North West River, N Karatta
2013 Aquatic Ecosystem Condition Report
- Permanent flowing stream in autumn and spring 2013
- Moderately diverse macroinvertebrate community with several rare, sensitive and flow-dependent species present
- Water was fresh, clear and low in nutrients in autumn but enriched with nitrogen and phosphorus in spring
- Riparian vegetation comprised gums and wattles over native sedges and rushes
About the location
North West River is a moderately sized stream on the south-western end of Kangaroo Island. It rises at an elevation of about 250m in the middle of the island near the eastern end of Flinders Chase National Park, and flows in a south-south-easterly direction for nearly 20 km, before joining with North-East River to form the Stun’Sail Boom River. This river then flows for several more kilometres before discharging into the Southern Ocean. The major land use in the 5,941 hectare catchment upstream from the site sampled was nature conservation (94%), with smaller areas also used for plantation forestry, other minimal uses, grazing modified pastures, transport and communication, and residential living. The site was located in the middle of the catchment off a fire track from Church Road, about 16 km east-north-east from the Department for Environment, Water and Natural Resources office and Flinders Chase Visitor Centre.
The river was given a Good rating because the site sampled showed evidence of relatively minor changes in ecosystem structure and function. There was, however, some evidence of human disturbance due to nutrient enrichment in spring and the presence of introduced marron.
A moderately diverse community of at least 22 species of macroinvertebrates was collected or seen from the river (9 species in autumn and 17 in spring), 7 m wide and over 1.5 m deep, in autumn and spring 2013. The river consisted of still to slow-flowing pools in both seasons sampled. The community was not dominated by any species but included low to moderate numbers of water mites (Oxidae, Australiobates, Coaustraliobates, Procorticacarus, Albia and Arrenus), amphipods (Austrochiltonia), dixid flies, mosquitoes (Aedes), biting midges (Bezzia), chironomids (Paramerina, Paralimnophyes dark, Cricotopus, Tanytarsus, Rheotanytarsus and Chironomus), mayflies (Thraulophlebia inconspicua), waterbugs (Micronecta and Microvelia), stoneflies (Newmanoperla thoreyi) and caddisflies (Lectrides varians). Numerous introduced marron were also seen throughout the site, and the density of holes in the banks indicated that they were a commonly occurring species in this stream. Several notable species were recorded, including the rich range of hygrobatid mites (Australiobates, Coaustraliobates and Procorticacarus) and the presence of rare and sensitive species of mayfly and stonefly. The latter species generally only co-occur in a few fresh, permanently flowing streams in the Southern Mt Lofty Ranges. The chironomid (Rheotanytarsus) is another flow-dependent species that forms distinctive cases, fringed with a net, to assist larvae in filtering particles from the water flowing past their tubes.
The water was fresh (salinity ranged from 263-481 mg/L), generally well oxygenated (55-83% saturation), clear and slightly coloured, and with variable concentrations of nutrients such as phosphorus (0.01-0.06 mg/L) and nitrogen (0.38-0.85 mg/L); higher concentrations were recorded during the spring period.
The sediments were dominated by detritus, sand and clay, with a smaller amount of silt also present. Samples taken from below the surface were grey silts and showed no evidence that the sediments had recently been anaerobic or lacking in oxygen. There was also 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 only a small amount of phytoplankton (chlorophyll a ranged from 0.14-0.54 μg/L) and no filamentous algae recorded or seen at the site in 2013. Aquatic plants covered over 10% of the channel in spring, including submerged (Hydrocotyle) and emergent species (Juncus and Triglochin). The narrow riparian zone was dominated by patches of gums and wattles over sedges, sedges and bracken. The surrounding vegetation near the river comprised dense eucalypt woodland with a native understorey.
Special environmental features
North West River provides a permanently wet, freshwater habitat in a catchment dominated by native vegetation. The aquatic life recorded at the site in 2013 included several rare, sensitive and flow-dependent species and a suite of more tolerant macroinvertebrates and plants. This indicates that the river is among the most biologically significant on the island.
Pressures and management responses
|Moderate nutrient inputs from diffuse sources in the catchment (potentially leading to 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.|