An ecosystem assessment framework to guide management of the Coorong

Abstract

Located south-east of Adelaide in South Australia, the Coorong, Lower Lakes and Murray Mouth (CLLAMM) comprise one of Australia’s largest wetland systems. The region is of great habitat significance for migratory waterbirds. It is listed as a Wetland of International Significance under the Ramsar Convention and recognised as an ‘icon site’ under the Murray- Darling Basin Authority program, The Living Murray. The region also supports a varied economy based on agriculture, fisheries and tourism, and it is the traditional country of the Ngarrindjeri Nation. Lakes Alexandrina and Albert are two connected large freshwater lakes which comprise the Lower Lakes. The River Murray flows into Lake Alexandrina, which in turn is separated from the Coorong by a series of barrages constructed in the late 1930s. Flows from Lake Alexandrina to the Coorong can be controlled by opening gates in the barrages. The Coorong is connected to the sea via a narrow channel that is subject to infilling and scouring on a seasonal basis depending on whether the water is flowing through the barrages or not. The Murray Mouth section of the Coorong extends from this connecting channel northwestwards to Goolwa Barrage and southeastwards to the south end of Tauwitcherie Barrage, where the Coorong widens into the North Lagoon. A narrow and shallow channel in the vicinity of Parnka Point separates the North Lagoon from the South Lagoon, which is the third major component of the system. A network of drains (the Upper South East Drainage Scheme) also discharges small volumes of water to the southern end of the South Lagoon. Coorong water levels vary at daily to seasonal time scales creating habitats including periodically inundated mudflats, which are ideal for shorebirds. Typically, the Coorong has a strong salinity gradient along its 110 km length. When the barrages are flowing, water can be near fresh at its Mouth end and when they are not seawater salinity prevails in this region. Towards the southeastern end of the Coorong, salinity increases steadily due to evaporation. Frequently, salinity in the South Lagoon is substantially above that of seawater (hypersaline condition). The salinity variation representing estuarine, marine and hypersaline conditions supports different ecological communities. Modifications to the flow regime of the River Murray have resulted in a significant change to the ecological character of the region since its listing as a Ramsar wetland in 1985. Currently, consumptive use of water across the Murray-Darling Basin has reduced the average annual stream flow at the Murray Mouth by 61% (from 12,333 GL/year to 4733 GL/year). In combination with a regional drought in the Basin, inflows to the Coorong have been especially low since 2002, with no inflow recorded since 2006. This has resulted in the siltation of the Murray Mouth channel and the extreme hypersalinisation of the South Lagoon, where salinity is currently in excess of four times seawater. These changes to the water regime of the River Murray have been linked to a decline in abundance of a number of fish and waterbird species in the Coorong. While a number of management initiatives are underway to address the environmental decline of the region, few tools have been available to help design and optimise long-term management intervention strategies.