Nepean Nearshore Marine Biounit
2011 Aquatic Ecosystem Condition Report
Condition overview
Key points:
- On the map, zoom in and click on the dots to view underwater video at each site.
- Sites located around the Bay of Shoals and outside Spit had seagrasses that were dense and largely intact
- Sites located around Kingscote and Western Cove were in degraded with sparse and patchy seagrass cover
- All locations monitored were observed to have dense epiphytes growing on seagrass leaves, which is likely to be causing significant stress on seagrass habitats
Area map
About the biounit
The Nepean Biounit extends from North Cape through to Cape Willoughby encompassing approximately 114 km of Kangaroo Island coastline.
There are 3 main coastal towns within the Nepean Biounit, American River is the smallest with 216 residents, while the largest population is centred at Kingscote with 2034 residents, however the populations in these towns swell significantly during peak holiday seasons. Kingscote is currently the only town to have a Community Wastewater Management System which treats sewage from approximately half of the town. The treated wastewater is used to irrigate the golf course.
The Penneshaw local community does not have access to natural fresh water and relies on desalinated water. The small desalination plant uses conventional reverse osmosis technology to produce fresh water and a brine wastewater with elevated salinity is discharged into the nearshore marine environment.
The land use adjacent to the biounit is largely agricultural with the Cygnet River draining the largest catchment on the island. This river flows into the marine environment at Brownlow and brings large nutrient and sediment loads into the nearshore waters. The Cygnet catchment runoff has been linked to the loss of seagrass throughout Western Cove.
The Nepean biounit, has large areas of shallow, warm waters which have reduced flushing. This is likely to result in favourable conditions for algal grow that could increase the biological effects of excess nutrients.
The Nepean biounit was expected to be in Fair condition, based on an assessment of threats to the nearshore habitats.
In summary
The condition of habitats in waters between 2 – 15 m deep throughout the Nepean biounit was assessed based on monitoring data collected during autumn and spring 2010. There are large areas within the biounit that are deeper than 15 m which are not included as a part of this evaluation.
The Nepean biounit was observed to be in Good condition during 2011 and has not changed since the previous assessment in 2010.
In some areas there were dense and intact seagrass meadows while in other areas the habitats were degraded. Throughout the biounit there were many areas that were under significant stress due to nutrient enrichment which is likely to be causing excessive growth of algae on the seagrass leaves (epiphytes). Loss of seagrass cover could occur if this scenario is prolonged.
Findings
A total of 10 sites were monitored during autumn and spring in 2011; 66 % of the sites were covered in seagrass, while 24 % were covered in unvegetated sand. Reef covered less than 10 % of the sites.
The results indicated that the biounit was in Good condition and has not changed since the previous assessment in 2010. The average habitat condition for the biounit was 67 out of 100 which shows little biounit wide variability compared to the 2010 value of 62 out of 100. The monitoring showed that there were some areas that had dense and continuous Posidonia sp. seagrass. These sites were located in Bay of Shoals and outside Spit which had habitat condition values of 99 and 100 out of 100 respectively. There were sites that were in poor condition which included both inner and outer Western Cove and Kingscote with habitat condition values of 47, 0 and 50 respectively.
A number of sites in the biounit showed variability in seagrass coverage which can in part be attributed to more ephemeral seagrass species including Heterozostera spp. and Halophila spp. In most cases where large changes in cover were noted the amount of seagrass was relatively stable around the intra-annual variability. However one site Western Cove outer (m0054) had 43% cover of Heterozostera spp. seagrass in autumn of 2010, which reduced to 5% in spring and in both autumn and spring in 2011 the site was totally bare sand. While Heterozoetera spp. are known to be an ephemeral seagrass the 2010 surveys indicated very dense epiphyte loads on the existing seagrass and in 2011 there were frequent observations of dense opportunistic macroalgae in both autumn and spring.
There was a small area of reef monitored at the Frenchmans site. This reef had 36% macroalgae and there was 11% cover of turfing algae. These were slightly less than the observations in 2010 although they are likely to represent small scale variability rather than actual change in reef coverage.
All seagrass habitats throughout the biounit were covered in dense growth of epiphytic algae which is again consistent with the 2010 results. The sites near Kingscote were the most affected including inside Spit, Kingscote and Western Cove inner. The density of epiphytes was slightly worse during spring compared to autumn but it is likely that the seagrass habitats could be under prolonged stress due to these dense epiphyte loads throughout the year. In addition to epiphytes on seagrasses, there were observations of opportunistic macroalgae throughout Western Cove.
