Cygnet River, West from Pardana
2013 Aquatic Ecosystem Condition Report
- Permanently wet stream in autumn and spring 2013
- Moderately diverse macroinvertebrate community with one sensitive species but no rare or flow-dependent species recorded
- Water was fresh, turbid in autumn and strongly coloured in spring, and high in nitrogen
- Riparian vegetation comprised native vegetation
About the location
Cygnet River is the largest stream on Kangaroo Island. It rises near the centre of the island at an elevation of about 245 m north from the Playford Highway, and flows in an easterly direction for over 60 km before eventually discharging into Nepean Bay to the south of Kingscote. The major land uses in the 5,807 hectare catchment upstream from the site sampled were plantation forestry (37%) and grazing modified pastures (31%), with minor areas also used for other minimal uses, cropping, roads, nature conservation, irrigated cropping and dams. The site sampled was located in the upper catchment off Turkey Lane Road, about 9 km west 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 nitrogen enrichment and the extent of fine silt deposited in the channel, as well as the presence of introduced marron holes in the banks of this watercourse.
A moderately diverse community of at least 23 species of macroinvertebrates was collected or seen from the river (12 species in autumn and 16 in spring), 5-6 m wide and over 1 m deep, in autumn and spring 2013. The river consisted of a still to slow-flowing channel in both seasons sampled. The community was not dominated by any species but included low numbers of worms, mites, amphipods, isopods, beetles, chironomids, mosquitoes, mayflies, waterbugs, dragonflies and caddisflies. Most of these macroinvertebrates were generalist and tolerant species that are commonly found from similar habitats on the island and the wetter parts of South Australia. The only sensitive species collected were the mayflies (Thraulophlebia inconspicua). However, no rare, habitat specialist or flow-dependent species were recorded during 2013. The site supported an unusually rich range of water mites that included specimens from the families Hydryphantidae, Unionicolidae and Limnisiidae; only the latter family are uncommonly collected on the island.
The water was fresh to moderately fresh (salinity ranged from 620-1,327 mg/L), well oxygenated (105-108% saturation), slightly acidic in autumn but neutral in spring (pH 6.4-7), turbid in autumn but clear and slightly coloured in spring, and with low to moderate concentrations of phosphorus (0.02-0.03 mg/L) but high nitrogen concentrations (0.83-1.06 mg/L).
The sediments were dominated by detritus and sand, with smaller amounts of silt and clay also present. Samples taken from below the surface were grey clays, silts and sands, and showed no evidence that the sediments were anaerobic or lacking in oxygen. Over 5 mm of silt was deposited in the middle of the channel which must have been sourced from further upstream because there was no evidence of any erosion at the site. The only animal droppings seen in the vicinity of the river were from kangaroos.
There was a large growth of phytoplankton (chlorophyll a 20 μg/L) recorded in autumn, which would have contributed towards the highly turbid river in that season; only small amounts of phytoplankton were evident in spring. No signs of any significant filamentous algal growths were detected at the site but nearly 10% of the channel was covered by rushes (Juncus). The 10 m wide riparian zone was dominated by gums and wattles over native shrubs and understorey plants. The surrounding vegetation near the river comprised dense eucalypt woodland.
Special environmental features
Cygnet River provides a permanently wet, freshwater habitat in its upper reaches and supports a moderately diverse community dominated by generalist and tolerant aquatic macroinvertebrates but also including a few sensitive and uncommonly collected 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.|