Cygnet River, Koala Lodge GS
2013 Aquatic Ecosystem Condition Report
- Permanently wet flowing stream in autumn and spring 2013
- Sparse macroinvertebrate community with no rare, sensitive or flow-dependent species
- Water was moderately fresh, turbid and highly coloured, and very high in nutrients
- Riparian vegetation comprised native gums over weeds and introduced grasses
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 25,029 hectare catchment upstream from the site sampled were grazing modified pastures (65%) and other minimal uses (16%), with minor areas also used for cropping, nature conservation, plantation forestry, roads, land in transition, dams, residential living, intensive animal production, and irrigated perennial horticulture. The site sampled was located at a gauge station in the lower catchment of the river, accessed by an unnamed track off the Playford Highway, just north from Cygnet River and the Kingscote aerodrome.
The river was given a Poor rating because the site sampled showed evidence of major changes in ecosystem structure and moderate changes to the way the ecosystem functions. There was evidence of human disturbance due to the extent of nutrient enrichment and the presence of introduced marron and weeds in this river.
A sparse community of at least 17 species of macroinvertebrates was collected or seen from the river (7 species in autumn and 13 in spring), 7-8 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 mites, amphipods, beetles, mosquitoes, chironomids, waterbugs, dragonflies and caddisflies. Marron holes were seen in the banks, indicating that this introduced crayfish has been released and established large numbers in this river. All macroinvertebrates recorded from this site were generalist, tolerant species that are widely found from similar habitats elsewhere on the island and the wetter parts of the State. No rare, sensitive or specialised flow-dependent species were collected. A juvenile native galaxiid was the only fish seen at the site in 2013.
The water was moderately fresh (salinity ranged from 2,312-2,833 mg/L), well oxygenated (82-100% saturation), slightly turbid and strongly coloured, and with particularly high concentrations of nutrients in autumn, including phosphorus (0.03-0.23 mg/L) and nitrogen (1.02-3.03 mg/L).
The sediments were dominated by detritus, sand and silt, with a smaller amount of filamentous algae also present. Samples taken from below the surface were black silts and ash that were sulphidic in spring, indicating that the sediments were anaerobic and lacked oxygen for at least part of the year. Over 1 cm of fine sediment was present in the channel in spring, presumably deposited following winter flooding. There was a small amount of bank erosion affecting nearly 10 m of the site, which appeared to have been caused by a recent fire. The only animal droppings seen in the vicinity of the river were from kangaroos.
There was a large growth of phytoplankton recorded at the site in autumn (chlorophyll a ranged from 8.6-100 μg/L), which would have contributed towards the turbidity and highly coloured appearance of the river in that season. Filamentous algae (mostly Spirogyra) was only observed in autumn, when it covered nearly 10% of the channel. A similar area was also covered by an emergent aquatic plant (Typha). The riparian zone extended over 30 m wide in places and was dominated by scattered River Red Gums over a weedy understorey of mostly introduced grasses. The surrounding vegetation near the river comprised mostly rural backyard plantings.
Special environmental features
Cygnet River provides a permanently wet, freshwater habitat that supports a sparse community of generalist and tolerant aquatic macroinvertebrates and aquatic plants.
Pressures and management responses
|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.|