Sixth Creek, near Castambul
2013 Aquatic Ecosystem Condition Report
- Permanent flowing stream in autumn and spring 2013
- Diverse macroinvertebrate community with many rare, sensitive and flow-dependent species present
- Water was fresh, clear and showing the effects of nutrient enrichment
- Riparian vegetation was narrow and dominated by gums and reeds over introduced weeds and grasses
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 discharging 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 due to nutrient enrichment and degraded riparian habitat but the stream supports a large number of rare, sensitive and flow-dependent macroinvertebrate species.
A diverse community of at least 48 species of macroinvertebrates was collected from the creek (27 species in autumn and 36 in spring), 4.8-4.9 m wide and up to 42 cm deep, in autumn and spring 2013. The creek consisted of slow-flowing pools connected by shallower, faster-flowing riffle habitats in both seasons sampled. The community was dominated by moderate numbers of amphipods (Austrochiltonia australis) in the pools and blackflies (Austrosimulium furiosum, Simulium melatum and Simulium ornatipes) in the riffles. It also included smaller numbers of colonial hydrozoans, limpets, introduced snails (Physa and Potamopyrgus), leeches, mites, shrimp, springtails, beetles, dixid flies, mosquitoes, craneflies, chironomids, mayflies, waterbugs, damselflies, dragonflies, stoneflies and caddisflies. Many of these were rare, sensitive and/or flow-dependent species, including the above-listed blackflies, mayflies (Offadens sp. 5 and Atalophlebia australasica), stoneflies (Dinotoperla evansi and Illiesoperla mayii), dragonflies (Austrogomphus and Hemigomphus gouldii) and caddisflies (Ulmerochorema membrum, Taschorema evansi, Taschorema complex, Lingora aurata and Cheumatopsyche modica). The other macroinvertebrate collected were more commonly found, generalist, opportunistic and tolerant species that have a wide distribution across the wetter parts of the State. The only fish seen at the site were a few introduced trout in spring.
The water was fresh (salinity ranged from 295-347 mg/L), well oxygenated (80-122% saturation) and clear, and with low nutrient concentrations such as phosphorus (0.01 mg/L) and nitrogen (0.19-0.49 mg/L).
The sediments were dominated by detritus and filamentous algae in the pools and the riffles were dominated by boulder, cobble, pebble, detritus, and algae in spring. Samples taken from below the surface were grey sands that released sulphide when tested in autumn, indicating that the sediments were anaerobic or lacking on oxygen for at least part of the year. There was no evidence of any significant bank erosion and there was no sign of any stock accessing the creek at the site sampled; the lower section of the creek appears to be well fenced to prevent animals damaging the banks of the stream.
There was only a small amount of phytoplankton present during the year (chlorophyll a ranged from 0.5-1.8 μg/L) but large growths of filamentous algae (Cladophora and Spirogyra) extended over more than 35% of the creek in spring. A similar area was also covered by aquatic plants in spring, comprising introduced watercress (Rorippa) and dock (Rumex), and native macrophytes such as sedges (Cyperus), reeds (Phragmites), rushes (Juncus) and buttercups (Ranunculus). The narrow, riparian zone was generally less than 5 m wide at the site and comprised a few scattered gums over reeds, introduced weeds (mostly broom and bamboo) and grasses. The surrounding vegetation near the creek comprised dense native woodland that extended up the steeply-sided gorge that the stream flows through before discharging into the River Torrens.
Special environmental features
Sixth Creek provides a permanently flowing, freshwater stream that has consistently supported a large number of rare, sensitive and flow-dependent species since the mid 1990’s. It is among the most ecologically significant and best condition streams in the Southern Mount Lofty Ranges and the State. The lower reaches of Sixth Creek is also one of the few streams in the region that supports a rare, flow-dependent beetle called a waterpenny (Sclerocyphon); unrelated sampling of this stream confirmed its continued presence in low numbers, despite it not being collected as part of this program.
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.|
|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.|
|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.|
|Some nutrient inputs to the creek from numerous diffuse sources (leading to algal growth)||The Adelaide and Mount Lofty Ranges NRM Board’s 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.|
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.