Dawson Creek, near Strathalbyn
2010 Aquatic Ecosystem Condition Report
- Permanently wet, moderately fresh, slow-flowing creek in autumn and spring.
- Moderately diverse macroinvertebrate community with at least one rare and sensitive species.
- Obvious signs of moderate to gross nutrient enrichment.
- Riparian vegetation consists of introduced grasses, reeds and gum trees.
- Moderately eroded banks and large silt deposit in the channel.
About the location
Dawson Creek is a small stream in the southern Mount Lofty Ranges. It rises near Burslem Hill and flows in an easterly direction towards Strathalbyn, where it discharges into the Angas River. The major land uses are sheep and cattle grazing. The monitoring site was located off Dog Trap Road, over five kilometres west of Strathalbyn.
The creek was given a Fair rating because the site sampled showed moderate changes in ecosystem structure and some changes to the way the ecosystem functions. There was evidence of human disturbance, including nutrient enrichment, fine sediment deposition and a degraded riparian zone.
A moderately diverse community of about 31 species of macroinvertebrates was collected from the flowing stream, up to three metres wide and 50 cm deep, in autumn and spring 2010. The creek was a slow-flowing channel in autumn but consisted of a slow to fast-flowing channel in spring. The community was dominated by species that are generally tolerant to poor water quality such as amphipods (Austrochiltonia australis), blackfly larvae (Austrosimulium furiosum), chironomids (including Tanytarsus and Chironomus) and caenid mayflies (Tasmanocoenis tillyardi). It also included smaller numbers of flatworms, introduced snails (Potamopyrgus and Physa), worms, isopods, freshwater shrimps, beetles, dixid fly larvae, biting midges, soldierflies, waterbugs, odonates, stoneflies and caddisflies.
One rare species of stonefly (Riekoperla naso) was found at the site in spring, and an uncommonly collected dytiscid beetle (Limbodessus) was recorded in autumn. The only flow-dependent species recorded in spring were the blackfly larvae and stonefly but dytiscid beetles (Platynectes decempunctatus) were also collected in autumn when there was little flow in the creek. The only fish seen at the site was a threatened Mountain Galaxias (Galaxias olidus).
The water was moderately fresh (salinity of 2,894 mg/L in autumn and 2,347 mg/L in spring), well oxygenated (68–76% saturation) and clear, with moderate to high concentrations of nutrients such as nitrogen (0.42–0.76 mg/L) and phosphorus (0.03–0.08 mg/L).
The sediments were dominated by detritus, silt, sand, boulders and clay; samples taken from below the surface were black and anaerobic, indicating that too much organic matter had entered the creek in the past. Large deposits of silt, over 10 cm thick, covered the creekbed in autumn. Over 10 metres of bank showed signs of erosion due to previous flood damage.
A large amount of phytoplankton was present in spring and less than 10% of the site was covered by filamentous algae (Cladophora) in autumn. Over 35% of the site was covered by aquatic plants, including both submerged (Chara and Stuckenia) and emergent species (Phragmites, Callitriche, Cyperus, Isolepis, Baumea, Juncus, introduced Rorippa, Typha and Rumex). The riparian zone consisted of introduced grasses and sedges under a few scattered gum trees. The surrounding vegetation was grazing land with a few large gums trees.
Special environmental features
Dawson Creek provides habitat for a rare and sensitive stonefly, an uncommonly collected beetle and a threatened native fish (Mountain Galaxias). Recent fish surveys in the catchment have also collected another threatened species, the Southern Pygmy Perch, from further downstream in the lower reaches of the creek (M Hammer, Aquasave Consultants, 2009).
Pressures and management responses
|Livestock have direct access at the site and upstream, causing sediment erosion and adding excessive nutrients (which leads to habitat disturbance, algal growth and aquatic weeds).||The SA Murray–Darling Basin NRM Board acknowledges the significant impacts that livestock have on aquatic environments and seeks to provide free technical advice and incentives to land managers for fencing and other works as funding permits. Funding incentives are limited in value and extent and require land managers to volunteer to be involved.|
|Limited riparian vegetation at the site and upstream, providing minimal buffer protection from catchment landuses (reducing habitat quality).||The SA Murray–Darling Basin NRM Board recognises that the management of riparian vegetation requires a long-term, integrated approach to achieve ecosystem benefits. The board therefore provides free technical advice on a range of topics for land managers and various incentives for works as funding permits.|
|Insufficient natural water flows resulting from water extraction and climate variability (reducing ecological integrity.||The SA Murray–Darling Basin NRM Board is working with the Department for Water and the community to develop a water allocation plan and licensing system which aim to balance social, economic and environmental needs for water. The objective for providing water to the environment is to maintain and/or restore self-sustaining water-dependent ecosystems which are resilient in times of drought.|