Ravine des Casoars, South from Cape Borda
2013 Aquatic Ecosystem Condition Report
- Permanent flowing stream in autumn and spring 2013
- Diverse macroinvertebrate community with several rare, sensitive and flow-dependent species present
- Water was moderately fresh, clear and high in nitrogen
- Riparian vegetation comprised native vegetation with no weeds
About the location
Ravine des Casoars is located on the western end of Kangaroo Island within the Flinders Chase National Park. The stream rises in the middle of the park at an elevation of about 255 m to the south from the Playford Highway, and flows in a westerly direction for nearly 20 km before discharging into the Southern Ocean, south from Cape Borda. The upper reaches of this stream comprise several branches with all but one located within the boundaries of the park. The major land use in the 9,912 hectare catchment upstream from the site sampled was nature conservation (92%), with minor areas of grazing modified pastures, other minimal uses, plantation forestry and roads also present. The site sampled was from the lower reaches off a track from the West Bay Road, about 4 km south from Cape Borda.
The stream was given a Good rating because the site sampled showed evidence of relatively minor changes in ecosystem structure and function. There was some evidence of human disturbance due to nitrogen enrichment and road runoff from the unsealed West Bay Rd depositing fine sediment into the watercourse. The presence of what appeared to be marron holes in the banks also indicates that this introduced crayfish has been released into the stream in recent years. Despite this, the stream supported a rich assemblage of aquatic species, including several significant macroinvertebrates for the island.
A diverse community of at least 31 species of macroinvertebrates was collected or seen from the stream (19 species in autumn and 16 in spring), 2.8-7.5 m wide and up to 22 cm deep, in autumn and spring 2013. The stream consisted of still to slow-flowing shallow pools connected by faster-flowing, shallower riffle habitats in both seasons sampled; riffles comprised 40% of the site in autumn but less than 5% in spring. The community was not dominated by any species but included low numbers of limpet snails, worms, mites, amphipods, isopods, beetles, craneflies, blackflies, chironomids, mosquitoes, biting midges, mayflies, waterbugs, stoneflies and caddisflies. Holes in the banks indicate that marron probably also inhabited the site. The only rare and sensitive species collected were blackflies (Austrosimulium furiosum and Simulium melatum), a mayfly (Thraulophlebia inconspicua) and a stonefly (Dinotoperla evansi). A few flow-dependent species were also collected, including a dytiscid beetle (Platynectes), another blackfly (Simulium ornatipes) and the above-listed rare and sensitive species. The other members of the community were generalist and tolerant species that have been recorded from a wide range of streams throughout the wetter parts of the State.
The water was moderately fresh (salinity ranged from 1,102- 1,459 mg/L), well oxygenated (91-114% saturation), clear and slightly coloured, and with low concentrations of phosphorus (0.01 mg/L) but high nitrogen concentrations (0.54-0.6 mg/L).
The sediments from riffle habitats were dominated by boulder, bedrock, cobble and sand whereas the edge habitats mostly comprised detritus over the similar coarse sediment types. Samples taken from below the surface were grey sands and showed no evidence that the sediments were anaerobic or lacking in oxygen. There was evidence of a small amount of erosion affecting about 10% of the banks, which appeared to have been caused by past flood damage. The only animal droppings seen in the vicinity of the stream were from kangaroos.
There were no significant growths of phytoplankton or filamentous algae detected during 2013. Over 35% of the stream was covered by aquatic plants, including extensive growths of rushes (Juncus) and patches of submerged charophytes (Chara) and emergent sedges (Baumea). The riparian zone was dominated by gum trees and wattles over bracken, rushes and other native understorey plants. The surrounding vegetation near the stream comprised dense eucalypt woodland which included tall gum trees and large numbers of small regenerating gums.
Special environmental features
Ravine des Casoars provides a permanently wet, freshwater habitat that supports a rich community of aquatic macroinvertebrates, in comparison with most other streams on the island. The stream also provides habitat for several rare, sensitive and flow dependent species, as well as a number of tolerant and generalist species.
Pressures and management responses
|Moderate nutrient inputs from diffuse sources in the catchment (which may lead to extensive growth of algae and aquatic weeds)||The Kangaroo Island NRM Board has funded the fencing of significant areas of riparian vegetation in the catchment and continues to work with landowners to increase the fencing of watercourses.|
|Introduced crayfish (marron) that have the potential to impact on aquatic biodiversity||This information is not available at the moment but it will be updated as soon as possible.|
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