Yankalilla Nearshore Marine Biounit
2017 Aquatic Ecosystem Condition Report
On the map, zoom in and click on the dots to view underwater video at each site
- Seagrass condition has generally improved throughout the region with the exception of Hallett Cove, which showed a substantial decrease in seagrass cover since 2010.
- Reef condition in Yankalilla has decreased due to reduction in canopy algae and increase in turfing algae at all sites.
About the biounit
The Yankalilla biounit covers the nearshore waters between 2-15 m across much of southern metropolitan Adelaide and the Fleurieu Peninsula. The biounit experiences low to moderate wave energy that increases towards the south.
There are a number of small, coastal towns with low permanent populations that increase substantially during holiday periods. The adjacent land has mixed usage between industry in the Lonsdale area to agriculture in the southern parts.
The Adelaide Desalination plant was completed in 2011 (stage 1) and 2012 (stage 2) and can contribute up to 50 % of Adelaide’s drinking water requirements. The hypersaline brine discharge from the plant is discharged back into Gulf St Vincent.
The Christies Beach wastewater treatment plant discharges treated wastewater into nearshore waters adjacent Horseshoe reef. Another wastewater treatment plant in Aldinga discharges much smaller volumes into the Christies Beach WWTP. The remaining small towns treat sewage through community wastewater treatment systems or septic tanks that may influence groundwater quality.
There are a number of small rivers and creeks, including the Onkaparinga River and Bungala Creek that transport runoff from the surrounding land into the marine environment. The runoff from creeks and stormwater drains can bring pollutants from roads and agricultural land that may impact on the condition of the marine environment.
The O’Sullivans Beach boat harbour and the Wirrina Marina are both potential sources of litter from vessels as well as oil leaks and fuel spills. Marinas and boat harbours require sporadic dredging to maintain usability and can increase turbidity or release pollutants from sediments. Marinas also provide large roosting areas over water for a number of shore birds; these aggregation points contribute to the nutrients that are flushed out to the near shore marine environment with tidal movements.
Yankalilla was expected to be in Fair condition based on a desktop assessment of threats.
The Yankalilla biounit was rated as Poor during 2017. The condition of Yankalilla has not changed since the last assessment in 2011.
Seagrass cover was greater at sites in the south of the biounit around Carrickalinga and Rapid Bay, which recorded over 90 % cover of Amphibolis and Posidonia seagrasses. Hallett Cove showed a decrease in seagrass cover since 2010 monitoring but this has been relatively stable since 2011. Seagrass epiphytes were typically low at all sites and reflects both the generally low seagrass cover and higher wave energy of the biounit.
Rocky reef makes up a considerable proportion of some sites in Yankalilla. Overall, reef condition was variable with greater cover of canopy algae on reef at southern sites including Carrickalinga which showed dense canopy of robust brown macroalgae including Ecklonia radiata and Sargassum spp. The reefs that had been monitored in 2011 showed a decrease in canopy algae and more bare substrate or turfing algae along with some marine debris was observed in the 2017 monitoring.
A total of sites were monitored during 2017 and included 4 new sites in the south of the biounit to provide better spatial representation. A number of sites had mixed habitats and for the biounit, 36% of the sites were seagrass habitat, reef was 13% and unvegetated sand was 51%.
Seagrass condition for the biounit was determined to be 14 out of 100 and largely stable since 2011. There are a number of different species in Yankalilla that grow in higher wave energy environments including among reef. One species, Posidonia coriacea was present at Maslin Beach and Southport Outer in low cover (<2%). The greatest seagrass cover was at Rapid Bay, Carrickalinga and Myponga Beach reflects the sheltered waters that allow meadows of seagrass to grow. Cover has declined at Hallet Cove since 2011.
Epiphytes were low throughout Yankalilla due to the higher abrasive forces on seagrass leaves in higher wave energy environments. The epiphyte load for all sites has decreased from 8/100 in 2011 to 5/100 in 2017.
Reef condition is site specific, but overall canopy algae cover has declined and cover of turfing algae has increased since 2011. Reef at Carrickalinga had the highest canopy algae cover (70%) followed by reef at Myponga Beach that had a canopy algae cover of 33.3%. The lower canopy cover at Myponga is due to the boulderous, reef characteristics, but may also be affected by the septic tanks in the shack community and runoff from pasturelands through the Myponga Creek.
Water quality results show that nutrients are low and have not changed substantially from 2011. The higher wave energy and more open coastline of the Yankalilla biounit increases flushing rates, and pulses of runoff from stormwater will be quickly diluted.
Pressures and management responses
Nutrient load discharged (over several decades) by the wastewater treatment plant at Christies Beach.
The Adelaide Coastal Water Quality Improvement Plan (ACWQIP) has targets for reducing nutrient discharges from the Christies Beach wastewater treatment plant (WWTP).
In order to accommodate urban growth and improve environmental performance, SA Water has heavily invested in upgrading the Christies Beach WWTP. This upgrade has improved effluent quality and has enabled the development of the Southern Urban Reuse Scheme and large scale irrigation of vineyards. This investment has resulted in a significant reduction in nutrient loads discharges to marine waters from the plant.
Sediment and coloured runoff from the land entering nearshore marine waters
The EPA through the Australian Government National Landcare Program funded Catchment to Coast project has been working with councils and community groups to install rain gardens that slow the release of stormwater and provide longer for sediment to drop out of the water. This project titled ‘Catchment to Coast focus for water quality improvement across urban Adelaide has implemented several strategies of the ACWQIP, including building community capacity and education to clean up stormwater and promoting greater use of water sensitive urban design in Adelaide. A partnership with Water Sensitive SA has helped to promote information and training for local councils, developers and community members in the construction and maintenance of rain gardens.
However, it is important to note that seagrass is slow growing and it takes a long time to recover from disturbance, so improvement in condition relative to these strategies may not be apparent for decades.
The District Council of Yankalilla is working with the AMLRNR Seascape Program through hosting The Southern Fleurieu Coastal Conservation Officer (CCO) and having shared work plans for on-ground works. The main task of the CCO is to implement actions outlined in the Southern Fleurieu Coastal Action Plan (SFCAP). These actions are reviewed annually and have led to the following plans being created: Bungala River Estuary Action Plan, Small Fleurieu Estuaries Condition Report, Yankalilla Stormwater Management Plan and the Nearshore Marine Habitats Values, Threats and Actions Report.
Council has been working with AMLRNR Seascapes program to implement allocated actions from the Southern Fleurieu Coastal Action Plan and Bungala River Estuary Action Plan. An example of this work has been the natural area restoration activities at council reserves - Bungala Park and the Normanville Jetty Caravan Park. At these sites significant weed control and revegetation for biodiversity and erosion control has been undertaken. This work will continue to extend through to the small estuaries to implement actions outlined in the Small Fleurieu Estuaries Condition report.
Nearshore Marine Habitats Values, Threats and Actions Report has a major threat as stormwater which council has been addressing through the Yankalilla Stormwater Management Plan (funded by Council and AMLRNR Seascapes program). DC Yankalilla retro fitted biofiltration areas at Carrickalinga through revegetation at the sand dune stormwater outlets as a community project last year. Future projects to further improve stormwater runoff include rain gardens at Normanville.
Adelaide Desalination Plant
The Adelaide Desalination Plant monitors the environment in the gulf. This gives real-time data on water temperature, salinity and the speed of the currents. To ensure minimal impact on the marine environment.