Restoration of Ruppia tuberosa
in the Coorong
Kor-jent van Dijk and Michelle Waycott
DEPARTMENT OF ENVIRONMENT, WATER AND NATURAL RESOURCES, UNIVERSITY OF ADELAIDE
The Coorong is a permanent coastal lagoon that forms part of the terminal lake system at the River Murray mouth. It runs for 100km parallel to the sea and is an inverse estuary, with a constricted opening at the River Murray mouth. The Coorong is a unique ecosystem with a strong salinity gradient increasing from the north lagoon (estuarine/marine) near the mouth, to the southern lagoon, which is hypersaline (exceeding 200 g/L during the millennium drought).
The Coorong and Lower Murray Lakes were designated as a Wetland of International Importance under the Ramsar Convention in 1985, in recognition of its role in supporting a wide range of habitat types (estuarine, marine and hypersaline) and vast numbers (tens of thousands) of waterbirds, including ducks, swans, pelicans, terns, grebes, and migratory shorebirds.
Ruppia tuberosa is the dominant macrophyte in the Coorong, fulfilling the role of cornerstone species by providing food for waterbirds and habitat to a diverse fish community. R. tuberosa is able to withstand the extreme range of salinity, particularly the hypersalinity occurring in the southern Coorong.
R. tuberosa distribution and density declined in the southern lagoon between 2000 and 2010 (a period described as ‘the Millennium drought’) and it was completely absent in 2010. Very high salinity and low water levels are widely acknowledged as the main environmental causes for this loss of R. tuberosa. Salinity in the Lower Coorong reached a maximum recorded salinity of >200 TDS g/L in 2000 – 2010. After the millennium drought (2000-2010) the River Murray flows returned (late 2010) and water levels in the southern Coorong were appropriate again for R. tuberosa to grow. Ruppia tuberosa began to recover after the 2010-2011 floods in the southern lagoon and distribution increased in the years after with the adjusted water allocations. Even so, R. tuberosa did not closely recover to pre-drought densities.
Given the importance of R. tuberosa for the Coorong ecosystem a translocation program was established to facilitate its recovery. In autumn 2013 the program was started where R. tuberosa seeds were sourced from the margins of Lake Cantara and moved to recovery sites. With a small excavator, seeds were harvested by scraping a thin layer (~1.5 cm) of sediment (with seeds) from the dried-up lake margins during autumns of 2013 and 2014. The sediments were piled up on the banks and was shovelled by hand into sand-bags and transported to four recovery sites where the bags were evenly distributed over the donor-site and emptied. A total of 280 (14,080 bags) and 450 tonnes (30,100 bags) of sediment were translocated in 2013 and 2014 respectively. Bags were translocated to Policeman Point and Woods Well in 2013 and to Fat Cattle Point, Jacks Point and Seagull Island in 2014. An estimated area of around 20 ha and 41 ha were treated during the two restoration years. The restoration efforts were successful in that R. tuberosa did recolonize the areas transplanted.
While the restoration has helped recovery in the South lagoon, the process of recovery has been slow, and up until 2016 water levels have not been high enough to successfully complete the reproductive cycle. In particular, seed and turion density remain low compared to historical values. Ongoing monitoring of the system will identify if there is increased Ruppia recovery in the Coorong over longer time frames.