Cockatoo Creek, south from Sandy Creek
2011 Aquatic Ecosystem Condition Report
- Permanently wet, flowing, moderately fresh to saline creek in autumn and spring 2011
- Diverse macroinvertebrate community with only two sensitive or flow-dependent species
- Obvious signs of moderate nutrient enrichment
- Riparian vegetation dominated by native trees, reeds and introduced grasses and weeds
- Large deposit of fine sediment in the creek.
About the location
Cockatoo Creek is a moderately sized stream in the Southern Mount Lofty Ranges that rises west from Williamstown and flows in a westerly direction into the South Para River near Gawler. The monitoring site was located on a track off Woodlands Road in Cockatoo Valley, about 1.5 km south from Sandy Creek. The major land uses in the 2,494 hectare catchment are water supply (Barossa Reservoir), rural residential living, stock grazing and remnant native vegetation.
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, fine sediment deposition and degraded riparian habitats.
A diverse community of at least 43 species of macroinvertebrates was collected from the flowing creek, 2.6-3.7 m wide and up to 77 cm deep, in autumn and spring 2011. Small areas of fast-flowing riffle habitats were present in autumn but the creek was only just flowing when sampled again in spring. The community was dominated by generalists and species tolerant to poor water quality and moderate salinity such as chironomids and amphipods (Austrochiltonia). It also included smaller numbers of flatworms, nematodes, introduced and native snails, worms, mites, isopods, yabbies, springtails, beetles, craneflies, mosquitoes, biting midges, dixids, soldierflies, mayflies, waterbugs, odonates, stoneflies and caddisflies. The only sensitive species collected was a single stonefly (Austrocerca tasmanica) in spring; it normally frequents flowing, freshwater streams from the wetter parts of the Mount Lofty Ranges. The only other species of note was another flow-dependent species, a single dytiscid beetle (Platynectes decempunctatus) that was collected in autumn. The salinity of the creek probably contributed to the lack of more notable species from the site sampled. No fish were recorded from the site in 2011.
The water was moderately fresh to saline (salinity ranged from 2,694 mg/L in autumn to 3,209 mg/L in spring), well oxygenated (65-85% saturated), slightly turbid and coloured, and with generally moderate concentrations of nutrients such as nitrogen (0.41-0.53 mg/L) and phosphorus (0.03-0.04 mg/L).
The sediments were dominated by detritus and silt, with algae, sand and clay also present. Samples taken from below the surface were anaerobic and sulphidic, indicating that the sediments lacked oxygen and provided a harsh environment for burrowing animals to live in. A large deposit of silt, over 10 cm deep, covered the creekbed and a moderate amount of bank erosion, over 10 m wide, was noted in autumn as a result of recent flood damage.
About 10% of the channel was covered by filamentous algae (including Spirogyra and Cladophora) and more than 65% of the creek was covered by submerged (Chara) and emergent plants (Cyperus, Isolepis, Juncus, Phragmites and Typha). The riparian vegetation was largely comprised gums and wattles over reeds and a range of introduced grasses and weeds. The surrounding vegetation was mostly cleared grazing land with a few scattered gum trees and olives, but also included a patch of eucalypt woodland that extended down to one side of the creek.
Special environmental features
The presence of the stonefly record is significant because this species is likely to be persisting around its upper salinity tolerance. The site also provides important habitat for a wide range of more generalist species of macroinvertebrates.
Pressures and management responses
|Insufficient natural water flows in the creek resulting from water extraction and climate variability (reducing ecological integrity).||Through water allocation planning the Adelaide and Mount Lofty Ranges NRM Board seeks to manage a sustainable water supply for the region so that there is enough water available for everyone (including the environment) even in drought conditions.|
|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.