Benara Creek, Lake Bonney SE
2014 Aquatic Ecosystem Condition Report
- Permanently wet, non-flowing channel in autumn and spring 2014
- Sparse macroinvertebrate community with no rare or sensitive species
- Enriched with nutrients which support large algal and plant growths
- Riparian vegetation consists of remnant low coastal woodland over weeds and introduced grasses
About the location
Benara Creek is a small stream in the lower South East with a catchment area of about 66 square kilometres. It rises at an elevation of nearly 20 metres above sea-level about eight kilometres south of Tantanoola, and drains in a south-westerly direction where it discharges into the southern part of Lake Bonney SE. The major land uses are sheep and cattle grazing, wind power generation, and small areas of pine plantations and remnant vegetation. The monitoring site was located on Three Chain Road, about 14 kilometres north-west of Kongorong and two kilometres east from Lake Bonney SE.
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 evidence of human disturbance, including nutrient enrichment and vegetation clearance throughout most of its’ grazed catchment; the same site was rated in a Good condition in 2009 when the site was dry but it was noted at the time that it may rate in a poorer condition when wet due to the extent of stock grazing in the catchment.
A sparse community of at least 23 species of macroinvertebrates (17 in autumn and 12 in spring) was collected from the shallow, non-flowing channel in autumn and spring 2014. The wetted channel extended up to 2.6 metres wide and 33 centimetres deep in places.
The community was dominated by moderate numbers of yabbies (Cherax), amphipods (Austrogammarus) and dytiscid beetles (Liodessus) in autumn and by waterbugs (Anisops) and chironomids (Chironomus) in spring. It also included smaller numbers of snails, swamp yabbies (Geocharax), yabbies (Cherax), isopods, mites, beetles, mosquitoes, dragonflies and damselflies. The only rarely collected macroinvertebrate recorded at the site were swamp yabbies in autumn; they typically inhabit wetland habitats but are occasionally collected from freshwater streams and drains in the lower South East. The other macroinvertebrates collected were generalist, tolerant and opportunistic species, tolerant to poor water quality and with a widespread distribution in the region and State.
The water was fresh (salinity ranged from 440-576 mg/L), generally poorly oxygenated (23-44% saturation), clear and slightly coloured, and with high concentrations of nutrients such as nitrogen (1.5-2.8 mg/L) and phosphorus (0.06-0.1 mg/L).
The sediments were dominated by detritus, sand and silt. Samples taken from below the surface were blackened, sulphidic and anaerobic silts and clays, indicating that the sediments lacked oxygen and were a harsh environment for most benthic species to live in.
Up to 90% of the water’s surface was covered by a floating duckweed (Spirodella) and small patches of dock (Rumex) were the only other aquatic plants seen at the site. A moderate to large growth of phytoplankton was present (chlorophyll a 5.5-8.9 Âµg/L) but no filamentous algae was seen in 2014.
The riparian zone extended over 5 metres wide and comprised native tea tree, wattles and a few gums over a wide range of weeds and introduced grasses. A similar assemblage of plants occurred on the surrounding landscape adjacent to the site but a few hundred metres upstream and downstream of the site the catchment was cleared sheep grazing land with scattered gums over introduced grassland.
Special environmental features
Benara Creek provides habitat for tea trees (Leptospermum) and swamp yabbies (Geocharax), both uncommon records for the region.
Pressures and management responses
|Limited water flow||The Drainage Network in the region supports nearly 200 regulators for water conservation and adaptive flows management practices. The freshwater weir pools of some regulators in the Lower South East are now known to support colonies of threatened aquatic species. The South Eastern Water Conservation and Drainage Board has undertaken preliminary investigations to identify additional biological hot spots in the Lower South East, and further investigations may be undertaken. This may lead to the installation of additional regulators to retain water as drought refuge at these key drain locations.|
|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 doesn’t impede access for management and maintenance.|