Willson River, West from Woodleigh
2013 Aquatic Ecosystem Condition Report
- Permanently flowing stream in autumn and spring 2013
- Moderately diverse macroinvertebrate community with no rare or sensitive species, and one salt tolerant flow-dependent species
- Water was saline, clear and high in nutrient concentrations
- Riparian vegetation comprised native trees over weeds
About the location
Willson River is the largest stream on the Dudley Peninsula at the eastern side of Kangaroo Island. It rises a few kilometres south-east from Penneshaw at an elevation of about 170 m and flows in a southerly direction for nearly 20 km before discharging into the Southern Ocean at Mouth Flat Beach, on the southern coastline of the island. The major land uses in the 3,304 hectare catchment upstream from the site sampled were grazing modified pasture (83%) and other minimal uses (9.7%), with minor areas used for cropping, roads, irrigated pastures and horticulture, dams and residential living. The site sampled was located in the lower reaches off Willson River Road, about 12 km south from Penneshaw.
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 nutrient enrichment, high salinity, the presence of introduced snails, and the dominance of weeds in the understorey of the riparian zone.
A moderately diverse community of at least 23 species of macroinvertebrates was collected or seen from the river (17 species in autumn and 13 in spring), 1.8-3.5 m wide and up to 51 cm deep, in autumn and spring 2013. The river consisted of still to slow-flowing pools connected by faster-flowing riffle habitats in both seasons sampled. The community was dominated by moderate numbers of amphipods (Austrochiltonia) and waterbugs (Micronecta) in the edges of pools and blackflies (Simulium ornatipes) and amphipods in the riffles. It also included low numbers of introduced snails (Potamopyrgus), mites (Recifella and Arrenurus), yabbies, beetles, mosquitoes (Aedes), biting midges (Bezzia and Culicoides), chironomids, waterbugs (Microvelia), damselflies and dragonflies. All were tolerant generalists that are frequently found from other saline, nutrient enriched streams in South Australia. The site lacked any rare and sensitive species, and the only flow-dependent macroinvertebrate collected was the most common and widely distributed species of blackfly (Simulium ornatipes).
The water was saline (salinity ranged 5,062-9,202 mg/L), well oxygenated (86-102% saturation), clear, and with low to moderate concentrations of phosphorus (0.02-0.03 mg/L) and high nitrogen concentrations (1.44-3.28 mg/L); the latter included a large amount of oxidised nitrogen (0.53-2.46 mg/L), which typically occurs near a groundwater discharge point in a stream.
The sediments were dominated by detritus, sand, filamentous algae, pebble and boulder in the edges and by boulder, cobble, algae, gravel and detritus in the riffles. Samples taken from below the surface were grey sands and stones, and appeared to be well-aerated when inspected in 2013. The presence of blackened rocks, however, indicates that the sediments have been anaerobic or lacking in oxygen in the past. A moderate amount of bank erosion was noted, with over 10 m of bank showing evidence of damage caused by past flood events. The only animal droppings seen in the vicinity of the river were from kangaroos.
There was a large bloom of phytoplankton (chlorophyll a 40 μg/L) and small growth of cyanobacteria or blue-green algae (chlorophyll b 1.4 μg/L) recorded from the river in autumn. This declined to a chlorophyll a concentration of 7.4 μg/L in spring, when water levels were higher and flow more extensive. Filamentous algae (Cladophora and Enteromorpha) covered over 10% of the channel in autumn but was not detected in the spring survey. No floating or rooted aquatic plants seen at the site in 2013. The narrow riparian zone was dominated by gums over weeds, including sower sobs, bridle-creeper and boxthorn. The surrounding vegetation near the river comprised sheep grazing land and areas of cropping with a few scattered gums in the local landscape.
Special environmental features
Willson River provides a permanently wet habitat on the eastern end of the island and supports an aquatic community that is well adapted to live in saline, nutrient enriched streams. The high salinity limits the ability for a more diverse and significant assemblage of aquatic species to colonise this stream.
Pressures and management responses
|Widespread introduced weeds in the riparian zone at the site and upstream (reducing habitat quality).||This information is not available at the moment but it will be updated as soon as possible.|
|Saline groundwater inflows to the creek (reducing ecological integrity).||This information is not available at the moment but it will be updated as soon as possible.|
|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.|