Mitchell Gully Creek, near Rockleigh
2010 Aquatic Ecosystem Condition Report
- Permanently wet with saline, isolated pools present in autumn and spring.
- Sparse macroinvertebrate community with no rare or sensitive species.
- Obvious signs of moderate to gross nutrient enrichment.
- Riparian vegetation mostly consisted of introduced grasses and rushes.
About the location
Mitchell Gully Creek is a small stream in the eastern Mount Lofty Ranges. It rises near Whalley Hill and flows in an easterly direction where it ultimately discharges into Salt Creek. The major land use is sheep grazing.
The monitoring site was located off Rockleigh Road, less than one kilometre east of Rockleigh and about 10 kilometres south-east of Harrogate.
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, large deposits of silt and a degraded riparian zone.
A sparse community of about 26 species of macroinvertebrates was collected from the isolated pools, up to 12 metres wide and over one metre deep in places, in autumn and spring 2010. The community was dominated by large numbers of saline tolerant snails (Coxiella) and dytiscid beetles (Antiporus, Necterosoma, Lancetes and Eretes). It also included smaller numbers of mites, springtails, hydrophilid beetles, craneflies, biting midges, chironomids, waterbugs, odonates and leptocerid caddisflies. No sensitive or rare species were found. The only fish recorded was the introduced mosquitofish (Gambusia).
The water was saline (salinity of 16,560 mg/L in autumn and 7,163 mg/L in spring), well oxygenated (112–126% saturation) and clear, with moderate to high concentrations of nutrients such as nitrogen (1.59–2.1 mg/L) and phosphorus (0.04–0.06 mg/L).
The sediments were dominated by detritus, algae and clay; samples taken from below the surface were black and sulfidic, indicating that too much organic matter had entered the creek in the past. Silt deposits over 10 centimetres thick covered the creekbed in places in autumn; subsequent sampling in spring indicated most of this had been transported downstream during winter flows because the silt deposits were less than one centimetre thick in the middle of the channel.
A large amount of phytoplankton was present in the pools and more than 10% of the site was covered by filamentous algae (Cladophora). Over 10% of the site was also covered by aquatic plants, including both submerged (Chara) and emergent species (Juncus, Bolboschoenus and Cyperus).
The narrow riparian zone consisted of rushes and introduced grasses, and a few pepper trees and samphire. The surrounding vegetation was grazing land with a few scattered gum trees.
Special environmental features
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.|
|Saline groundwater inflow (reducing ecological integrity).||Saline groundwater inflows may be exacerbated by two things; vegetation clearing and resultant increase in rainfall recharge, or the extraction of surface water reducing the dilution factor in natural saline discharge zones. The SA Murray–Darling Basin NRM Board’s Land Management Program strategically invests in salinity ‘hotspots’ by providing incentives to land managers to plant perennial pasture/fodder crops/ or revegetation to reduce recharge. The NRM Board works with various agencies to minimise any further vegetation clearing which may impact on the catchment’s water balance. The NRM Board seeks to manage and provide for environmental flows to allow natural dilution of saline waters through the development of Water Allocation Plans and Water Affecting Activity policies across the region.|