Encounter Nearshore Marine Biounit
2021 Aquatic Ecosystem Condition Report
- On the map, zoom in and click on the dots to view underwater video at each site
- Seagrass was in generally in good condition, but has slightly declined
- 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 within the biounit increases substantially during holiday periods, this 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.
Using reduced site data, in 2015 the Encounter biounit was in Very Good condition, based on monitoring conducted over autumn.
A reduced number of sites were monitored in 2021 (5) compared to 2015 (10). For comparable analysis, only sites that were monitored in both years were used to calculate the latest scores. Video from a total of 5 sites was analysed from waters between 2 – 15 m deep during autumn 2021 to assess the condition of the Encounter biounit. There are large areas within the biounit that are deeper than 15 m, that are not included as a part of this evaluation.Many of the sites within the biounit were made up of mixed complex habitats of seagrass and reef. Overall the biounit experienced 6% loss in total seagrass coverage with no sites experiencing new growth. Sites located within the island sheltered areas of Encounter bay were dominated with Posidonia Sp., whereas the more exposed sites around Victor Harbour, McCracken and South of The Bluff were dominated by Amphibolis Sp. and reef. At all sites where seagrass was recorded epiphyte loads were reduced when compared to the previous monitoring period.
Of the 5 sites monitored in Encounter, 21% of the habitat was made up of seagrass, 40% reef and 39% unvegetated sand.
Throughout the biounit, Amphibolis sp. was intermixed with rocky reef species, it was established amongst the sand which had accumulated on the reefs. Posidonia sp. was sometimes seen intermixed with Amphibolis, typically in the sand at the base of reef habitat and throughout the reefs in voids with high sedimentation. The seagrass has shown a decline of 6% across the biounit, due to the monitoring program design this is likely natural variability and not considered significant for condition assessment. The seagrass monitored throughout the biounit was generally in good health with very little to no epiphyte.
The reef habitats monitored were typically found to be in a good condition, with greater than 80% average coverage of large canopy forming algae (up 50% from 2015) and very low amounts of turfing algae (down 50% from 2015) . Granite Island (m0509) has remained predominantly bare sand since 2015. It is subject to high turbidity, however, there is some very sparse brown algal species present. Interestingly Causeway (m0510) which is less than 0.6 nautical miles away, in the same bay, had very low turbidity. This could be attributed to the fact that Causeway has around 50% seagrass coverage.
Analysis of pigment sizes of phytoplankton communities and Fp ratios show that there are less micro pigments and more nano/pico pigments, this suggest that the biounit is in mesotrophic-oligotrophic conditions, similar to 2015. Soluble nutrients were low throughout the biounit and showed a slight reduction from 2015. Total Nitrogen was down 0.04mg/L and Total Phosphorous was down 0.005mg/L.
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) 18 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.
Since 2015, SA Water has expanded its supply of recycled water and now supplies an avocado farm adjacent to the WWTP. In addition, the recycled water distribution network in Victor Harbor has been expanded supplying additional customers. This beneficial use of recycled water diverts treated water that would have otherwise been discharged to the Inman River and ultimately the marine environment.
Granite Island Causeway Project Upgrade
No repsonse provided.