Canunda Nearshore Marine Biounit
2015 Aquatic Ecosystem Condition Report
- On the map, zoom in and click on the dots to view underwater video at each site
- The biounit is subject to high wave energy
- Seagrasses were in generally in good condition, but did not form large continuous meadows
- Reefs were typically in good condition with extensive brown canopy algae
- Turbidity was very high at a number of sites and may be impacting on ecosystem condition.
About the biounit
The Canunda biounit is in the Otway bioregion and begins at Cape Jaffa and extends to Cape Banks, near Carpenters Rocks. The southwesterly orientation of Canunda exposes it to high wave energies. Offshore reefs attenuate much of the energy, resulting in moderate wave energy reaching sandy and rocky shores. The dominant coastal marine habitat is limestone reef with bare sand and seagrass meadows accounting for relatively minor components of the area.
The southeast region of South Australia is a highly modified landscape. Since 1863, broad-scale land clearance and the creation of an extensive cross-catchment drainage system discharges nutrient and sediment laden water from agricultural catchments directly to the marine environment via dozens of drains. Robe and Beachport are two major towns within Canunda. Populations in coastal towns have been increasing with the sea-change phenomenon and both also experience an influx of tourists during holiday periods. The pulse of increased population increases pressure on the sewerage network and can impact on the marine environment.
Sewage from Canunda is treated at community wastewater management systems at Robe, Beachport and Southend with treated wastewater being used for irrigation of dunes and public parks.
The Robe Marina houses a commercial fishing fleet focused mostly on southern rock lobster. The marina experiences high boat traffic and requires regular dredging to maintain safe navigational waters, which can increase turbidity. Swing moorings are used in Beachport and other locations, for the commercial fishing fleet, these moorings can scour the seafloor if placed on seagrass will result in seagrass loss.
The Canunda biounit was predicted to be in Good condition, based on an assessment of threats to nearshore habitats.
Twenty sites were monitored in waters between 2 – 15 m deep during autumn and spring in 2015 to assess the condition of the Canunda biounit. There are large areas within the biounit that are deeper than 15 m, which are not included as a part of this evaluation.
The dominant seagrass was Amphibolis sp. which is capable of growing in relatively high energy environments and at many sites it was growing in sand patches on top on the reefs. A number of sites around Robe were too turbid for reliable analysis. The turbidity may be reducing the amount of light available for plant and algal habitats, however it is unknown whether this is a natural occurrence or is due to human impacts in the region.
The findings suggest that the nearshore marine habitats are generally in Fair condition: habitat structure has been impaired with impacts from nutrient enrichment or suspended sediment. The habitat changes are likely to be altering ecosystem function including resilience, biodiversity, productivity and sediment stability. Detrimental effects may extend to numerous sites or small areas where longer term recovery is required.
Of the habitats monitored in Canunda, 13% were classified as seagrass, 30% was reef and 57% was unvegetated sand. A feature of this bioregion was dense Amphibolis sp. seagrass growing within sandy patches on top of the reef. While this is not uncommon, it does limit our ability to interpret the seagrass habitat using our existing conceptual models.
Seagrasses reduce wave energy allowing sediments to settle faster and can help to protect beaches from erosion. This beach erosion has become problematic at Beachport, in part due to seagrass loss, and the local council, in conjunction with the Department for Environment, Water and Natural Resources (DEWNR) spends significant money for ongoing shoreline protection and restoration.
The documented loss of seagrass in Rivoli Bay linked to nutrient pollution and high turbidity from the drain discharges (eg Drain M, Wear et al. 2006) is likely to be continuing to have an impact on coasts. It is also likely that remaining seagrass meadows, particularly in Rivoli Bay, have become de-stabilised in the high-energy environment resulting in further seagrass loss due to erosion.
In general, rocky reef habitats were typically comprised of moderate cover of large brown algae (Canunda average 32% cover) and diverse communities of understorey algae including Caulerpa sp. particularly the more exposed sites such as Wright Bay (m0527) and Stinky Beach (m0536).
A number of sites within Guichen Bay (eg Robe Esplanade: m0532, Robe: m0533) were so turbid that video assessment could not be used. Water chemistry results showed that across both seasons the average turbidity was above 1.6 NTU and as high as 3.4 NTU at m0533 in spring. The high turbidity limits the ability to understand whether these habitats have been impacted or whether this is a natural state.
At the time of sampling, the nutrient concentrations were consistently very low with soluble nitrogen below the limit of reporting, suggesting plants and algae were limited in growth by the lack of nitrogen. Phytoplankton results (chlorophyll a) were subsequently low with an average of 0.51 µg/L with the exception of a number of sites near Southend (Rivolli Bay south: m0541, and Southend: m0542) which were all elevated during autumn (Chl a: 2.04 and 2.15 µg /L respectively).
Pressures and management responses
|Numerous drains from the agricultural catchments discharges runoff with high nutrients and suspended sediments to coastal waters and may be inhibiting seagrass regrowth where it has historically been lost.||
Natural Resources SE, on behalf of the Limestone Coast Local Government Association, is currently implementing a large scale on-ground works project focused on improving the quality of coastal habitats along the full length of the coast from the Coorong to the Victorian border. This work includes extensive revegetation along drainage reserves, roadsides and coastal foredunes. This work will focus on coastal dune stabilisation, contributing to beach and dune stability.
Further the project includes substantial beach clean-up with marine debris removed from 50 kms of beach every year.
|Urban stormwater runoff from Robe, Beachport and Southend||
The Wattle Range Council has installed stormwater management systems in the Commercial Hardstand and Wash-down area of the Marina to manage the water that is used during the maintenance of the commercial fishing fleet as well as a smaller system to handle the wash down from the recreational vessels on the recreational boat ramp.
The stormwater side entry pits are cleaned annually with the sludge being treated through Councils CWMS treatment plant.
All new subdivisions have stormwater retention basins required, which collect stormwater before dissipating it into the underlying sand.
|Septic tank leakage may be reaching the marine environment through groundwater.||At Beachport, the Wattle Range Council has constructed a full sewer system to reduce environmental risks from septic systems.|
|Dredging of the mouth of the Robe marina may increase turbidity in adjacent waters.||Council has reduced sand dredging from the marina channel to once every three years with the sand dredged deposited on Main Beach to try to reduce turbidity.|
Download the 2015 habitat and water quality data
Download the Methods Report for the nearshore marine ecosystems monitoring, evaluation and reporting program.
Wear, R. J., Eaton, A., Tanner, J. E., Murray-Jones, S. 2006. The impact of drain discharges on seagrass beds in the South East of South Australia. Final report prepared for the South East Natural Resource Consultative Committee and the South East Catchment Water Management Board. South Australian Research and Development Institute (Aquatic Sciences) and the Department of Environment and Heritage, Coast Protection Branch, Adelaide. RD04/0229-3.