Onkaparinga River, south from Mylor
2011 Aquatic Ecosystem Condition Report

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 including nutrient enrichment, degraded riparian vegetation and fine sediment deposition in the channel but the stream still provided habitat for some rare and sensitive macroinvertebrate species.

A moderately diverse community of at least 30 species of macroinvertebrates was collected from the flowing river, 1.6-18 m wide and over 1 m deep in places, in autumn and spring 2011. The river consisted of a slow-flowing channel and small areas of fast-flowing riffle habitats. The community was dominated by generalists and species tolerant to poor water quality such as amphipods, worms and isopods. It also included smaller numbers of native and introduced snails, limpets, yabbies, springtails, beetles, blackflies, chironomids, mayflies, waterbugs, stoneflies and caddisflies. Several rare and sensitive species were collected, including an amphipod (Family Perthiidae), chironomid (Riethia), blackfly (Austrosimulium furiosum), mayflies (Offadens and Atalophlebia australasica), caddisflies (Taschorema evansi, Ulmerochorema membrum, Triplectides similis and Anisocentropus) and a stonefly (Dinotoperla evansi). The site also provided habitat for several species normally associated with flowing water, including a caddisfly (Cheumatopsyche sp. 2) and many of the above-listed rare and sensitive species. No fish were seen at the site in 2011.

The water was fresh (salinity ranged from 247-283 mg/L), well oxygenated (88-90% saturated), generally clear, and with moderate to high concentrations of nutrients such as nitrogen (0.78-0.97 mg/L) and phosphorus (0.09 mg/L).

The sediments were dominated by detritus, boulder, silt, algae, sand and clay in the slow-flowing channel and boulder, bedrock and algae in the riffles. Samples taken from below the surface were silt grey in appearance and sulfidic, indicating that the sediments were anaerobic or lacked oxygen. A large amount of silt, 1-5 cm deep in places, covered the channel and about 10 m of bank showed evidence of erosion due to past flood damage.

A moderate amount of phytoplankton occurred in the river and over 35% of the channel was covered by filamentous algae (mostly Cladophora) in autumn. Aquatic plants covered around 10% of the river and included both submerged (Myriophyllum) and emergent species (Juncus, Cyperus, Isolepis and Rumex). The riparian vegetation was dominated by woody weeds (willows and ash trees) over introduced grasses and other weeds, with only a few River Red Gums present along the banks of the river. The surrounding vegetation was eucalypt woodland with a weedy understorey dominated by introduced grasses.

The Onkaparinga River provides permanent, flowing habitats that support a wide range of aquatic species, including several rare, sensitive and flow-dependent species.

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