On the map, zoom in and click on the dots to view underwater video at each site
Seagrass cover has declined throughout the biounit.
Reef condition has declined due to a decrease in canopy algae and an increase in turfing algae.
Area map
About the biounit
The Nepean biounit extends from North Cape through to Cape Willoughby on the north coast of Kangaroo Island and encompasses Nepean Bay. The northeast coast is protected from prevailing southwesterly winds and swells, but strong currents rush through the Back Stairs Passage between the mainland and the island creating a higher energy environment.
Kingscote has the largest population in the Nepean Biounit, while Penneshaw and American River are also important townships with populations that swell significantly during peak holiday seasons. Kingscote and American River have Community Wastewater Management Systems to treat sewage, with the treated wastewater used to irrigate green spaces. Other coastal towns rely on septic tanks to manage sewage, which can result in nutrients entering groundwater that may flow towards the sea.
A small desalination plant supplements drinking water to Penneshaw. This desalination plant draws in marine water and through reverse osmosis removes the salts, resulting in a brine with elevated salinity discharged into the nearshore marine environment.
Agricultural land dominates the catchment around Nepean, 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 with heavy rain. The Cygnet catchment runoff has been linked to extensive loss of seagrass throughout Western Cove.
Oyster leases line the mouth of American River on the north shore of Eastern Cove and benefit from the increased tidal movement into and out of the narrow channel into American River.
The waters of Kangaroo Island are popular holiday destination location for boats that anchor in the sheltered waters of Nepean Bay. Some of these vessels may discharge waste within the bay, which may pose a risk to nutrient enrichment, but also recreational swimmers.
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 Good condition, based on the assessment of threats to water quality.
In summary
The Nepean biounit was rated as Poor during 2017. The condition has declined since 2011 and the biounit showed widespread loss of seagrass.
All of the sites in Nepean Bay have lost seagrass and have substantial epiphyte loads or opportunistic algae indicating widespread eutrophication. The most sheltered sites in Bay of Shoals, and Western Cove appear the worst and have suffered considerable seagrass loss.
The condition of reef has also declined with a decrease in canopy algae and an increase in turfing algae.
There is significant nutrient enrichment in the Nepean biounit that is impacting on marine ecosystem condition.
Findings
A total of 10 sites in Nepean were monitored in 2017; 41% of habitats were seagrass, while 2% were reef and 57% unvegetated sand.
Across Nepean the average seagrass cover monitored was 15%, this has significantly decreased from 42.5% and 49.3% in 2011 and 2010 respectively. In 2017, seagrass cover was greatest at Outside Spit (>70 %), while all other sites had less than 20 % cover. Seagrass has been almost entirely lost at Bay of Shoals, Kingscote and Western Cover outer. Where seagrass was absent, unvegetated sand in some locations had microalgal mats growing suggesting substantial nutrient enrichment. Algal mats were most notable at Inside Spit in the north of Nepean Bay.
The northwest of Nepean Bay had very sparse or patchy seagrass with high loads of epiphyte or opportunistic algae. Epiphytes and nuisance algae can prevent light that is essential for photosynthesis of seagrass leaves. In Bay of Shoals, nuisance algae almost entirely covered the seabed with only occasional seagrass leaves protruding suggesting this has been particularly affected.
The seagrass epiphyte load for the biounit was 27/100 for 2017 which has decreased from 34/100 in 2011, which is related to the widespread decline in seagrass cover removing the substrate for attachment. The opportunistic nuisance algal cover was 26/100 for the biounit, but the highest was 73/100 at Nepean Offshore.
Only one site monitored in Nepean has reef: Frenchmans, large brown canopy algae was less than 30% and turfing algae or bare substrate was 32% suggesting the reef is somewhat degraded. The cover of bare or turfing algae has increased since 2011 where it was less than 15%.
Water quality results did not show any outstanding levels of nutrients and were comparable to results from 2011. However, nuisance algae quickly assimilate nutrients from the water column especially in shallow, well lit waters of bays. Turbidity was reasonably high and could reflect nutrient rich runoff from high rainfall events, or due to the continual inflow of water from the Backstairs Passage.
There have been wide scale decline in the condition of nearshore marine habitats in Nepean when compared to 2011. The documented loss of seagrass in Nepean since the late 1990’s has consistently been attributed to nutrient enrichment from runoff from agricultural land reaching the marine environment through the Cygnet River. Greater than average rainfall in recent years led to high flow from the Cygnet, which drains high nutrient and sediment loads from the catchment into Nepean Bay. Hydrodynamic studies have shown that water movement is very slow within the Bays and wind, particularly during the summer months, pushes water north and potentially into Bay of Shoals. During winter any discharges from the Cygnet are likely to flow towards Western Cove.
Nutrient enrichment and seagrass loss has been highlighted throughout this region by numerous studies since the late 1990s. Without extensive action on nutrient loads entering Nepean Bay habitats will further degrade and any remaining seagrass will be lost.
The loss of seagrass has been shown to negatively impact commercial and recreational fish production, sand movement and erosion, which can have a significant impact to the local ecosystems, social values and economies.
Pressures and management responses
Pressures
Management actions
Agricultural runoff discharging through the Cygnet River.
The Kangaroo Island Natural Resources Management Board (KINRM) has been addressing Cygnet River nutrient and sediment loads through Australian Government-funded Catchment to Coast (C2C) programs since 2009. Catchment to Coast and other on-ground works activities focus on revegetation, 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. The Kangaroo Island Water Affecting Activity Policy regulates activities near watercourses that may increase sediment export. Nutrient and sediment loads are measured through water quality and flow monitoring in the upper and lower Cygnet River. Seagrass condition and extent has been monitored in Nepean Bay from 2009 to further inform trends in seagrass condition and extent within Nepean Bay.
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 has conducted monitoring into the quality of brine being discharged.
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. In 2016, the Environmental Management Plan was revised with more stringent environmental monitoring. The system has also been updated with the addition of aerobic treatment to the process.
The council has increased focus on re-use of water and has eliminated issues with unplanned discharge of treated irrigation water to wetland areas.
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 has established a CWMS at American River and also at Penneshaw. Together the systems have removed in excess of 600 onsite systems and redirected waste water via a full sewerage scheme.
Urban runoff entering Nepean Bay in high rainfall events.
The Kangaroo Island Council has installed a gross pollutant trap at intersection of Telegraph and Esplanade in Kingscote 2017, reducing stormwater pollutants entering Nepean Bay.