Winninowie Nearshore Marine Biounit
2018 Aquatic Ecosystem Condition Report
- 2018 monitoring assessed 6 sites within the biounit
- The overall condition of Winninowie has reduced from fair in 2012 to poor in 2018.
- Miranda recorded substantial seagrass loss with almost 40% reduction in cover.
- Epiphyte growth on seagrass remained high
About the biounit
The Winninowie biounit occupies the far northern reaches of northern Spencer Gulf; extending from Point Lowly on the Eyre Peninsula, to Ward Point on the Yorke Peninsula, north to Port Augusta at the top of the gulf. The orientation of the coast, relatively shallow depth and distance from the mouth of the gulf, typically results in low wave energies and restricted water movement which can result in areas with reduced flushing. This is likely to result in favourable conditions for algal growth that could increase the effects of nutrient discharges. The geomorphology of the coastline is reasonably complex with numerous small bays and tributaries linking extensive mangrove flats to narrow, deeper channels where strong currents and tides in excess of 3 m occur.
The City of Port Augusta is located in the northeast of the biounit and is the most northerly town on Spencer Gulf. It has a population of approximately 6,560 people based on the 2016 Census data. The city’s sewage is collected and treated through two wastewater treatment plants (WWTP). The plant at Port Augusta East discharges effluent with high nutrient concentrations into the nearshore coastal waters near Hospital creek just south of Port Augusta, while Port Augusta West has no marine discharge. There are also a number of community waste management systems (CWMS) and individual septic systems to service satellite communities and shacks.
The two large coal-fired power stations in Port Augusta; Playford and Northern power stations closed in 2016 ceasing all thermal discharges into the marine environment north of Port Paterson. As a result an increase in condition of the upper reaches of the gulf is expected, but this may take decades to be seen.
Sundrop Farms has been farming tomatoes in greenhouses since 2010 and discharges brine from a solar desalination plant into Spencer Gulf. The brine is discharged via a number of duckbill diffusers located at the end of the now disused power station thermal effluent infrastructure.
The waters of Fitzgerald Bay are licenced for the sea cage aquaculture of yellowtail kingfish (Seriola lalandii) (YTK). However, fish farming largely ceased in Fitzgerald Bay since 2012. Prior to this time, farming in the bay peaked at just over 2000 tonnes of fish.
Based on the assessment of threats to the nearshore habitats, Winninowie was expected to be in Fair condition.
The condition of habitats in waters between 2–15 m deep throughout the Winninowie biounit was assessed based on monitoring data collected during autumn 2018. There are some small areas within the biounit that are deeper than 15 m which are not included as a part of this assessment.
Winninowie was observed to be in Poor condition. In some areas there were dense and intact seagrass meadows while in other areas the habitats were degraded with sparse and patchy seagrass and there were many areas that were under significant stress due to nutrient enrichment. Nutrient enrichment including 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 6 sites were monitored during autumn in 2018 to assess the condition of Winninowie; 22% of the habitats monitored were covered in seagrass, while unvegetated sand accounted for the remaining 78% of seafloor habitat, no rocky reef was encountered in the sites assessed. The observed habitats monitored are generally consistent with existing habitat mapping suggesting this monitoring is a good representation of the system.
Seagrass condition throughout the biounit was variable. Adjacent Ward Point, seagrass was dense and continuous, consistent with results from 2012. Seagrass at Miranda reduced significantly from 77% cover in 2012 to 38% in 2018 and appeared unhealthy with dense epiphytes covering seagrass leaves. The EPA AECR in 2012 found Miranda had extensive epiphytes and blanketing cover of algae, which may have contributed to the losses observed since that time. In other areas, seagrass was sparse or patchy and generally in a degraded condition including Port Augusta and Fitzgerald Bay inner. Groundwater flow from agricultural lands are likely to move towards the coast and discharge at unknown locations brining nutrients into the gulf. However, there has been no major changes to adjacent land use or discharges that would offer an explanation for this seagrass loss. Additionally, Miranda is shallow and may experience lower flushing than sites within the main channel.
Water chemistry showed results typical of gulf waters, however high and variable ammonia at Miranda was observed ranging from 0.01–0.04 mg/L which is over three times the biounit average. Total nitrogen was also elevated at Port Augusta, the northern most site monitored in Northern Spencer Gulf.
The entire biounit demonstrated indicators of nutrient enrichment. Seagrasses in many locations were covered epiphytic algae as well as opportunistic algae covering spaces between seagrass patches. The reduced flushing is likely contributing to the elevated nitrogen in the upper reaches of the gulf and the loss of seagrass and elevated ammonia at Miranda is concerning. If these conditions are sustained over time further habitat loss could occur.
Sea surface temperatures in Southern Australia have been increasing since the mid 1980’s and broadscale indicators such as phytoplankton data (Chlorophyll a) has shown a gradual increase from 2012 to 2018 within the Winninowie biounit. These regional scale changes may also be contributing to the changes seen in the current monitoring data.
If habitats are degraded or lost there can be impacts on the productivity of fisheries, nutrient assimilation and carbon storage, erosion and sand movement on beaches resulting in a negative impact on the marine environment and local communities.
Pressures and management responses
Nutrient loads discharged by the Port Augusta East wastewater treatment plant into the nearshore coastal waters just south of Port Augusta
Stormwater runoff from urban areas discharge nutrient and sediment loads to the nearshore waters of the biounit
Septic tanks treat sewage at shacks along the coast which in sandy soils transports nutrients into nearshore waters
Desalination effluent at Sundrop Farms