Yonga Nearshore Marine Biounit
2012 Aquatic Ecosystem Condition Report
- On the map, zoom in and click on the dots to view underwater video at each site.
- Seagrass habitats along the west and east coast in the southern portion of the biounit were dense and largely intact.
- Seagrass at sites adjacent Whyalla and Port Pirie were generally degraded. While habitats in areas remote from the large towns in this biounit were typically healthy
- Almost all habitats monitored throughout the biounit were under stress from seagrass epiphyte growth and opportunistic macroalgae. It is possible that seagrass could be lost if this stress continues.
About the location
The Yonga Biounit occupies the southern extent of the northern Spencer Gulf bioregion. It extends from Victoria Point (north of Franklin Harbour) to Point Lowly on the Eyre Peninsula to Ward Point and Point Riley on the Yorke Peninsula. Yonga predominantly experiences low wave energy, although slightly higher wave energy occurs south of Shoalwater Point, where the coast faces the south to south westerly winds and occasional ocean swell (Edyvane, 1999). A number of small bays along both sides of the biounit restrict water movement.
The region has been a major heavy industrial hub for decades with a lead smelter at Port Pirie since 1881 being the major industry for that town. While a major steelworks at Whyalla has been located on the shores of False Bay since the 1950’s. These industries have historically discharged metal rich effluent into the coastal waters of Yonga which has resulted in widespread contamination of the sediments and biota. In recent years the loads of metals have reduced and there is some evidence to suggest some improvement in the contamination status of the region, but it is unlikely that the region will ever fully recover. The extent of contamination and biological impacts of the metals are well described in scientific literature (see references under Further Information). This report assesses the condition of the Yonga biounit with a specific focus on the impact of nutrients and poor water clarity and as such do not specifically address the metal contamination. The EPA have more recently undertaken specific studies to assess the risk of metals in this region and have reported these elsewhere (Gaylard, Thomas and Nelson 2011, under Further Information).
Port Pirie, on Yonga’s north eastern shore, and Whyalla, on its north western shore are the two major urban centres with approximate populations of 14,000 and 21,000 respectively. Both centres have wastewater (sweage) treatment plants (WWTP) which discharge effluent into coastal waters. There are also a number of community waste management systems (CWMS) and individual septic systems to service satellite communities and shacks scattered throughout the biounit.
Much of the surrounding land is used for agricultural purposes although the urban footprint and associated runoff is increasing for both centres. The largest watercourse entering Yonga is the Broughton River and may carry substantial loads of nutrients and sediment especially after heavy rainfall events.
Yonga 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.
Yonga 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 the Yonga biounit was assessed based on monitoring data collected during autumn and spring 2012. There are large areas within the biounit that are deeper than 15 m which are not included as a part of this assessment.
Yonga was observed to be in Good condition. In general the habitats in many areas in close proximity to large towns were found to be degraded, while many areas remote from human activity were found to have dense and intact seagrass meadows. Throughout the biounit there were many areas that were under significant stress due to nutrient enrichment which is likely to be causing excessive growth of algae on the seagrass leaves (epiphytes), which if prolonged, can result in seagrass loss over time.
It is important to note that this report assessed condition of the ecosystem and that these reports do not assess the suitability or quality of waters for aquaculture, food quality and fish health. For details about water quality affecting seafood quality please refer to the South Australian Seafood Quality Assurance Program (SASQAP).
A total of 50 sites were monitored during autumn and spring in 2012 to assess the condition of the biounit; 52% of the habitats monitored were covered in seagrass, while unvegetated sand accounted for 48%of the benthic habitat and rocky reef was encountered at a small number of sites, making up less than 1% of the area surveyed.
Seagrass coverage throughout the biounit was variable. Some areas had dense and continuous seagrass meadows, particularly in the southern part of the biounit on the west coast near Lucky Bay and the east coast near Port Broughton. While seagrass was very sparse or patchy and generally in a degraded condition close to urban centres especially throughout Germein Bay and Whyalla.
The entire biounit showed numerous indicators of nutrient enrichment. Seagrasses in many locations were covered in a thick covering of epiphytic algae and there were frequent observations of snot weed (Hincksia sordida) and sea lettuce (Ulva spp.) which all suggest excess nutrients throughout the biounit.
These findings suggest that the nearshore marine habitats in parts of biounit are under stress due to nutrient enrichment, which if sustained over time could result in habitat loss. If habitats are lost this can impact on the productivity of fisheries, nutrient assimilation, erosion and sand movement on beaches and wave attenuation and it can have a negative impact affect marine biodiversity.
Pressures and management responses
|The Whyalla steelworks discharges ammonia rich effluent into False Bay||
|Nutrient loads discharged by the Whyalla and Port Pirie wastewater treatment plants into poorly flushed mangrove creeks||
|Stormwater runoff from urban areas discharges nutrient and sediment loads to coastal waters||This information is not available at the moment but it will be updated as soon as possible.|
- Download the Methods Report for the nearshore marine ecosystems monitoring, evaluation and reporting program.
- Edyvane, K. S. (1999). Conserving Marine Biodiversity in South Australia - Part 2 - Identification of areas of high conservation value in South Australia, Primary Industries and Resources SA, South Australian Research and Development Institute Aquatic Sciences. SARDI (Aquatic Sciences).
- Ward, T.J. (1987) Temporal variation of metals in the seagrass Posidonia australis and its potential as a sentinel accumulator near a lead smelter. Marine Biology 95, 315–321.
- Ward T.J. & Young, P.C. (1981) Trace metal contamination of shallow marine sediments near a lead smelter, Spencer Gulf, South Australia. Australian Journal of Marine and Freshwater Research 32, 45–56.
- Ward, T.J. & Hutchings, P.A. (1996) Effects of trace metals on infaunal species composition in polluted intertidal and subtidal marine sediments near a lead smelter, Spencer Gulf, South Australia. Marine Ecology Progress Series 135, 123–135.
- Gaylard S., Thomas, S., & Nelson M. (2011) An assessment of the current status of bioavailable metal contamination across South Australia using translocated mussels Mytilus galloprovincalis. Transactions of the Royal Society of South Australia, 135, 39–54.
- Corbin, T. & Wade, S. (2004) Heavy metal concentrations in razorfish (Pinna bicolor) and sediments across northern Spencer Gulf. (Environment Protection Authority, Adelaide).