South West River, West from Kelly Lodge
2013 Aquatic Ecosystem Condition Report
- Permanent flowing stream in autumn and spring 2013
- Sparse macroinvertebrate community with a few sensitive and flow-dependent species present
- Water was moderately fresh, clear and high in nitrogen
- Riparian vegetation comprised patches of gums and wattles over sedges and rushes
About the location
South West River is a moderately sized stream located on the south-western end of Kangaroo Island. The river rises at an elevation of about 235 m south from the West End Highway, and flows in a southerly direction for over 20 km before discharging into the Southern Ocean in Hanson Bay. The major land uses in the 8,583 hectare catchment upstream from the site sampled were plantation forestry (34%), other minimal uses (25%), grazing modified pastures (21%) and nature conservation (16.5%), with minor areas of roads, cropping, lakes, dams and residential living also present. The site sampled was located downstream from the South Coast Road, about 4 km west from Kelly Lodge.
The river was given a Fair rating because the site sampled showed evidence of moderate changes in ecosystem structure and some changes to the way the ecosystem functions. There was evidence of human disturbance due to the extent of nutrient enrichment of the river and presence of fine silt covering the bottom of the watercourse.
A moderately diverse community of at least 19 species of macroinvertebrates was collected or seen from the river (11 species in autumn and 10 in spring), 2.6-2.8 m wide and up to 35 cm deep, in autumn and spring 2013. The river consisted of still to slow-flowing shallow pools connected by tiny areas of faster-flowing, shallower riffle habitats in autumn but it lacked riffles in spring. The community was not dominated by any species but included low numbers of mites, amphipods, beetles, mosquitoes, chironomids, blackflies, mayflies and dragonflies occupying the pool habitats at the site sampled. The riffles were too small to effectively sample but a number of stoneflies, yabbies, marsh beetles and worms were seen on the sticks and fine sediments in the fast-flowing riffles in autumn. The majority of the community were generalist and tolerant species but at least four sensitive and/or flow-dependent species were present at the site; they included a blackfly (Austrosimulium furiosum), chironomid (Rheotanytarsus), mayfly (Thraulophlebia conspicua) and small unidentified stoneflies.
The water was moderately fresh (salinity ranged from 1,119-1,421 mg/L), well oxygenated (67-85% saturation), clear and slightly coloured, and with low to moderate concentrations of phosphorus (0.02-0.04 mg/L) but high nitrogen concentrations (0.99-1.29 mg/L).
The sediments were dominated by detritus and sand, with smaller amounts of silt and clay also present. Over 1 cm of fine silt was present in the middle of the channel in autumn. Samples taken from below the surface were grey sands which had a strong anaerobic odour and tested positive for sulphides in spring when water level was low; this indicates that the sediments lacked oxygen and represented a harsh environment for most benthic species to be able to live in. There was no evidence of any significant areas of bank erosion at the site and the only animal droppings seen in the vicinity of the stream were from kangaroos.
There was a moderate amount of phytoplankton present in the river in autumn but much lower concentrations were evident in spring (chlorophyll a ranged from 1.4-6 μg/L). No significant growths of filamentous algae were seen in 2013. Less than 10% of the river was covered by a few aquatic plants, including rushes (Juncus) and Water Ribbons (Triglochin). The narrow riparian zone (<5 m wide on each bank) was dominated by patches of gums and wattles over rushes, sedges (Carex) and bare soil. The surrounding vegetation near the river comprised dense eucalypt woodland.
Special environmental features
South West River provides a permanently wet, freshwater habitat and supports a few sensitive and flow-dependent species of macroinvertebrates.
Pressures and management responses
|Moderate nutrient inputs from diffuse sources in the catchment (leading to increased 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.|