Orontes 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 around the middle of the biounit Port Julia to Wool Bay has increased.
- Reef condition has substantially decreased with a decrease in canopy algae and increase in turfing algae or bare substrate.
About the biounit
The Orontes biounit covers the majority of the eastern Yorke Peninsula and extends from Troubridge Hill to Ardrossan. Most of the biounit experiences low wave energy and the adjacent land has a low population with minimal industry discharges compared to the eastern side of Gulf St Vincent.
There are a number of coastal towns adjacent the Orontes biounit, although the permanent population of each town is small, the populations swell considerably during holiday seasons. The pulse of increased population can put pressure on small town infrastructure including septic systems that leach nutrients into growndwater that can flow to the marine environment. The largest town, Ardrossan, treats and manages sewage through a community wastewater management system which reuses treated wastewater for irrigation on the golf course.
The adjacent land is largely used for agriculture of crops and sheep livestock. Three small ports in Orontes facilitate grain, dolomite and lime export. There are a number of aquaculture leases around Stansbury and Port Vincent that are dedicated to farming Pacific oysters (Crassostrea gigas). A newly established native oyster (Ostrea angasi) reef has been established near Rogues Point and is intended to increase tourism interest in the area.
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 growth that could increase the biological effects of excess nutrients.
The Orontes biounit was expected to be in Very good condition based on the 2011 Aquatic Ecosystem Condition Report.
The Orontes biounit was rated as Good condition during 2017. The score has decreased since 2011 where it was classified as Very good, but this is likely due to natural variation with only small changes in condition.
The marine ecosystem in Orontes was largely stable since 2011 with seagrass cover and condition largely unchanged. Reef habitat has shown a decline in condition and this has contributed to the condition rating in the present assessment.
Some sites such as Coobowie Bay Sth and Orontes Bay Sth had dense seagrass, however it often corresponded to high epiphyte load indicating nutrient enrichment. One site, Pine Point Nth had seagrass with a high epiphyte load and also had considerable reef with marine debris present. The widespread occurrence of epiphyte suggests that the biounit is under stress from nutrient enrichment.
Water chemistry results show a reduction in nutrients compared to 2011 and this may explain the ubiquitous nature of rapidly growing epiphytes that use nutrients to grow. Epiphytes and turbid water both restrict light that seagrasses and canopy algae need to photosynthesise and grow.
A total of sites were monitored during 2017 and included one extra site added to give better spatial representation in the south of the biounit; 87% of the sites were covered in seagrass, 2% was reef and 11% was unvegetated sand.
The average condition of the ecosystem in the Orontes biounit was 52 out of 100. Some sites have dense continuous seagrass cover, but most sites appear to be under stress and have considerable epiphyte cover or seagrass in poor condition. The epiphyte load varied between sites and was 49 out of 100 for the biounit. All sites with long-lived seagrass species had an epiphyte load greater than 50 out of 100, the few sites comprised mostly of ephemeral seagrass species had much lower epiphyte load as would be expected for seagrasses with rapid leaf turnover. Where the water is shallow, seagrass is still growing under substantial epiphyte cover due to the high light regime eg Coobowie Bay Sth, but seagrass is typically sparser at sites in the north of the biounit.
A reduction in water chemistry results compared to 2011 and corresponding high epiphyte load where seagrass was present may be indicative that epiphytes are rapidly using nutrients from the water column. The seagrass leaves at Pine Point Nth are short for the species (Posidonia) in Orontes and the presence of epiphytes might suggest that epiphyte loads on seagrass has caused breakage of the leaves. All sites had some seagrass cover indicating that seagrass would normally grow there in continuous meadows in the absence of human pressures. However, bare sand at sites with low (<20%) seagrass cover shows signs of degradation with sedimentation, opportunisitc macroalgae, microphytobenthos and high bioturbation and may indicate that seagrass has been or is being lost in some areas.
There was marine litter among the reef at Red Cliffs and Pine Point Nth comprised of bottles, tyres and fishing line. Although not part of the present condition assessment, marine litter can leach toxins and trap or injure marine life and therefore it is noted as part of the ambient nature of this program.
The overall condition of Orontes is stable. The prevalence of epiphytes throughout the biounit suggest ongoing nutrient enrichment pressures that can limit recovery or cause further impact if not addressed. Councils and stakeholders are addressing the pressures through a number of management actions.
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 Yorke Peninsula Council continues to investigate the expansion of the current community wastewater management scheme (CWMS) at Port Vincent, Stansbury and Coobowie. All three of these communities are currently listed with the LGA for funding assistance to expand the current CWMS schemes to cover the whole townships.
Stormwater runoff from urban coastal areas, discharging nutrients and sediments to the nearshore marine waters.
The Yorke Peninsula Council continues to require maximum retention and use of stormwater when allotments are developed.