The nutrient concentrations were generally similar to what has been observed at reference locations. The amount of phytoplankton in the water was slightly elevated particularly during autumn with an autumn average of 0.83 ug/L and a maximum of 1.40 ug/L at inside Spit compared to an average of 0.59 ug/L observed in reference locations.
These findings indicate that the nearshore marine habitats within the Nepean biounit are under significant stress due to widespread dense epiphyte loads on all seagrass habitats monitored. It is possible that the low marine current conditions and high water residence times typical in Nepean Bay are increasing the effect of nutrient inputs.
On going stress to these marine habitats could result in habitat loss. If habitats are lost there could be:
- Impacts on the productivity of fisheries
- Increased erosion and sand movement on beaches
- Reduced wave attenuation along the coast
- A negative impact on marine biodiversity
Pressures and management responses
Pressures | Management responses |
---|---|
High nutrient loads in runoff discharged from the Cygnet River into the nearshore waters of Western Cove | The Kangaroo Island Natural Resources Management Board (KINRM) is addressing the Cygnet River nutrient loads through the Australian Government-funded Catchment to Coast (C2C) project, which commenced in 2009 and will continue until 2018. Catchment to Coast focuses on fencing of riparian vegetation and construction of in-stream remediation works such as formed creek crossings in order to exclude livestock from riverbanks and watercourses, thereby reducing erosion and runoff. On-ground works are guided by a catchment model developed by the EPA that identifies sub-catchments that contribute the greatest nutrient and sediment loads to the Cygnet River. Outcomes are measured and assessed through weekly and monthly water quality and flow monitoring at two sites in the Cygnet River and monitoring of seagrass condition and extent at seven sites in Western Cove. |
There is a community wastewater management system (CWMS) adjacent to the Cygnet Estuary. This has potential for nutrient transport to the estuary and nearshore marine waters via groundwater flows. | The Kingscote community wastewater management system (CWMS) is operated by the Kangaroo Island Council. Two of the three CWMS facultative lagoons were relined in 2010 and a third storage lagoon is planned to be relined by about 2015 in two stages. An aerobic waste water treatment plant will also be constructed and commissioned at this time. Groundwater monitoring has indicated that infrastructure upgrades have been successful in preventing nutrient from this facility entering the Cygnet Estuary and Nepean Bay. |
Stormwater runoff from the Kingscote area, discharging nutrients and sediments to Nepean Bay |
The Kangaroo Island Natural Resources Management Board (KINRM) has submitted a proposal for funding under the Caring for our Country Target Area Grants for a four-year project that will deliver plans for integrated stormwater management in Kingscote. The project involves collaboration between KINRM, Department for Environment, Water and Natural Resources and the Kangaroo Island Council to establish strategies to retain and reuse stormwater and design and install filtration mechanisms to remove pollutants and sediments. In 2009/10 additional emergency storage tanking was installed at Pump Stations, to supplement the existing overflow tanking. W&G Engineers designed and supervised the construction, to increase the capacity of these tanks with regard to power outages. Natural Resources Kangaroo Island also has an Emergency Manual that highlights actions to connect generators/pumps to maintain low levels in pump stations for periods of extended power outages. |
Failing and/or high density of onsite wastewater treatment (septic) systems in some coastal towns. This is probably most significant in the American River, Island Beach and Penneshaw areas. Overflowing septic systems contribute nutrients to nearshore marine waters through shallow sub-surface or occasional overland flows. |
The Kangaroo Island Council commissioned the American River community wastewater management scheme (CWMS) when it was completed in 2010, and this includes a waste water treatment plant and facultative lagoons. American River properties have progressively been connected to a CWMS with remaining developed properties with septic trenches connecting by June 2014. The Penneshaw CWMS remains in consultation between Council/ LGA/Community action groups- Stage 1 implementation is expected in 2014. An Island Beach CWMS is on a long term plan & lodged with the LGA in 2009. |
The Penneshaw desalination plant discharges small volumes of wastewater with elevated salinity. | The Penneshaw desalination plant is a relatively small plant. It was built and is operated by SA Water to provide potable water for the residents of and visitors to Penneshaw. The desalination plant is operated in accordance with EPA licence conditions. Sea water is treated to remove salt and in doing so, a small amount of ‘brine’ is created and discharged back to the sea. SA Water monitors the quality of brine being discharged as well as the quality of water off shore of the discharge. |
Further information
- Download the 2011 habitat and water quality data
- Download the Methods Report for the nearshore marine ecosystems monitoring, evaluation and reporting program.