On the map, zoom in and click on the dots to view underwater video at each site.
Sheltered habitats alternated between mixtures of seagrasses including dense Posidonia spp. and Heterozostera spp., through to Halophila spp and fine branching green algae.
There were some indicators of nutrient enrichment present during autumn, including seagrass epiphytes, which were prevalent on the Posidonia australis seagrass.
Area map
About the Biounit
The Yanerbie Biounit is located in the Eyre bioregion and extends from Cape Bauer near Streaky Bay through to the Talia Caves located to the south of Venus Bay. The biounit is a combination of open coasts that experience high wave energy, cliffs and rocky headlands through to enclosed embayments that experience low wave energy and sheltered conditions of Venus Bay and Baird Bay.
The biounit has a diverse mix of habitats reflective of the different environments. Unvegetated sand dominates the known habitats on exposed coasts, while the sheltered bays are covered in extensive seagrass, particularly Heterozostera spp., and Posidonia australis spp. and are important nursery areas for fisheries. Rocky reefs in this area are typically dominated by large brown algae Ecklonia radiata and Cystophora spp. and the red algae Osmundaria spp. and Haliptilon spp. occurring in shallower waters.
Venus and Baird Bays are shallow, sheltered bays which open to the ocean through narrow mouths. This restriction is likely to result in lower water exchange with the ocean, which can exacerbate the effect of nutrient enrichment. Habitats outside of these bays are exposed to the southern ocean swells.
The largest town in the biounit is Venus Bay with 139 permanent residents, while Port Kenny, Baird Bay and Sceale Bay all have smaller populations. The population of the biounit substantially increases over holiday periods with the region being a popular recreational fishing location. All townships in this biounit use on-site septic tanks to dispose and treat sewage. In high densities, on-site septic tanks have the potential to introduce nutrients into shallow groundwater which can flow to the coast. The influx of people during holiday periods adds strain to the capacity of on-site septic systems, particularly at the Caravan Park.
The adjacent land use is primarily cereal and modified pasture crops, there are also numerous coastal conservation parks which can act to buffer land based surface runoff, but given the very low rainfall of the area, surface water runoff from agricultural land is likely to be minimal.
The assessment of identified threats to the nearshore habitats predicts that the Yanerbie is likely to be in Very Good condition.
In summary
Habitats within Venus Bay showed a transition related to distance from the mouth of the Bay that may be broadly associated with the salinity. Posidonia australis was found to be prevalent around Port Kenny (m0421) with a gradual transition towards Heterozostera spp. mixed with intermittent fine branching green algae in the middle of the Bay shifting to a mix of fine branching green algae and Heterozostera sp. becoming sparse at west Venus Bay (m0425).
There were some indicators of nutrient enrichment during autumn, including seagrass epiphytes, which were prevalent on the Posidonia australis seagrass. Ambient nutrients and phytoplankton were also elevated during autumn.
Findings
The condition of habitats in waters between 2–15 m deep throughout the Yanerbie biounit was assessed based on monitoring data collected during autumn and spring 2014. There are some areas within the biounit that are deeper than 15 m which are not included as a part of this assessment.
The isolation and exposure of Yanerbie resulted in only 4 sites being assessed in autumn and spring of 2014 and all of these sites were located within Venus Bay. Caution has been applied when extrapolating the condition assessment from Venus Bay to the rest of Yanerbie due to the vastly different exposure and likely habitats.
Habitats within Venus Bay were a mix of different species, but there was a transition in composition which was likely related to distance from the mouth. Posidonia australis was prevalent around Port Kenny (m0421) with a gradual transition towards Heterozostera spp. mixed with intermittent fine branching green algae in the middle of the Bay. This shifted to fine branching green algae dominant with Heterozostera sp. becoming sparse. This transition was broadly associated with the increase in salinity observed across the bay ranging from approximately 38 ppt at Port Kenny, through to 43.9 ppt farthest from the open ocean at West Venus Bay (m0425). The salinity gradient was strongest during autumn after the evaporation from the summer months at approximately 4 ppt across the Bay, while the gradient was much lower during spring sampling at approximately 1 ppt.
Seagrass epiphyte loads were consistently higher in autumn than in spring and were higher at Port Kenny. This may be related to the presence of Posidonia australis seagrass which has a long leaf turnover rate compared to Heterozostera spp. allowing more time to accumulate epiphytes. Nutrient concentrations during autumn were elevated with consistently higher total and dissolved nitrogen concentrations and elevated phytoplankton in the water during autumn compared to spring.
The habitats outside of Venus Bay and Baird Bay are likely to be dominated by unvegetated sand and rocky reef communities which are exposed to the high wave energy of the southern ocean. These areas will be prioritised for future monitoring to enable a more accurate assessment for the biounit.
This assessment of condition indicates that Yanerbie was in Good condition which suggests that the habitats are largely intact and only slightly impaired with initial symptoms of nutrient enrichment.
The findings presented here are explained more thoroughly in the Eyre & Murat Assessment Report.
Pressures and management responses
Pressures
Management responses
Urban runoff from Venus Bay and Baird Bay can transport nutrients and sediment into nearshore waters
The District Council of Elliston regularly review the stormwater management around Venus Bay and Port Kenny. Issues of concern are identified and appropriate treatments are considered.
Agricultural runoff from the surrounding lands can bring nutrients and sediment into nearshore waters during large rain events
The current low agricultural activity within the surrounding land of Venus Bay and Port Kenny is liekly to reduce the likelihood of this pressure having a significant effect.
Failing and/or high density of onsite wastewater treatment (septic) systems in some coastal towns. This is probably most significant in the Venus Bay, Baird Bay and Port Kenny areas. Overflowing septic systems contribute nutrients to nearshore marine waters through shallow subsurface or occasional overland flows.
New applications for waste water systems are subject to Council Development Plan requirements.