Encounter Nearshore Marine Biounit
2015 Aquatic Ecosystem Condition Report
- On the map, zoom in and click on the dots to view underwater video at each site
- The biounit is subject to high wave energy and strong currents
- Seagrass was in generally in good condition
- Reefs were in good condition with extensive brown canopy algae
- Turbidity was very high at a number of sites and may be impacting on ecosystem condition
About the biounit
The majority of the Encounter biounit is within the Coorong bioregion, however a small proportion is within the Gulf St Vincent bioregion. For simplicity, the biounit has been included in the Coorong Bioregional assessment. The Encounter biounit extends from Rapid Head on the Fleurieu Peninsula to the outer western edge of Horseshoe Bay at Port Elliot. The area is characterised by high wave energy and strong tidal movement, especially in the Backstairs Passage between Kangaroo Island and Cape Jervis.
Victor Harbor, Encounter Bay and Hayborough are relatively large towns made up of predominantly permanent residencies and holiday homes in the Encounter biounit. The population increases substantially during holiday periods throughout the biounit, and is likely to be increasing the pressures of urban runoff and wastewater systems on the marine environment.
Sewage for the Victor Harbor region is treated at a wastewater treatment plant, which discharges into the Inman River, then into the ocean between Victor Harbor and Encounter Bay. The Hindmarsh and Inman Rivers also contribute nutrients and sediment to the marine environment from agricultural catchments. Discharges from the Murray River and Coorong carry runoff from the highly modified landscape of the Murray-Darling Basin and increases the turbidity in the marine environment.
The Encounter biounit was predicted to be in Very Good condition, based on an assessment of threats to the nearshore habitats.
Ten sites were monitored in waters between 2 – 15 m deep during autumn and spring in 2015 to assess the condition of the Encounter biounit. There are large areas within the biounit that are deeper than 15 m, which are not included as a part of this evaluation.
Seagrass and reef habitats were often intermixed resulting in complex habitats. Where seagrass was monitored it was generally in good condition with high cover and had minimal signs of nutrient enrichment in the form of epiphytes and opportunistic algae, however a number of sites within the biounit may be under stress due to high turbidity which can block light to plants and algae.
The monitoring suggests that seagrass and reef habitats throughout the biounit were likely to be in Good condition with only slightly impaired with initial symptoms of disturbance due to nutrient enrichment or suspended sediment. There may be some initial changes to ecosystem function and detrimental effects are likely to be localised and where recovery is possible.
Ten sites were monitored in Encounter and these were made up of 25% seagrass habitat, 36% reef and 39% was unvegetated sand.
Throughout the biounit, seagrass was intermixed with the rocky reefs, typically growing amongst the sand accumulated on the reef. Seagrass was not found to grow in dense meadows as would be expected in other Bioregions, and may reflect the higher wave energies and rocky substrates common throughout Encounter. Where seagrass was recorded, it was generally in good condition with high cover of a range of species but particularly Amphibolis spp., and Posidonia spp. The greatest cover of seagrass (91%) was found at Rapid Head South (m0500), similarly (72%) at Land End (m0502). Reef habitats monitored were typically found to be in good condition with most sites being dominated by more than 40% cover of canopy algae and very low proportion of turfing algae. Only one site, adjacent Granite Island (m0509), was comprised wholly of bare sand.
While reef and seagrass were generally in good condition, there were a number of sites that appear to be under pressure. For example, sites Granite Island (m0509) and Causeway (m0510) closer to the mouth of the Hindmarsh River and the Coorong had high turbidity 1.2NTU and 1.3NTU, respectively. The high turbidity prevented good quality observations of habitat condition and was likely to be restricting light available to plants and algae, adding stress to the habitats.
Soluble nutrients were typically very low with most measurements below the limit of reporting. One site, Rapid Head south (m0500) showed consistently elevated organic nitrogen in autumn and spring, which may be related to Yoho creek or could be due to nearby sealions (Neophoca cinerea).
Pressures and management responses
|The Inman and Hindmarsh Rivers discharges runoff from a highly modified agricultural landscape into coastal waters adjacent to the town of Victor Harbor.||The District Council of Yankalilla hosts a NRM Funded Coastal Estuarine and Marine Officer. The primary focus of this role is to implement the Southern Fleurieu Coastal Action Plan. The works undertaken include the revegetation and protection of coastal environments and estuaries from erosion threats.|
|Urban stormwater runoff from Victor Harbor||
Yankalilla Council is undertaking the development of a Stormwater Management Plan (SMP) which had as part of its brief the assessment and identification of land based actions and improvements to reduce adverse impacts from existing catchments and identified residential growth areas on the receiving marine environment.
Yankalilla Council has recently altered its practice in the way it provides vehicular access to Normanville beach, instead of importing a clay/ sand mixture to provide a summer access ramp, which would eventually wash away. It is using a reuseable plastic track system to lay down a pathway to the beach.
|Victor Harbor wastewater treatment plant discharges into the marine environment via the Inman River.||
SA Water implemented an Environment Improvement Program for the Victor Harbor wastewater treatment plant (WWTP) 10 years ago which included relocating the plant, significantly improving treatment to reduce nutrients in the discharge and implementing reuse from plant.
Monitoring of the receiving environment since the upgrade has showed substantial improvement in the environmental performance of the WWTP with improvements in water quality in the downstream receiving environment of Inman River.
SA Water in conjunction with Trility, the current owners and operators of the Victor Harbor WWTP are looking at ways to build on the past improvements and have committed, as part of the EPA licence for the plant, to an Options Assessment looking at further reducing discharges to the Inman River. The options study is due to be submitted to the EPA in 2017.