The Dutton biounit extends from Salt Creek, north of Tumby Bay to Arno Bay on the central western side of Spencer Gulf (see map). The shoreline of the biounit has an overall easterly orientation resulting in primarily offshore winds and low wave energy.
The Dutton Biounit contains the township of Port Neill which has a permanent population of approximately 194 people. A Community Wastewater Management Scheme (CWMS) was completed at Port Neill in early 2017. This CWMS will reduce the pressure from septic tanks that discharges into the shallow groundwater, particularly in sandy soils. This groundwater is likely to flow towards the coast which is put under significant pressure during holiday periods when the town’s population swells
The land use adjacent Dutton is predominately agriculture with cereal (grain) crops and sheep farming. Salt creek near Tumby Bay is a small creek that intermittently flows to the sea. This creek was assessed by the EPA in 2010, and was found to be in very poor condition, with obvious signs of nutrient enrichment. Rain events will bring sediments and nutrients into Tumby Bay, but the low average rainfall means this is rare.
Sea-cage aquaculture of Yellowtail Kingfish (YTK) is a significant industry at Arno Bay. Similarly aquaculture of Southern Bluefin Tuna is significant in the Jussieu biounit, offshore from Port Lincoln. Modelling of these discharges suggests that nutrient rich water flows into the Dutton biounit which may affect its condition. Notwithstanding this, the desktop risk assessment of potential threats to water quality for Dutton predicted it was likely to be in good condition.
In summary
A total of 3 sites were monitored during autumn 2016; 74% of the habitats monitored were seagrass, with bare sand making up the remainder. Overall the results indicate the biounit was in Very Good condition which is better than the predicted condition of good.
Seagrass habitats were generally dense and continuous Posidonia spp. with a very small amount of Amphibolis spp.. This indicates the system was in very good condition with only minor effects of nutrients or suspended sediments on the seagrass. While the habitats were intact, the seagrass was covered in moderate load of epiphytes which may be early warning signs of nutrient enrichment.
Findings
This AECR assessed the condition of habitats in waters between 2–15 m deep throughout the Dutton biounit sampled in autumn 2016. There are large areas within the biounit that are deeper than 15 m which are not included as a part of this assessment.
The results showed that all sites monitored supported dense and continuous seagrass meadows of Posidonia spp. with Outside Lipson (m0143) having an average of 90 % cover, while inside Lipson (m0147) had 55% cover. Seagrass at these sites had a moderate cover of epiphytes growing on the leaves, particularly Port Neill (m0148). Epiphyte growth can be an indicator of excess nutrients. Prolonged epiphyte growth on seagrass can lead to stress to the seagrass.
Water quality throughout the biounit was consistent between sites, with low turbidity ranging from 0.17–0.24 NTU, which is consistent with a nearshore ecosystem in very good condition. Other nutrients and phytoplankton were similar throughout the biounit.
The observed habitat structure is considered to be very good, but variability can be seen between sites, which may be normal, or could be early warning signs of change from an Excellent condition.
Having said that, the current condition reflects a system in very good condition maintaining natural structure and function and any variability is likely to be quickly reversible. The system is likely to be resilient and recovery of any localised impacts are likely to be rapid.
Pressures and management responses
Pressures
Management actions
Stormwater runoff from the Port Neill area, discharging nutrients and sediments to the nearshore marine waters
The Eyre Peninsula NRM Board have funded and completed a stormwater retention area to reduce surface runoff from a paved parking area in Port Neill. Community revegetation and jute matting stabilisation completed over recent years has also assisted with stabilisation of degraded coastal areas, reducing the potential for erosion and subsequent runoff.
Nutrient and sediment rich runoff from agricultural lands can enter the marine environment from Salt Creek in high rainfall events
The District Council Tumby Bay have developed and will implement their stormwater management plan including installing water sensitive urban design infrastructure. The Eyre Peninsula NRM Board will support Tumby Bay District Council to install water sensitive urban design infrastructure.
Failing and/or high density of onsite wastewater treatment (septic) systems in Port Neill.
The District Council Tumby Bay have completed construction of a Community Wastewater Management Scheme in the first half 2017. Approximately 20–25% properties will be connected as at 30 June 2017. The intent is to have all Port Neill township properties connected by 30 June 2018.
Sea cage aquaculture contributes high nutrient loads into the nearshore marine waters which can be transported into the biounit
There is currently no aquaculture activity being conducted within the Dutton biounit as at 20 June 2016.
The Aquaculture (Zones–Port Neill) Policy 2011 limits the maximum biomass of sea cage aquaculture animals that can be grown in the zoned areas located within the biounit and is based on a carrying capacity model to determine the suitable amount of fish for each particular region without causing adverse environmental impacts.
Refer to the Jussieu and Franklin biounit AECR for information regarding the management of this pressure within these two biounits where aquaculture activity is occurring.
Observed Condition
Expected Condition