Orontes Nearshore Marine Biounit
2011 Aquatic Ecosystem Condition Report
- On the map, zoom in and click on the dots to view underwater video at each site.
- Seagrass habitats between Port Vincent outer, Stansbury and Wool Bay were largely dense and intact.
- Seagrasses around Rogues Point, Klein Point and Edithburgh were generally moderate to sparse in condition.
- Evidence that nearly all habitats monitored throughout the biounit were under stress from nutrient enrichment and dense epiphyte growth on seagrass leaves. It is possible that seagrass could be lost if this stress continues
About the biounit
The Orontes biounit extends from Troubridge Hill to Ardrossan on the eastern side of Yoke Peninsula (see map). The biounit covers approximately 100 km and the shoreline has an overall easterly orientation into Gulf St Vincent and typically has low wave energies.
Ardrossan is the largest coastal town adjacent Orontes, with a population of just over 1100, while Stansbury, Edithburgh and Port Vincent are also significant coastal townships. During holiday seasons the population of these towns swells considerably. Ardrossan is the only coastal town adjacent the Orontes biounit that treats and disposes of sewage through a community wastewater management system which reuses 100 % of wastewater for irrigation on the golf course. The smaller townships and shack communities use onsite wastewater treatment (septic) systems for sewage management.
The main industry along the Yorke Peninsula is agriculture (broadacre cropping and sheep livestock), and there are 3 small ports within Orontes that facilitate the export of grain as well as dolomite and limestone.
A number of aquaculture leases that farm Pacific oysters (Crassostrea gigas) are located in Coobowie Bay, Stansbury and Port Vincent and a small abalone (Haliotis rubra, H. laevigata, H. cycobates) lease operates off Port Giles.
Orontes is sheltered from large waves experienced on the eastern side of the gulf and 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.
Orontes was expected to be in Good condition, based on an assessment of threats to the nearshore habitats.
The condition of habitats in waters between 2 – 15 m deep throughout Orontes was assessed based on monitoring data collected during autumn and spring 2011. There are large areas within the biounit that are deeper than 15 m which are not included as a part of this evaluation.
Orontes as a whole was in Very Good condition and has not changed since the previous assessment in 2010 and has not changed since the previous assessment in 2010. The seagrass habitats were generally dense and intact although many were under stress due to thick epiphyte loads on seagrass leaves and it is possible that seagrass habitats could degrade if this stress continues.
A total of 16 sites were monitored during autumn and spring in 2011 to assess the condition of the biounit; 85% of the sites were covered in seagrass, while 12% were covered in unvegetated sand. There was only a small amount of rocky reef or small algae encountered in the sites assessed (< 4%).
The results showed that seagrass coverage throughout Orontes was variable with over half the sites showing dense and continuous seagrass meadows in excellent condition, particularly between Port Vincent outer and Coobowie. While in other areas the seagrass was very sparse or patchy and generally degraded such as Rogues Point, Klein Point and Edithburgh.
Throughout Orontes the seagrasses were covered in dense epiphytes on the seagrass leaves. This was particularly evident towards the north of the biounit including Rouges Point, around Pine Point and Port Julia during autumn. Other locations which showed consistently dense epiphyte loads include Pine Point, Stansbury and Coobowie. It is likely that at many locations throughout the biounit the seagrasses are under considerable stress and may result in degradation if this stress continues.
The amount of nitrogen in the water column was very low with both the dissolved and total fractions of nitrogen significantly lower than observed in reference locations, which is also significantly lower than the results of the 2010 survey. The total phosphorus concentrations were significantly greater than seen at reference locations. The amount of phytoplankton in the water was similar to what is typical in areas considered reference but was higher in autumn compared to spring.
The turbidity of the water was significantly greater in the Orontes biounit compared to locations considered reference and was considerably higher in spring than in autumn, which is consistent with the 2010 data although the magnitude of the difference is greater in 2011.
It is possible that the low wave energy typical of this biounit may be increasing the effect of excess nutrients, creating more favourable conditions for epiphyte and algal growth (for example at Coobowie). Further increase in nutrient loads throughout these areas may cause a decline in ecosystem condition over time.
Pressures and management responses
|Failing and/or high density of onsite wastewater treatment (septic) systems in some coastal towns. This is probably most significant in the Stansbury, Port Vincent and Edithburgh areas. Overflowing septic systems contribute nutrients to nearshore marine waters through shallow sub-surface or occasional overland flows.||The District Council of Yorke Peninsula is investigating the expansion of the current community wastewater management scheme (CWMS) at Port Vincent. A CWMS at Coobowie to replace the onsite septic systems will also be investigated.|
|Stormwater runoff from urban coastal areas, discharging nutrients and sediments to the nearshore marine waters.||The District Council of Yorke Peninsula requires maximum retention and use of stormwater when allotments are developed.|
|There are oyster farms near Coobowie Bay, Stansbury and Port Vincent do filter out phytoplankton from the water column although the overall effect on nutrient loads remains uncertain.||
Primary Industries and Regions SA research has indicated that sedimentation due to oyster waste production has negligible impacts to the surrounding environment (see Wear et al 2004).
Wear, R., Theil, M., Bryars, S., Tanner, J. and de Jong, S. (2004). Environmental risk assessment of intertidal shellfish aquaculture in South Australia. South Australian Research and Development Institute (Aquatic Sciences). Adelaide. SARDI Publication No. RD04/0155. 75 pp.