Onkaparinga River, W from Woodside
2013 Aquatic Ecosystem Condition Report
- Permanently wet, connected channel in autumn and spring 2013
- Moderately diverse macroinvertebrate community dominated by commonly found species and including two rare and/or sensitive species
- Water was fresh, clear, strongly coloured, and high in nutrients
Riparian vegetation dominated by native trees over weeds and introduced grasses
About the location
Onkaparinga River is one of the largest streams in the Southern Mount Lofty Ranges. It rises at an elevation of about 500 m east from Lobethal and flows in a general south-westerly direction, where it eventually discharges into Gulf St Vincent at Port Noarlunga. The major land use in the 2,430 hectare catchment upstream from the site was stock grazing (51%), with smaller areas used for irrigated horticulture and pastures, other minimal uses, residential towns (eg Woodside, Charleston and Lobethal), cropping, nature conservation, roads, industrial use and dams. The site was located off Tiers Road, a couple of hundred metres west from the township of Woodside.
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 cattle accessing the river, nutrient enrichment and the dominance of weeds in the understorey of the riparian zone, but the site still provided habitat for two rare and sensitive species of macroinvertebrates.
A moderately diverse community of at least 37 species of macroinvertebrates was collected or seen from the river (14 species in autumn and 29 in spring), 1.2-3.1 m wide and up to 61 cm deep, in autumn and spring 2013. The river consisted of a connected, non-flowing channel connected by tiny areas of slow-flowing, shallow riffle habitats in autumn, whereas in spring the water level was lower and no riffles were present. The community was not dominated by any species but comprised low numbers of limpets, native and introduced snails (including Physa), worms, mites, amphipods, springtails, beetles, mosquitoes, marsh flies (Sciomyzidae), chironomids, waterbugs and caddisflies. The presence of yabby holes in the banks indicated that this species was common at the site, despite not being collected during either field sampling period. Immature stoneflies (probably from the Family Gripopterygidae) were seen in the riffle habitats in autumn and represented the only sensitive, flow-dependent species seen at the site during 2013. The only rare macroinvertebrate collected were a few amphipods from the Family Perthiidae. The other species comprised a wide range of generalist, opportunistic and tolerant species with a widespread distribution in the region.
The water was fresh (salinity ranged from 418-562 mg/L), well oxygenated (79-104% saturation), clear and with moderate to high concentrations of nutrients such as phosphorus (0.03-0.04 mg/L) and nitrogen (0.62-0.95 mg/L).
The sediments were dominated by detritus, with smaller amounts of pebble, gravel, sand, silt, clay, and filamentous algae in spring, also present. Samples taken from below the surface were grey clays and silts, and showed no evidence to indicate that the sediments were anaerobic or lacking in oxygen. There was a small amount of bank erosion affecting about 10 m of the site, which appeared to have been caused by cattle accessing the steep, clay-based banks of the river. Cattle manure was also present on the banks in places but none was seen within the channel.
There was a small to moderate amount of phytoplankton recorded from the river (chlorophyll a ranged from 0.7-6.7 μg/L) and a filamentous alga (Spirogyra) covered more than 10% of the channel in spring. Over 35% of the river was covered by aquatic plants, including extensive growths of cumbungi (Typha) and knotweed (Persicaria), and a few rushes (Juncus) and Water Ribbons (Triglochin procera). The riparian zone was dominated by River Red Gums and wattles over weeds, introduced grasses and a few rushes and dock plants. The surrounding vegetation near the river comprised cleared cattle grazing land with a few scattered gum trees, although there was a small area of open acacia woodland on one bank near the site.
Special environmental features
The only significant environmental value noted at the site in 2013 was the presence of amphipods from the Family Perthiidae; few records of this family have been made from the region but it has been collected from the Onkaparinga River in the past.
Pressures and management responses
|Widespread introduced weeds in the riparian zone at the site and upstream (reducing habitat quality).||The Adelaide and Mount Lofty Ranges NRM Board has several pest plant (weed) mitigation and control programs. They work closely with landholders to control weeds on their property and to help stop the spread to other properties and waterways.|
|Livestock having direct access at the site and upstream (causing sediment erosion and adding excessive nutrients).||The Adelaide and Mount Lofty Ranges NRM Board’s land management program encourages and promotes managing land to improve water quality. This includes incentives for waterway and wetland fencing to exclude or limit stock from entering riparian zones.|
|Nutrient inputs to the creek from numerous diffuse sources (leading to extensive growth of algae and aquatic weeds)||The Adelaide and Mount Lofty Ranges NRM Board’s land management program encourages and promotes managing land to improve water quality. This includes working with industry and landholders to ensure efficient use of fertilisers and discuss ways to reduce runoff of nutrients into waterways.|
This aquatic ecosystem condition report is based on monitoring data collected by the EPA. It was prepared with and co-funded by the Adelaide and Mount Lofty Ranges NRM Board.