Sixth Creek, near Castambul
2015 Aquatic Ecosystem Condition Report
Permanently wet freshwater creek which was flowing in autumn and spring 2015
Diverse macroinvertebrate community with many rare, sensitive and flow-dependent species
Emerging signs of nutrient enrichment
Riparian vegetation consists of mostly introduced species with native vegetation beyond
About the location
Sixth Creek is a large stream in the Southern Mount Lofty Ranges that rises at an elevation of about 550 m near Summertown and Uraidla as Deep Creek, and flows northwards where it merges with other tributaries to form Sixth Creek and eventually discharges into the Torrens River near the site sampled. The major land uses in the 30,230 hectare catchment were stock grazing (52%) and native vegetation (16%), with smaller areas also used for irrigated horticulture and pastures, nature conservation, plantation forestry, residential, roads and dams. The site was located in the lowest reach of the creek near the junction of Corkscrew and Gorge roads, about 2 km west from Kangaroo Creek Reservoir.
The creek was given a Good rating because the site sampled showed evidence of relatively minor changes in ecosystem structure and function. There was evidence of human disturbance including emerging signs of nutrient enrichment, sediment deposition and the presence of weeds in the riparian zones but the stream provides habitat for several rare and sensitive species of macroinvertebrates.
A highly diverse community of at least 55 species of macroinvertebrates was collected from this flowing creek, approximately 4 m wide and 35 cm deep, in autumn and spring 2015. The creek consisted of slow-flowing pools with fast-flowing riffle sections between the pools in both autumn and spring 2015. The site was dominated by Hydrobiidae snails, amphipods, blackfly larvae (Austrosimulium and Simulium), non-biting midge larvae, mayflies, stoneflies, and the waterbugs Micronecta and Microvelia. Other species collected in smaller numbers included a range of generalist and pollution tolerant species, such as snails, beetles, six different families of fly larvae including a diverse range of non-biting midges, dragonflies and caddisflies. Some sensitive and rare species, as well as flow-dependent species, were also collected, including the riffle beetle Simsonia, the water penny Sclerocyphon, mayflies (Offadens, Atalophlebia and Thraulophlebia), stoneflies (Dinotoperla and Illiesoperla), and the caddisflies Apsilochorema, Taschorema, Cheumatopsyche and Lingora. The introduced snails Potamopyrgus and Physiella as well as yabbies were also seen in the creek.
The water was fresh (salinity ranged from 256-344 mg/L), well oxygenated (87-92% saturation) and clear, with low concentrations of nutrients such as nitrogen (0.21-0.44 mg/L) and phosphorus (0.008-0.012 mg/L). Some froth was noticed at the site in spring.
The sediments were dominated by cobbles and pebbles with some silt and detritus also present. Samples taken from below the surface were sandy grey and showed no evidence of being anaerobic, or lacking oxygen. Small deposits of silt covered the streambed to a depth of between 1 and 5 cm in places. No significant areas of bank erosion were seen.
A small amount of phytoplankton (chlorophyll a ranged from 0.82-0.96 µg/L) and filamentous algae (Spirogyra and Cladophora) covered more than 35% of the site. More than 10% of site was covered by a range of emergent plants (Cyperus, Isolepis, Juncus, Ranunculus, Polygonum, Rorrippa, Triglochin and Rumex). The riparian zone consisted of willows and bamboo with a weedy and grassy understorey with the surrounding vegetation comprising dense native woodland.
Special environmental features
Sixth Creek provides important habitat for many species sensitive to pollution and dependent on the near permanent flows that occur in this creek, such as blackfly larvae, riffle beetles, water pennies, the mayflies Offadens, Atalophlebia and Thraulophlebia, the stoneflies Dinotoperla and Illiesoperla and the caddisflies Apsilochorema, Taschorema, Cheumatopsyche and Lingora.
Pressures and management responses
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.
Large nutrient inputs to the creek from numerous diffuse sources (leading to extensive growth of algae and aquatic weeds)
The Adelaide and Mount Lofty Ranges NRM Board land management program encourages and promotes managing land to improve water quality. This includes working with industry and landholders to ensure efficient use of fertilisers and discuss ways to reduce runoff of nutrients into waterways.
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.
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.