Tributary of the Torrens River, 2 km north from Forreston
2011 Aquatic Ecosystem Condition Report
- Permanently wet, freshwater stream with slow-flowing habitats in autumn and isolated pools present in spring
- Sparse macroinvertebrate community with no rare or sensitive species but two flow-dependent species recorded in autumn
- Obvious signs of gross nutrient enrichment
- Riparian vegetation mostly introduced grasses and weeds under a few gum trees
- Some bank erosion and fine sediment deposits in the stream
About the location
Tributary of the Torrens River is a small stream in the Southern Mount Lofty Ranges that rises near ‘Flamenco Park’ and flows southwards along the eastern side of the Forreston Range, where it discharges into the Torrens River to the north of Gumeracha. The monitoring site was located off Northsworthy Road, about 2 km north from Forreston. The major land uses in the 78 hectare catchment are rural residential living and irrigated vines, with smaller areas used for remnant native vegetation, forestry and stock grazing.
The creek 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 considerable evidence of human disturbance including nutrient enrichment, degraded riparian habitats and fine sediment deposition in the channel.
A sparse community of at least 28 species of macroinvertebrates was collected from the creek, 1.5-1.8 m wide and up to 28 cm deep, in autumn and spring 2011. The creek consisted of slightly flowing, connected pools in autumn but contracted to non-flowing, isolated pools in spring. The community was dominated by generalists and species tolerant to poor water quality such as introduced snails (Physa), waterbugs (Micronecta, Agraptocorixa and Anisops), dytiscid beetles and leptocerid caddisflies. It also included native snails, worms, mites, amphipods, hydrophilid beetles, mosquitoes, biting midges, chironomids, mayflies and odonates. Many of these groups are highly mobile species and are often recorded from temporary streams throughout South Australia. No rare or sensitive species were collected and only two flow-dependent species, a dytiscid beetle (Platynectes decempunctatus) and chironomid (Rheotanytarsus), were found at the site in very low numbers in autumn. No fish were seen in 2011.
The water was fresh (salinity ranged from 445-623 mg/L), generally well oxygenated (51-93% saturated), clear in autumn but slightly turbid in spring, and with high concentrations of nutrients such as nitrogen (2.29-2.59 mg/L) and phosphorus (0.14-0.19 mg/L).
The sediments were dominated by detritus, silt and clay, with cobble, boulder, gravel and sand also present. Samples taken from below the surface were black in colour and sulphidic in spring, indicating that the sediments were anaerobic and lacked oxygen when the pools were drying. A large deposit of 5-10 cm of silt covered the bed of the tributary and nearly 10 m of bank showed evidence of erosion caused by cattle and occasional flood damage.
An extensive phytoplankton bloom was present at the site in both seasons sampled and minor growths of filamentous algae (mostly Spirogyra) were noted in spring. Aquatic plants covered about 10% of the channel and included submerged (Crassula) and emergent macrophytes (Juncus, Eleocharis, Isolepis and Rumex). The narrow riparian zone was dominated by introduced grasses and weeds, and included a few scattered River Red Gums. A large number of small eucalypt seedlings that had germinated in the riparian zone, in response to a high flow event in winter, was also noted during the spring survey. The surrounding vegetation was pasture and vineyards with a few isolated gum trees in the local catchment.
Special environmental features
None detected apart from the presence of a small number of flow-dependent macroinvertebrate species in autumn.
Pressures and management responses
|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.|
|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.|
|Limited riparian zone vegetation at the creek and upstream (reducing habitat quality, increasing sediment erosion).||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 revegetation programs around waterways and wetlands and stock exclusion as well as educating landholders about the importance of riparian vegetation in managing soil erosion.|
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.