Cygnet River, Huxtable Forest Gauge Station
2013 Aquatic Ecosystem Condition Report
- Permanently wet flowing stream with riffle and pool habitats present in autumn and spring 2013
- Diverse macroinvertebrate community with several rare, sensitive and/or flow-dependent species present
- Water was moderately fresh, slightly turbid and high in nutrients
- Riparian vegetation comprised native trees and understorey plants
About the location
Cygnet River is the largest stream on Kangaroo Island that rises from the centre of the island at an elevation of about 245 m north of the Playford Highway, and flows in an easterly direction for over 60 km until it discharges into Nepean Bay to the south from Kingscote. The major land uses in the 15,794 hectare catchment upstream from the site sampled were grazing modified pastures (55%), other minimal uses (18%) and plantation forestry (11%), with minor areas also used for cropping, nature conservation, roads and services, dams, residential living, intensive animal production and irrigated horticulture. The site sampled was located at a gauge station in the middle of the catchment, off an unnamed track from Bark Hut Road, about 6 km north-east from Parndana.
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 and presence of marron in the river.
A diverse community of at least 35 species of macroinvertebrates was collected or seen from the river (19 species in autumn and 25 in spring), 1.3-6.9 m wide and up to 38 cm deep, in autumn and spring 2013. The river consisted of still to slow-flowing pools connected by smaller areas of faster-flowing riffle habitats in both seasons sampled. The community was not dominated by any species but included low numbers of worms, hygrobatid mites (including Procorticacarus), amphipods, introduced marron, beetles, craneflies, mosquitoes (including Anopheles and Aedes), biting midges, blackflies, long-legged flies, chironomids, mayflies, waterbugs, stoneflies and caddisflies. Most were commonly found, generalist taxa that are tolerant to poor water quality. However, several rare, sensitive and/or flow-dependent species were also collected, including the long-legged fly (Family Dolichopodidae), mayfly (Thraulophlebia inconspicua), two blackflies (Austrosimulium furiosum and Simulium ornatipes), two chironomids (Eukiefferiella and Cardiocladius) and two stoneflies (Dinotoperla evansi and Riekoperla naso). The only fish seen at the site were some small native galaxiids.
The water was moderately fresh (salinity ranged from 1,589-1,673 mg/L), well oxygenated (75-114% saturation), slightly turbid and coloured, and with moderate to high concentrations of nutrients such as phosphorus (0.02-0.03 mg/L) and nitrogen (0.84-1.09 mg/L).
The sediments were dominated by detritus, pebble and sand, with smaller amounts of gravel, silt, clay and cobble also present. Boulders and cobbles were also present in the riffle habitats. Samples taken from below the surface were grey sands but showed no evidence to indicate that the sediments were anaerobic or lacking in oxygen. There was no sign of any significant areas of bank erosion recorded in autumn but more than 10m of bank had collapsed in the river later in the year, presumably due to winter flood damage. The only animal droppings seen in the vicinity of the river were from kangaroos.
There was a moderate amount of phytoplankton (chlorophyll a 5.2-5.3 μg/L) but no evidence of any filamentous algal growths recorded in 2013. Less than 10% of the channel was covered by only a few rushes (Juncus). The riparian zone extended over 10 m wide in places and was dominated by patches of River Red Gums, wattles and casuarinas over rushes and yaccas. The surrounding vegetation near the river comprised areas of dense eucalypt woodland and pine forests.
Special environmental features
Cygnet River provides a permanently wet, freshwater habitat and supports a wide range of macroinvertebrates that include tolerant, generalist, rare, sensitive and flow-dependent species.
Pressures and management responses
|Moderate to large nutrient and sediment inputs from diffuse sources in the catchment (leading to extensive growth of algae and aquatic weeds as well as increased turbidity and smothering of habitat).||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.|