Stony Creek, near eastern edge of Lake Bonney SE
2014 Aquatic Ecosystem Condition Report
- Permanently wet, flowing drain in autumn and spring 2014
- Diverse macroinvertebrate community with a few specialised flow-dependent species present
- Obvious signs of gross nutrient enrichment
- Riparian vegetation dominated by introduced grasses
About the location
Stony Creek is a moderately sized drain in the lower South East with a catchment area of about 340 square kilometres. It consists of a series of drains on the eastern side of the Woakwine Range that converge into the one drain that cuts across the range and then takes on more of a natural stream channel just before it discharges into Lake Bonney SE.
Stony Creek is an artificially constructed drain where the primary function is to remove surface water to improve agricultural productivity in the region (Department for Water 2010). Given its artificial character, the drain is not expected to be in a highly rated aquatic ecosystem condition, although it does provide significant habitat for many aquatic species in the region.
The major land uses are grazing and cropping, with wind-farm turbines installed along the eastern edge of Lake Bonney SE and some small areas of conservation parks and forestry in the upper catchment. The monitoring site was located in the lower reaches of the earthen bank-lined drain at the gauge station off Boundary Road, about 11 kilometres south from Millicent.
The drain 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 and poor riparian habitat but the site still supported a rich range of aquatic life.
A diverse community of at least 40 species of macroinvertebrates (29 in autumn and 30 in spring) was collected or seen from the slow to fast-flowing drain that ranged from 2.1-4.2 metres wide and up to 45 centimetres deep, during autumn and spring 2014. The community was dominated by amphipods, waterbugs (Paraplea), mayflies and caddisflies in the slow-flowing sections and by amphipods, blackflies, chironomids and caddisflies in the riffles. It also included flatworms, leeches, hydrobiid snails (Angrobia and Posticobia), introduced physid snails (Physiella), oribatid mites, nematodes, worms, yabbies, beetles, chironomids, biting midges, waterbugs, damselflies and dragonflies. Most were generalists and species tolerant to poor water quality but a few flow-dependent species were present, including blackfly larvae (Simulium ornatipes) and leptophlebiid mayflies (Atalophlebia australis). The site also supported two uncommonly recorded caddisflies from the region (Hellyethira simplex and Hydroptila scamandra) but lacked any regionally rare macroinvertebrates. The only fish seen at the site were a few threatened Southern Pygmy Perch.
The water was fresh (salinity ranged from 406-626 mg/L), well oxygenated (62-64% saturation) and clear, and with nutrient concentrations comprising high amounts of nitrogen (1.37-1.43 mg/L) largely due to oxidised nitrogen and low concentrations of phosphorus (0.01 mg/L).
The sediments were dominated by detritus and silt, with smaller amounts of filamentous algae, bedrock, cobble, boulder and sand also present; samples taken from below the surface were generally anaerobic, black silts that released sulfide when tested in spring, indicating that the sediments lacked oxygen and were a harsh environment for most burrowing species to be able to survive in. No significant bank erosion was seen and no stock appeared to have accessed the site during the year. The presence of silt deposits in the channel up to 5 centimetres deep was probably due to the decomposition of plants growing in the drain rather that from bank erosion further upstream.
A small to moderate amount of phytoplankton algae was recorded (chlorophyll a 2.2-5.3 Âµg/L) and filamentous algae (Cladophora) covered over 10% of the creek throughout the year. Several types of floating (Azolla and Spirodella), submerged (Chara and Callitriche) and emergent plants (Juncus, introduced Rorippa, Triglochin, Ranunculus and Typha) were growing prolifically over more that 65% of the edges and surface of the drain. These plant indicators are typical responses to constant and excessive nutrient enrichment and have been regularly recorded from all the well-watered, agricultural creeks and drains in the region.
The narrow (<5 metres wide) riparian zone was dominated by introduced grasses and weeds and included a few scattered low-lying wattles. The surrounding vegetation at the site was cleared cattle grazing land, which also included a few wattles in the local landscape.
Special environmental features
Stony Creek is noteworthy because it was one of the few waterways sampled from the region in 2014 that maintained flowing riffle habitats during both seasons sampled. The site was significant because it supported a diverse range of generalist and tolerant macroinvertebrates, as well as a few flow-dependent species. It also provided habitat for the Southern Pygmy Perch, a threatened fish in South Australia and a common resident in Lake Bonney SE.
Pressures and management responses
|Livestock having direct access (causing sediment erosion and adding excessive nutrients).||Drains have been constructed since the 1860s as an engineering solution to support agricultural development and it is South Eastern Water Conservation and Drainage Board practice to lease drain reserves for grazing in certain circumstances. Not all drains are subject to grazing and leases for grazing are only approved following an engineering and environmental assessment. Lease conditions require the lessee to fulfil pest plant, pest animal and CFS management requirements, thereby relieving the Board of these responsibilities.|
|Limited riparian zone vegetation (reducing habitat quality, increasing sediment erosion).||The South Eastern Water Conservation and Drainage Board has undertaken a limited revegetation program at key locations, and has the ability to undertake further revegetation works when resources allow. Revegetation at biological hotspots is recognised as a mechanism to reduce nutrient input and soil erosion, and can be undertaken if it does not impede access for management and maintenance.|