water in the channel and lower lakes. A small amount of green filamentous algae (Cladophora) was also present and covered nearly 10% of the channel. Small patches of dead Sago Pondweed (Stuckenia pectinata) were noted but no live plants were recorded; the high water level may have inundated any submerged plants and made detection impossible using the field methods applied in this assessment.
Sparse patches of Lignum (Muehlenbeckia cunninghamii) and Nitrebush (Nitraria) grew over introduced couch grasses in the riparian zone.
Open paddocks used for cropping dominated the surrounding area on Hindmarsh Island, although there were also some patches of native woodland consisting of eucalypts, wattles and paperbarks growing over 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, however, 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).|