Yonga Nearshore Marine Biounit
2018 Aquatic Ecosystem Condition Report
Condition overview
Key points:
- 2018 monitoring assessed 26 sites within the biounit
- The condition of Yonga has reduced from Good in 2012 to Fair in 2018
- Seagrass habits along the south west and south east coasts displayed losses of seagrass in varying degrees
- Almost 60% of seagrass was lost from False Bay outer, and 45% lost from Black Point Inner.
- 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.
Area map
About the biounit
The Yonga biounit occupies the southern portion 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. A number of small bays along both sides of the gulf are likely to experience reduced flushing.
The region has been a major heavy industrial hub for decades with a lead and zinc smelter at Port Pirie operating since 1881. Similarly a major steelworks has operated at Whyalla since the 1950s. These industries have discharged metal rich effluent into the coastal waters of Yonga, resulting in widespread metal 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.
The AECR program assesses the condition of the Yonga biounit with a specific focus on the impact of nutrients and poor water clarity and as such does not specifically address the metal contamination. The extent of contamination and biological impacts of the metals are well described in scientific literature and more recently the EPA are running a number of projects specifically concerned with this issue and these are reported these elsewhere, for example 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 populations of approximately 14,000 and 21,500 respectively. Both centres have wastewater (sewage) 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.
The surrounding land is largely used for agriculture although the urban footprint and associated runoff is increasing for both towns. The largest watercourse entering Yonga, the Broughton River, may carry substantial loads of nutrients and sediment, especially after heavy rainfall events, but this rarely flows to the sea.
Yonga has large areas of shallow, warm waters which have reduced flushing, which is likely to result in favourable conditions for algal grow that could increase the biological effects of nutrient enrichment.
Yonga was expected to be in Good condition, based on an assessment of threats to the nearshore habitats.
In summary
The condition of habitats in waters between 2–15 m deep throughout the Yonga biounit were assessed based on monitoring data collected during autumn 2018. 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 Poor condition. In general, seagrass was stable or showed a decline at most sites throughout the biounit. Throughout the biounit there were many areas under significant stress due to nutrient enrichment causing excessive growth of algae on the seagrass leaves (epiphytes).
Findings
A total of 26 sites were monitored during autumn 2018 to assess the condition of Yonga’s nearshore environment; 33% of the habitats monitored were seagrass, while unvegetated sand accounted for 67% of the habitats monitored, and no rocky reef was encountered. The observed seagrass cover is slightly less than the national benthic habitat mapping which suggests around 58% of the biounit is comprised of seagrass habitat.
Seagrass coverage was variable throughout the biounit. Some areas had dense and continuous seagrass meadows, particularly in the south western part of the biounit near Lucky Bay and the east coast near Port Broughton. However, the majority of areas the seagrass was sparse and/or patchy and generally in a degraded condition.
In 2012, the biounit was found to be in good condition. Of the comparable sites, 18 supported seagrass habitats, six of these showed a significant loss of seagrass. Most notable was at False Bay Outer and Black Point Inner (both near Port Bonython) with losses of seagrass around 60% and 45% respectively. Only one site showed a significant increase in seagrass, located adjacent the steelworks, which increased by almost 20% since 2012. The remaining sites showed only slight changes in seagrass cover which is likely to reflect natural variability.
Generally South Australian gulf waters are very low in nutrients, particularly nitrogen. Notwithstanding the snapshot nature of water chemistry monitoring in this program, the Yonga biounit showed higher nitrogen in the form of both ammonia and total nitrogen than what would have been expected from gulf waters, and what was observed in 2012. This may be due to normal natural variability due to difference climatic, hydrodynamic and biogeochemical processes but this also is consistent with the broader habitat observations of nutrient enrichment and seagrass loss.
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 further 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, which can have a negative impact on marine biodiversity.
Sea surface temperatures in Southern Australia have been increasing since the mid 1980’s and broadscale indicators such as phytoplankton (Chlorophyll a) data has shown a slight increase from 2012 to 2018 within the Yonga biounit. These regional scale changes may also be contributing to the changes seen in the current monitoring data
Pressures and management responses
Septic tanks treat sewage at shacks along the coast which in sandy soils transports nutrients into nearshore waters.
Pressures |
Management responses |
Whyalla Steelworks discharges ammonia rich effluent into False Bay |
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Nutrient loads discharged by the Whyalla and Port Pirie wastewater treatment plants into poorly flushed mangrove creeks |
Whyalla City Council/NREP
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The lead and zinc smelter discharges metal rich effluent into First creek and has resulted in substantial metal contamination throughout the region |
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Stormwater runoff from urban areas discharges nutrient and sediment loads to coastal waters |
Whyalla City Council/NREP
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