Sturt Nearshore Marine Biounit
2017 Aquatic Ecosystem Condition Report
On the map, zoom in and click on the dots to view underwater video at each site
- Seagrass cover is generally dense and continuous throughout the biounit.
- Reef condition is generally good with high cover of canopy algae.
- Widespread epiphyte cover of seagrasses indicates habitat stress.
About the biounit
The Sturt biounit extends from West Cape to Troubridge Point on the southern coast of Yorke Peninsula. Sturt experiences faster currents due to the south dominated orientation of the shoreline and the funnelling of water through Investigator Strait. This results in greater flushing within the biounit than most of Gulf St Vincent. There are many small embayments that are protected from the strong currents and prevailing south westerly winds that provide sheltered water that supports seagrass.
The surrounding land throughout Yorke Peninsula has been heavily modified for agriculture, with the majority of land cleared for cropping and a relatively small proportion is used for livestock.
There are number of small townships or shack communities in Sturt, the largest being Marion Bay with a permanent population of less than 200. These communities swell considerably during holiday periods and can put considerable pressure on local wastewater management systems. Only Foul Bay has Community Waste Management Scheme with other properties managing sewage through septic systems that can leach nutrients into shallow groundwater and potentially flow towards the coast.
The Yorke Peninsula Council operates a small desalination plant at Marion Bay. This desalination plant has the capacity to generate 20 ML/a of desalinated water, of which 5.5 ML was used for various purposes within the township in 2010. The discharge of brine from this system is discharged along the Marion Bay jetty.
The assessment of identified threats to the nearshore condition of the Sturt biounit predicted that Sturt is likely to be in Very Good condition. Typically the habitat structure is expected to be largely natural, however there may be some initial symptoms of nutrient enrichment or short episodes of temporarily poor water clarity.
The Sturt biounit condition was rated as Good during 2017.
Seagrass typically formed dense and continuous meadows of long-lived species.
Rocky reef was generally healthy with high cover of diverse, canopy forming algae.
The condition of the nearshore marine environment in Sturt reflects the low input from industry or urbanised areas.
This is the first ecosystem condition report for the Sturt biounit. 4 sites were monitored to assess the condition of the nearshore waters: 86% of habitats were seagrass, 7% was reef and 7% was unvegetated sand.
Seagrass cover throughout the biounit was largely dense and continuous seagrass meadows, but cover was variable within some sites. Seagrass at Port Davenport varied from sparse to dense and supported both long-lived species of Amphibolis sp. and Posidonia sp. as well as colonising species of Halophila sp. The average cover at that site was 41% and was the lowest seagrass cover of any site in Sturt. The greatest cover was at Marion Bay which supported more than 90% cover of dense, continuous seagrass meadow comprised of a diverse blend of long-lived species.
Seagrass epiphyte load was reasonably high for the biounit; 37/100 and was highest at Port Moorowie (52/100). While epiphyte was prevalent at all sites, there are a broad range of epiphytic species that can colonise seagrass and at this stage the condition of the biounit does not seem to be impacted.
Rocky reef habitat was observed at Port Moorowie, which supported an average canopy algae cover of 30% comprising of mainly Cystophora spp. and turf or bare substrate accounted for 19%.
Water quality within the Sturt biounit suggested conditions are close to what would be considered background. This is in line with low inputs from human activity and high flushing from the Investigator Strait.
Pressures and management responses
Failing and/or high density of onsite wastewater treatment (septic) systems in some coastal towns
The Yorke Peninsula Council continues to investigate the expansion of the current community wastewater management scheme (CWMS).