Drain 44, near northern end of Lake Bonney SE
2009 Aquatic Ecosystem Condition Report
- Dry in autumn and shallow flowing channel in spring.
- Sparse macroinvertebrate community with only one flow dependent species and no rare or sensitive species.
- Obvious signs of moderate nutrient enrichment.
- Riparian vegetation mostly native species but some invasion by weeds and grasses.
About the location
Drain 44 is a large drain in the South East with a catchment area of about 150 km2. It rises at an elevation about 20 metres above sea level near the Mount Burr Range, receives drainage from Drain 29, 31, 32, 34, 35, 36 and 47 and flows in a southwesterly direction into the northern end of Lake Bonney SE.
Drain 44 is an artificially constructed drain where the primary function is to remove surface water and draining saline groundwater 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 cattle and sheep grazing, and cropping. The drainage network also receives stormwater from Millicent township and treated effluent from the Millicent Wastewater Treatment Plant. The monitoring site was located at the v-notch weir just upstream from Lake Bonney SE near the Frontage Road, about eight kilometres south–south–west of 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 considerable evidence of human disturbance including nutrient enrichment and poor riparian habitat, although the drain still provided a refuge for some notable native species.
A sparse community of about 20 species of macroinvertebrates was collected from the flowing channel in spring 2009, when the drain ranged from 3–4 metres wide and up to 45 cm deep; the drain was dry in autumn. The community was dominated by generalists and species tolerant to poor water quality such as chironomids and amphipod crustaceans. No sensitive or rare species were found and the only habitat specialist collected was a few flow-dependent blackfly larvae (Simulium ornatipes). Two threatened fish species were collected from the site.
The water was fresh (salinity of 833 mg/L), well oxygenated (139% saturation) and clear, with high concentrations of nutrients such as nitrogen (1.15 mg/L) and phosphorus (0.29 mg/L).
The sediments were dominated by algae, detritus and cobble, with smaller amounts of boulder, sand and silt; samples taken from below the surface were aerobic at a time when the drain was flowing strongly in spring. When flows reduce or cease to occur, the sediments are likely to become blackened and anaerobic when the high organic load in the sediments starts to decompose.
Floating (Spirodela) and emergent plants (introduced Rorippa and Rumex) were growing in the channel and on the water’s edge. Large amounts of phytoplankton and filamentous algae also occurred in the drain, with the latter covering over 35% of the channel.
The riparian zone consisted of a narrow and sparsely vegetated assemblage of bracken, casuarinas and introduced grasses. The surrounding vegetation was low native woodland dominated by paperbarks and gums over bracken and grasses.
Special environmental features
Drain 44 provides habitat for a flow-dependent blackfly larvae, which represented an uncommon macroinvertebrate record for the region. The site sampled also supported at least two threatened native fish species, the Southern Pygmy Perch and Dwarf Galaxias, in spring 2009. The lower section of Drain 44 provided the last known refuge for the Yarra Pygmy Perch in the local catchment as a result of the recent drought (S Slater, Department for Environment and Heritage, 2009); this threatened species was not detected in 2009.
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 doesnot impede access for management and maintenance.|
|Wastewater discharge, adding excessive nutrients and organic matter (leading to algal growth and aquatic weeds).||
SA Water Millicent Wastewater Treatment Plant
SA Water assess and undertake scheduled process improvement actions at the wastewater treatment plant, with the aim to reduce environmental risk and ensure operations are compliant with EPA licence conditions.