Dominant seagrass was Amphibolis spp. which in some areas produced continuous meadows, it was also seen on top of reef structures in between algal communities
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 portion of the biounit.
The southeast region of South Australia is a highly modified landscape. Due to broad-scale land clearance and the creation of an extensive cross-catchment drainage systems, nutrient rich sediment laden water from agricultural land is discharged 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 at some small coastal towns 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. The treated wastewater is reused 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 can result in losses if seagrass friendly mooring are not used.
Using reduced site data, in 2015 the Canunda biounit was in Very Good condition, based on monitoring conducted over autumn.
In summary
A reduced number of sites were monitored in 2021 (11) compared to 2015 (20). For comparable analysis, only sites that were monitored in both years were used to calculate the latest scores. Video analysis at 6 of the 11 sites was excluded due to high turbidity. Two sites were located on the Southern side of Robe bay, the rest were all in Rivoli Bay. The same six sites were also too turbid to monitor in 2015. 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 dominant seagrass species was Amphibolis, which in places created dense continuous meadows. It was also present on top of reef structures within communities of large branching brown algae and red algae species. Canunda biounit faces south-west where it is exposed to relatively high wave energies from the Southern Ocean.
Data analysis for the autumn 2021 monitoring period suggest that the marine environment is in a Very Good condition. Analysis of the 2015 data set suggests that the condition of the biounit has largely stayed the same, with the score staying at Very Good. Overall, the total seagrass percent cover stayed the same. All sites containing reef were made up of complex communities with little sand sedimentation. The presence of epiphyte and opportunistic algae were not recorded within the biounit.
Findings
The condition of habitats in waters between 2–15 m deep throughout the Canunda biounit was assessed based on monitoring data collected during autumn 2021. There are some areas within the biounit that are deeper than 15 m which are not included as a part of this assessment. A total of five sites were analysed during autumn 2021 to assess the condition of Canunda; 26% of the habitats monitored were classified as seagrass, 34% was reef and 40% was unvegetated sand.
The results showed that the percentage cover of seagrass for the biounit stayed the same, overall there was less than 1% difference between the two monitoring years. Individual sites such as Umpherston Bay (m0548) showed a loss in seagrass percent cover, whereas sites such as Bernouilli (m0526) showed a gain in seagrass cover. Some areas had dense and continuous meadows of Amphibolis, particularly in the sites South of Cape Jaffa such as Bernouilli (m0526). Apart from those areas of dense meadows, Amphibolis was generally sparse and in areas occupied the top of reef structures in between mixed communities of algae.
During 2021 monitoring, Rivoli Bay was too turbid to analyse the video. Water quality samples from the sites showed turbidity was 0.84 NTU. A depth profile at Southend (m0542) showed that there was a 15 NTU increase from the surface to the bottom. A turbicline of suspended sediment was noted in the bottom 1-2 m in many of the sites within Rivoli Bay. The documented loss of seagrass in Rivoli Bay linked to nutrient pollution and high turbidity from drain discharges (eg Drain M, Wear et al. 2006) has likely created an environment where sediments have become destabilised and are susceptible to resuspension. DEW’s South-East Flow Restoration Plan (SEFRP) was completed in early 2019. It diverts freshwater from Drain M at Callendale northwards along the Bakers Range Watercourse. Fresh water is then directed through the existing drainage network to fill wetlands in the Bakers Range and West Avenue Watercourses in the Upper South East. Since implemented it has reduced the amount of surface water run-off that is being discharged into Rivoli bay, in-turn, reducing nutrient loads. However, it is also likely that remaining seagrass meadows in Rivoli Bay, have become de-stabilised due to sediment erosion in the high wave energy environment resulting in further loss.
The 2021 monitoring period suggest that there was a slight increase in reef condition at Boatswain point. The findings showed that the average coverage of big brown algae went from 31% in 2015 to 67% in 2021, bare reef and turfing algae reduced 8%, other sites containing reef habitat largely stayed the same. Dominant reef species was Eckloniaand Cystophoraspp. Both species known to tolerate high wave energy environments well. Another indication of good reef health was the presence of Blue Throated Wrasse (Notalabrus tetricus) noted on multiple occasions during the 2021 survey.
Soluble nutrients at the time of sampling were consistently very low with soluble nitrogen at many of the sites below the limit of detection (<0.0015mg/l). Chlorophyll a data was consistently higher throughout the biounit in 2021 in comparison to 2015 and chlorophyll pigment size data indicates that the biounit is in mesotrophic conditions. Notwithstanding this, epiphytic algae was minimal within the biounit.
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.
The Limestone Coast Landscape Board has a variety of water security and sustainability projects. Specifically the elements of the projects relate to holding water in the landscape which will prevent less water reaching the ocean via the drainage network.
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 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.
Observed Condition
Expected Condition