Coorong nearshore marine biounit
2015 Aquatic Ecosystem Condition Report
- The biounit is subject to high wave energy
- Seagrass in the south of the biounit is largely dense and continuous, but some areas indicate nutrient enrichment and ongoing stress of land based discharges
- Mixed habitats of seagrass and reef were common at deeper sites.
About the biounit
The Coorong biounit is in the Coorong bioregion and spans from the outer western edge of Horsehoe Bay at Port Elliot to Cape Jaffa at the southern end of Lacepede Bay. The majority of coastal waters of the Coorong biounit are high energy with waves rolling over bare sand and breaking onto the sandy beaches of the Younghusband Peninsula. The seafloor gradient decreases in the southern end of the biounit allowing seagrasses to proliferate in the shallow waters of Lacepede Bay while reefs become prevalent at the outer, more exposed areas of the bay.
The towns of Port Elliot, Middleton and Goolwa are adjacent the northern section of the Coorong biounit. The population of these towns are steadily increasing with new permanent residents in the Sea-change movement. Tourists swell the population of these coastal towns during holiday periods. The growing urbanisation and flux of tourists is likely to be increasing stormwater runoff and possible strain on the sewerage systems resulting in more pollutants reaching the marine environment. In the south of the biounit is Lacepede Bay and the coastal towns of Kingston SE, and Cape Jaffa. Urban runoff from these small towns carrying nutrients and sediment reaches the coast during periods of heavy rainfall.
The southeast region of South Australia is a highly modified landscape. Since 1863, broad-scale land clearance and the creation of an extensive cross-catchment drainage system discharges nutrient and sediment laden water from agricultural catchments directly to the marine environment via dozens of drains. Agricultural runoff contributes nutrients and sediment to the marine environment often in very large and concentration discharge events.
The Coorong biounit was expected to be in Very good condition, based on an assessment of threats to nearshore habitats.
Twenty-one sites were monitored in waters between 2 – 15 m deep during autumn and spring in 2015 to assess the condition of the Coorong biounit. There are large areas within the biounit that are deeper than 15 m that are not included as a part of this evaluation.
Seagrasses dominated many of the shallow, sheltered sites in the southern end (ie around Lacepede Bay) with reef the more dominant habitat in deeper water. In the north, rocky reefs were often obscured by high turbidity, while reef in the south of the biounit appeared diverse with variable cover of canopy algae. Where present, seagrass habitats were generally dense and continuous however, at a number of sites high amounts of epiphytic algae was observed on the seagrass leaves.
Overall, these findings suggest that the nearshore marine habitats in the Coorong biounit are generally in Very good condition: the habitat structure is considered natural, but there are some detectable changes compared to excellent condition. Habitat changes are unlikely to be leading to changes in ecosystem function. Any detrimental effects are limited to small pockets and likely to be reversible.
Of the sites monitored, 63% of the habitat monitored was seagrass, 7% reef and 30% was unvegetated sand.
Seagrasses dominated the southern end of the biounit with expansive dense meadows generally in good condition. A study conducted in 2004/05 showed seagrass adjacent Blackford drain, Maria creek and Butchers Gap were all impacted by excess nutrients and there were areas of total lost (Wear et al, 2006). In this program, Blackford Drain (m0516) had lower cover of seagrass (49%) than adjacent sites, Maria north (m0518) and Port Caroline (m0517) with 84% and 76% respectively. Soluble and total nutrient concentrations were low throughout Coorong regardless of proximity to the drains, suggesting that if nutrient enrichment is still an issue of concern, nutrient and sediments enter the system episodically during high rainfall events. The poorer seagrass condition may also be a reflection of the time it takes for Posidonia spp. seagrass to recover from disturbance, known to be many years to decades, if at all.
Seagrass habitats throughout Coorong showed elevated seagrass epiphytes growing on leaves. This fast growing algae blocks light reaching the seagrass leaf and can result in loss. The sites around the Butchers Gap drain and south towards the Cape Jaffa marina were extensive seagrass meadows that were covered in dense epiphytes in both autumn and spring, suggesting nutrient pollution. If this is prolonged, seagrass habitats could be lost.
Rocky reefs were found to be in generally good condition, dominated by large brown canopy forming algae, which is typically seen as an indicator of good condition. Two sites at the northern end of the biounit: Horseshoe Bay (m0511) and Bashams (m0512) were dominated by low reef and were extensively covered (58-71%) in canopy forming brown algae. Notwithstanding this, both sites had very turbid water (average = 3.5 NTU in autumn) which resulted in habitats unable to be assessed during autumn. This turbidity is likely to be restricting light to plants and algae and if prolonged, could result in habitat loss.
Pressures and management responses
|The highly modified, impacted and regulated Murray River has significantly changed the quality and quantity of water discharged from the Coorong.|
|Discharge from Southeast agricultural drains brings considerable loads of nutrients to coastal waters.||The District Council Kingston uses stormwater bores in numerous locations throughout the town for its stormwater management rather than discharging to Maria Creek and into the sea.|
|Urban stormwater carries nutrients and pollutants to coastal waters.||
District Council of Kingston uses on-site stormwater management using stormwater bores, especially for commercial developments, for example: the recent re-development of BP service station by On The Run demonstrated developer and Council collaboration facilitating storm water infiltration to the grassed road reserve.
All new residential developments in the District Council of Kingston must have rainwater tank with storage capacity of minimum of 22,500 litres for domestic use thus reducing the amount of excess storm water run-off into Council’s storm water table.
At the Cape Jaffa Anchorage marina residential development, each allotment must include on-site overflow stormwater facilities including a filtration trench or similar structure prior to discharge to the street storm water system. Street stormwater system incorporates a swale on the road reserve, which due to the nature of sandy soil absorbs most of the excess run-off.
|Septic tank leakage may be reaching the marine environment through groundwater.||District Council of Kingston Development Plan requires that for any development to take place the minimum site level is 2.4 metres AHD (Australian Height Datum) and minimum finished floor level is 2.65 metres AHD. This requirement also applies to the septic tank location and reduces the amount of effluent that may discharge into the ground water and potentially to the sea.|
Download the 2015 habitat and water quality data
Download the Methods Report for the nearshore marine ecosystems monitoring, evaluation and reporting program
Wear, R. J., Eaton, A., Tanner, J. E., Murray-Jones, S. (2006). The impact of drain discharges on seagrass beds in the South East of South Australia. Final report prepared for the South East Natural Resource Consultative Committee and the South East Catchment Water Management Board. South Australian Research and Development Institute (Aquatic Sciences) and the Department of Environment and Heritage, Coast Protection Branch, Adelaide. RD04/0229-3.