Clinton 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 cover along the deeper northern margins of the Adelaide coast has declined.
- Seagrass cover has increased at a number of southern sites e.g. Somerton Park and Brighton.
- Reef condition has generally declined with reductions in canopy algae and increases in turfing algae.
About the biounit
The Clinton biounit covers the waters between 2 and 15 m deep from Ardrossan to Marino Rocks, but excludes the Adelaide Metropolitan biounit.
Clinton, particularly the head of the gulf, is sheltered from large waves experienced further south in the gulf, but the wave energy increases towards the southern section. The biounit is dominated by extensive seagrass meadows and supports the most extensive mangrove and tidal flats within the gulf. Clinton also has large areas of shallow, warm waters which have reduced flushing. This is likely to result in favourable conditions for algal grow that could increase the biological effects of excess nutrients.
Clinton occupies the northern reaches of Gulf St Vincent with substantial adjacent agricultural land. The southern portion of Clinton has been heavily urbanised. There are a number of small coastal towns that border Clinton; the largest being Ardrossan which bridges the Clinton and Orontes biounits. Port Wakefield is the next largest town and both of these centres treat sewage through a community wastewater management systems (although they do not discharge to the environment). Other small coastal towns including Port Clinton, Port Parham and Thompson Beach all use onsite wastewater treatment (septic) systems. Septic tanks can introduce nutrients into the shallow groundwater, which are likely to flow towards the marine environment where it can contribute to the nutrient load in nearshore marine waters. Clinton is also likely to receive significant nutrient loads from the Bolivar wastewater treatment plant which has been shown to flow to the north from Chapman creek into the Clinton biounit. It is also possible that pollutants will transport into Clinton from the adjacent the Adelaide Metro biounit.
The Clinton biounit was rated as Good during 2017. While there have been some improvements in condition at in Clinton, others have shown a decline.
In the north of the gulf, most sites had extensive seagrass meadows that showed only small changes in seagrass condition, likely due to natural variability. Towards the metropolitan region, results were largely consistent with the Adelaide metropolitan biounit with sites to the north of the Torrens River showing declines in condition since 2011. One site, Black Pole outer has encouragingly experienced a substantial increase in condition compared to 2011. That result is mirrored at a number of sites south of Glenelg which are showing an increase in seagrass cover.
Water chemistry results show that these same areas showing seagrass decline were also more turbid than sites further south. This suggests that turbidity may be limiting recovery of seagrass. Turbid waters restrict light getting to seagrass leaves and also affects the aesthetic value of swimmers, making it unattractive to swimmers.
A total of 24 sites were monitored in 2017; 92% of the sites were covered in seagrass, 8% was unvegetated sand and reef accounted for less than 1% of the sites.
The average condition of the Clinton biounit for 2017 was 60 out of 100. Clinton is a large biounit that spans from the top of the gulf, and covers the waters adjacent to the most urbanised portion of the coast. As such, the condition was variable between sites with some sites showing dense, continuous seagrass meadows, while others had sparse and patchy seagrass.
In general, the sites in north of Clinton were largely dense Posidonia sp. seagrass that showed only small changes in cover, likely reflecting natural variability. A number of sites to the south of the Torrens outlet around Somerton (inside and outside) and to a lesser extent Brighton showed increases in seagrass cover, from moderate to dense. Similarly, Black Pole outer also showed a substantial increase in seagrass cover and sites around Outer harbor are showing signs of recovery with increases in cover of the colonising species Halophila sp.
Port Parham south and Port Gawler outer showed significant decreases in cover since 2011, which may be in response to a large flow event in late 2016 from the Light River. Closer to the metropolitan area, deeper sites to the north of the Torrens River outlet (Tennyson, Henley and West Beach outer) generally still have extensive Posidonia sp. seagrass but have significantly reduced in cover compared to 2011.
An average epiphyte load of 26/100 for the biounit suggests that nutrient enrichment may still be an issue for seagrass condition. Seagrass epiphyte loads were highest at the top of the Gulf, especially at Port Clinton (89/100), which may reflect the low flushing times and low wave energy typical of that area. Areas with higher wave energy such as Somerton (inner and outer), and Brighton had lower epiphyte loads of 10 and 11/100 which may help to flush nutrients out, or increase abrasion reducing epiphyte loads.
The monitoring only covered small amounts of reef within Clinton, with reef assessed at three sites with Outer Harbor south, West Lakes and Somerton inner. Only Somerton inner was monitored in 2010 or 2011 and in 2017 this reef has lost a substantial proportion of the large canopy algae, while turfing algae or bare substrate has increased which suggests that those sites are degraded.
Water quality results show that most nutrient indicators have decreased since 2011. Sites in the north of Clinton eg Price and Ardrossan Nth had higher turbidity than sites towards the south including the Somerton sites and Brighton, which may reflect the higher flushing and wave energy in the south.
The overall condition of Clinton is stable. The long history of poor management of coastal marine health is being addressed through management actions under the Adelaide Coastal Waters Quality Improvement Plan (ACWQIP) and improvements in condition can be linked to those actions. However, it will take time for widescale improvement to be become obvious.
Pressures and management responses
Nutrient load discharged (over several decades) by the wastewater treatment plant at Bolivar.
The Adelaide Coastal Water Quality Improvement Plan (ACWQIP) has targets for reducing nutrient discharges from Bolivar wastewater treatment plant.
Since the mid-1990s, SA Water has made a significant investment in reducing nutrient discharge loads from the Bolivar Wastewater Treatment Plant. This included a major upgrade to the Bolivar plant including the development of a large water recycling scheme that delivers treated wastewater for irrigation to market gardens in the Virginia area. This investment has resulted in a significant reduction in nutrient loads from the plant. SA Water is now further expanding its recycled water system through the Northern Adelaide Irrigation scheme.
Failing and/or high density of onsite wastewater treatment (septic) systems in some coastal towns. This is probably most significant in the Port Parham, Thompson Beach, Webb Beach and Middle Beach areas. Overflowing septic systems contribute nutrients to nearshore marine waters through shallow sub-surface or occasional overland flows.
Middle Beach has a Septic Tank Effluent Disposal system with 98% of properties connected. Effluent is treated and then disposed of within the allocated wood lot.
Wastewater systems in Parham, Thompsons and Webb beach traditionally were septic soakage systems. New dwellings now require a wastewater engineer to design the system. The Adelaide Plains Council is also in the process of redesigning the discharge at the Parham camp site by moving the discharge point back another 200m from the high tide mark and disposing at ground level to reduce environmental harm.
Penrice Soda Holdings at Osborne contributed high nutrient loads (as ammonia) into the Port River over several decades. This nutrient enriched water has been transported into the nearshore marine waters of the Clinton biounit.
Penrice Holdings Products facility in the Port River closed in 2014. The ammonia discharged has dropped from approximately 670 tonnes per year to zero since the closure.