Gorge Creek, near Tepko
2010 Aquatic Ecosystem Condition Report
- Permanently flowing, saline creek in autumn and spring.
- Sparse macroinvertebrate community with no rare or sensitive species.
- Obvious signs of gross nutrient enrichment.
- Riparian vegetation was mostly introduced grass, rushes and weeds.
- Large deposit of silt in the channel and moderately eroded banks.
About the location
Gorge Creek is a small stream in the eastern Mount Lofty Ranges. It rises about eight kilometres east of Harrogate and drains in an easterly direction where it ultimately discharges into Reedy Creek. The major land uses are cattle and sheep grazing. The monitoring site was located off Kubenk Road, over five kilometres northwest of Tepko.
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 habitat.
A sparse community of about 22 species of macroinvertebrates was collected from the flowing creek, two metres wide and up to 29 cm deep, in autumn and spring 2010. The site was very unusual because it only consisted of fast-flowing riffle habitat and lacked any slow-flowing or still pool habitats. The community was dominated by large numbers of species that are tolerant to high salinity and poor water quality such as blackfly larvae (Simulium ornatipes) and chironomids (including Cricotopus, Procladius and Chironomus).
It also included smaller numbers of hydrobiid snails, mites, amphipods, springtails, beetles, mosquitoes, soldierflies, waterbugs, odonates and leptocerid caddisflies. No sensitive or rare species were found. The only flow-dependent species collected were the blackfly larvae and dytiscid beetles (Platynectes decempunctatus).
The water was saline (salinity of 5,517 mg/L in autumn and 7,696 mg/L in spring), moderately well to well oxygenated (48–85% saturation) and clear, with moderate to high concentrations of nutrients such as nitrogen (0.81–1.09 mg/L) and phosphorus (0.02–0.06 mg/L).
The sediments were dominated by detritus, algae and silt; samples taken from below the surface were black, sulphidic and anaerobic, indicating that too much organic matter had entered the creek in the past. A deposit of 5–10 cm of silt covered the creekbed in spring and about 10 metres of the banks showed signs of erosion in autumn due to stock damage. About 35% of the site was covered by filamentous algae (Cladophora) and 65% of the channel was covered by a range of emergent aquatic plants (Juncus, Bolboschoenus, Cotula, Phragmites and Isolepis). The riparian zone consisted of introduced grasses, rushes and weeds, and a few planted gum trees were present on one bank. The surrounding vegetation was grazed grassland.
Special environmental features
None detected apart from the permanently flowing habitats of this saline creek.
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.|
|Widespread introduced trees and weeds in the riparian zone at the site and upstream in the catchment (reducing habitat quality).||The SA Murray–Darling Basin NRM Board recognises the limitations of available funds relative to the scale of the degradation caused by introduced trees and weeds. The NRM Board provides free technical advice and community education to assist land managers in dealing with the integrated management of aquatic weeds. The NRM Board also has a targeted process, as directed by State Government, to strictly prioritise its investment in weed control activities as funds are limited. The NRM Board actively seeks funding opportunities for weed control; most opportunities are for locations where biodiversity outcomes can be achieved.|
|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.|