Wirrealpa Creek, Wirrealpa Spring
2012 Aquatic Ecosystem Condition Report
- Flowing pool and riffle habitats in autumn but dry in spring 2012
- Sparse macroinvertebrate community with several flow-dependent species present
- Water was moderately fresh, clear and obviously enriched with nutrients
- Riparian vegetation consisted of native trees and shrubs
About the location
Wirrealpa Creek is a small stream that rises south from Wirrealpa Hill in the Northern Flinders Ranges and flows south-east, where it discharges into Balcoracana Creek in the Lake Frome drainage basin, only during exceptionally wet years. The only land use in the 1,323 hectare catchment upstream from the site sampled is grazing natural vegetation. The monitoring site was located off a track from the Blinman to Wirrealpa Road, about 21 km east-north-east from Blinman.
The creek 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 including nutrient enrichment, bank erosion due to stock and feral animal damage and the presence of weeds in the riparian zone but the stream still provided habitat for some commonly occurring and flow-dependent macroinvertebrate species when surface water is present.
A sparse community of at least 18 species of macroinvertebrates was collected from the creek, 2.4 m wide and up to 26 cm deep, in autumn; the creek was dry in spring 2012. The creek consisted of shallow, flowing pool habitats connected by small faster-flowing riffle habitats in autumn. The community was dominated by moderate numbers of chironomids (Paramerina, Tanytarsus and Chironomus) and included smaller numbers of beetles (Sternopriscus, Platynectes, Eretes and Enochrus), mosquitoes (Anopheles), biting midges (Bezzia), blackflies (Simulium ornatipes), baetid mayflies (Cloeon), waterbugs (Microvelia, Agraptocorixa, Micronecta and Anisops), dragonflies (Hemicordulia) and caddisflies (Cheumatopsyche and Triplectides). All species collected were insect groups, capable of flying to alternative sites if the stream dries and/or burrowing into the wet sediments below the surface (e.g. mayfly and some of the beetles). The only habitat specialists collected were flow-dependent species such as the blackflies, dytiscid beetle Platynectes and caddisfly Cheumatopsyche. The mayfly was the only sensitive species collected that has a widespread distribution from permanent to semi-permanent freshwater streams in the region. The site was notable due to the absence of molluscs, mites, crustaceans and other non-insect groups, which indicates that this reach may dry too often for a more diverse assemblage of species to be able to inhabit this ephemeral watercourse.
The water was moderately fresh (salinity 2,608 mg/L), well oxygenated (125% saturation), clear, and with moderate concentrations of nutrients such as phosphorus (0.02 mg/L) and nitrogen (0.41 mg/L) in autumn.
The sediments were dominated by pebble and cobble, with boulder, filamentous algae, gravel, sand, silt and detritus also present; samples taken from below the surface were sulphidic, manure smelling, blackened sands in autumn, indicating that the sediments were anaerobic, or lacking in oxygen when wet. About 50 m of bank showed evidence of erosion caused by sheep and goats accessing the channel and banks of the creek, particularly when the stream was dry in spring.
A small amount of phytoplankton was recorded in autumn (chlorophyll a 1.9 Âµg/L) when a filamentous alga (Cladophora) extended over more than 10% of the channel. No aquatic plants were seen in the water or on the edge of the creek. The riparian vegetation was dominated by River Red Gums and acacias over a few sedge plants (Cyperus gymnocaulos) and weeds on the poorly vegetated banks (<25% vegetative cover). The surrounding vegetation at the site comprised low native woodland dominated by acacias, gums and acacias over native shrubs and understorey species.
Special environmental features
Wirrealpa Creek is a largely natural, ephemeral stream in the Northern Flinders Ranges that provides habitat for a sparse assemblage of aquatic insects which includes several flow-dependent species.
Pressures and management responses
|Livestock and feral animals (goats) have direct access at the site and upstream, exerting excessive grazing pressure, causing sediment erosion and adding excessive nutrients to the watercourse.||The SA Arid Lands Natural Resources Management Board recognizes that both direct and diffuse impacts on aquatic ecosystem condition can occur through direct stock access and excessive grazing pressure from stock and feral herbivores. Technical advice and incentives are offered to land managers in the region, as funding permits, to address these impacts through appropriate activities suitable for the context. In addition, projects are underway across the region to identify, prioritise and address impacts at key aquatic sites.|