Onkaparinga River, S from Bradbury
2013 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 due to nutrient enrichment and the dominance of woody weeds in the riparian zone but the stream provides habitat for at least two sensitive, flow-dependent species of macroinvertebrates.

A sparse community of at least 19 species of macroinvertebrates was collected or seen from the river (9 species in autumn and 14 in spring), 8-36 m wide and over 1 m deep in places, in autumn and spring 2013. The river consisted of a connected, slow-flowing channel in both seasons sampled; recent floods had raised the water level and scoured the river in May, and may have affected the community that was sampled in autumn. The community was not dominated by any species but comprised very low numbers of worms, amphipods, isopods, shrimp, yabbies, beetles, biting midges, chironomids, mayflies, waterbugs, dragonflies and caddisflies. The only sensitive and flow-dependent species collected were a single caddisfly (Triplectides similis) in autumn and several mayflies (Atalophlebia australasica) in spring. The other macroinvertebrates were common, generalist and opportunistic species that can tolerate a wide range of conditions, including streams with poor water quality and habitat structure. The site lacked any rare species and many groups of macroinvertebrates normally found in streams in the region were missing, including mites, snails, waterbugs, mayflies, stoneflies, and a wider range of beetles and flies. The absence of waterbugs in autumn was particularly notable because the shallow edges and deeper pools would normally be expected to support a wide range of species; this supports the possibility that recent floodwaters had affected the aquatic life inhabiting the river at that time.

The water was fresh (salinity ranged from 310-352 mg/L), well oxygenated (86-118% saturation), clear but strongly coloured, and with high concentrations of nutrients such as phosphorus (0.08-0.13 mg/L) and nitrogen (0.99-1.18 mg/L).

The sediments were dominated by detritus in autumn, with smaller amounts of clay, sand and silt also present; in spring, a wider range of sediment types were recorded, including pebbles, cobbles, boulders and gravel. Over 1 cm of fine silt covered the bed of the river in spring, indicating that significant deposition occurred following recent high winter flows in this river. Samples taken from below the surface were grey clays and sands, and showed no evidence to indicate that the sediments were anaerobic or lacking in oxygen. A moderate amount of bank erosion extended over more than 10% of the site, which appeared to have been caused by past flood damage. Goat droppings were seen on the banks but did not appear to have contributed towards any bank damage during 2013. The only other droppings seen near the river were from kangaroos.

A moderate to large amount of phytoplankton was recorded from the river (chlorophyll a ranged from 8.7-11.3 μg/L) but no filamentous algae was seen at the site in 2013. Less than 10% of the channel was covered by a few rushes (Juncus). The riparian zone was dominated by extensive stands of woody weeds that comprised willow trees over blackberries and broom; these plants provided 80-90% shading of the river, and would have contributed towards the generally poor plant growth seen in the understorey on the banks and also in the river. The surrounding vegetation near the creek comprised dense eucalypt woodland.

The only significant records from this site in 2013 were the presence of a sensitive and flow-dependent species of caddisfly and mayfly.

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