Cassini 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 where environmental conditions are favourable.
- Reef condition is generally good with high cover of canopy algae.
About the biounit
The Cassini biounit is located on the northwest coast of Kangaroo Island (KI), extending from North Cape in the east to Cape Borda. The north coast of KI is relatively well protected from prevailing south westerly winds but can be affected by westerly swells. Strong currents rush through the Investigator Strait between Yorke Peninsula and the island creating a high energy environment that is similar to a moderately high energy wave coastline. Cassini has some small sheltered sandy bays where seagrass has established, but much of the nearshore marine environment is rocky reef.
There are no major towns adjacent Cassini and the largest community is Emu Bay, which has about 100 houses and the population swells during peak holiday periods. There are also small communities at Stokes Bay, Snellings Beach and Western River Cove. The population in these communities increases considerably in holiday periods. The Stokes Bay camp ground has a sewage waste management system and toilet facilities at various coastal locations use sealed systems. The homes and shacks use septic systems to manage sewage.
The adjacent land is largely used for agriculture and has mostly been cleared to make way for sheep grazing and timber plantations. There are a number of creeks that discharge during rainfall events carrying nutrients and sediments into the sea. There is a land based abalone farm in Smith Bay that uses yhe local seawater and discharges wastewater back into the nearshore environment.
The Cassini biounit was expected to be in Very Good condition based on a desktop assessment of threats.
Cassini was first monitored in 2017 and was determined to be in Good condition. Pressures are low due to minimal human population on this part of Kangaroo Island, but these pressures increase during holiday periods.
Seagrass was generally healthy where it was found, being dense and continuous stands of several long lived species. Seagrass epiphyte load was low for the biounit. Emu Bay was found to have the highest seagrass epiphyte load, which is not surprising given the higher population in this area, but also the more extensive seagrass meadows in this bay. Where epiphyte was relatively high, seagrass did not appear to show signs of stress.
Rocky reefs were healthy with predominantly dense canopy algae and minimal turfing algae or bare substrate.
A total of 8 sites monitored in Cassini in 2017; 42% were seagrass, 28% was reef and 30 % was unvegetated sand.
Seagrass cover was greatest in the larger sandy bays including Emu Bay (>75 %). Most seagrass meadows encountered in Cassini were comprised of more than one long-lived seagrass species interspersed through the site. Seagrass cover was generally lower at sites further west in the biounit, likely due to higher wave energies experienced along the more exposed westerly coast of Cassini. Only a small amount of seagrass was recorded at Snug Cove, Western River Cove and Snellings Beach (<10 %). Although these waters are well protected from prevailing southwesterly winds, these coves are small and relatively shallow with seagrass being found most predominantly in deeper water. The species growing at Western River Cove is Posidonia coriacea which is known to exist in high energy environments characteristic of the deeper part of the cove.
Seagrass and reef were often present in the same site, occurring at 75 % of sites. At these sites, seagrass grew up to the edge and on sandy patches among the reef wherever conditions allowed.
Generally, reef habitats in Cassini were in good condition and throughout the biounit, canopy algae covered 59% while bare substrate or turf accounted for 20% of the cover. Extensive reef habitat was found at Stokes Bay accounting for 63% of the site. The reef here has extensive Cystophora spp. canopy algae suggesting a healthy reef.
Water quality throughout Cassini was good and likely to be close to background conditions for South Australia, which is not suspiring given the low human population and the limited rainfall prior to sampling. Sites further west showed slightly higher turbidity likely to be due to the higher wave and water movement.
Pressures and management responses
Creeks discharge sediment and nutrients from the surrounding agricultural land into the nearshore waters during rainfall events
The Kangaroo Island Natural Resources Management Board (KINRM) has an on-ground works program that can assist landholders in the delivery of mitigating actions such as revegetation and protection of riverbeds. The Kangaroo Island Water Affecting Activity Policy regulates activities that may increase sediment export into watercourses.
Failing and/or high density of onsite wastewater treatment (septic) systems in some coastal towns
Stokes Bay Camp ground Waste water management system actively monitored and managed in 2017 to improve health of the treatment and disposal system in close proximity to coast and creekline.
Day visitor facilities (toilets established at numerous north coast locations maintaining sealed systems with no wastewater soakage.
Coastal and near-coastal developments have the potential to increase sediment discharge into the coastal environment, cause damage to seagrass meadows and alter coastal hydrological processes.
The Kangaroo Island Natural Resources Management Board (KINRM) provides input into development assessments as part of a wider consultation and referral process. Conditions of approval may mitigate the risk of damage to the coastal environment.
- Download the 2017 habitat and water quality data
- Download the Methods report for the nearshore marine ecosystems monitoring, evaluation and reporting program