Drain 31, Millicent
2014 Aquatic Ecosystem Condition Report
- Permanently wet, non-flowing channel in autumn and spring 2014
- Moderately diverse macroinvertebrate community with two rare or sensitive species present
- Obvious signs of gross nutrient enrichment
- Riparian vegetation limited to introduced grasses and weeds
About the location
Drain 31 is a small drain in the lower South East with a catchment area of about 116 square kilometres. It rises at an elevation of about 20 metres above sea-level around Millicent, receives drainage from drains 32, 34 and 35, and flows in a southerly direction into Drain 44, which ultimately discharges into the northern part of Lake Bonney SE.
Drain 31 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 adjacent to the lower reaches of the drain are sheep grazing and cropping, whereas the upper reaches receive stormwater from Millicent township and treated effluent from the Millicent Wastewater Treatment Plant. The monitoring site was located off Lossie Road, on the southern outskirts of Millicent and upstream from the treatment plant discharge.
The drain 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 considerable evidence of human disturbance, including the effects of the drain realignment works in 2009, nutrient enrichment, and poor riparian habitat.
A moderately diverse community of about 29 species of macroinvertebrates (17 in autumn and 19 in spring) was collected from the slow to non-flowing channel, up to 5 metres wide and 49 centimetres deep, in autumn and spring 2014. The community was dominated by large numbers of hypogastrurid springtails (in autumn only) and included smaller numbers of hydras, flatworms, worms, leeches, introduced (Physiella) and native snails (Glyptophysa, Austropeplea and Angrobia), swamp crayfish, yabbies, beetles, mosquitoes, chironomids, waterbugs, mayflies and damselflies. Most species were generalist and tolerant species commonly found together in organically polluted watercourses. The only rare or sensitive species collected were the swamp crayfish (Geocharax), which are only found in the lower South East in South Australia, and a leptophlebiid mayfly (Thraulophlebia) that are normally associated with flowing, freshwater habitats. The co-occurrence of the swamp crayfish with yabbies (Cherax destructor) at the same site was an unusual finding since they normally occupy different habitats. Two threatened native fish were seen at the site during the spring survey, including Southern Pygmy Perch (Nannoperca australis) and Dwarf Galaxias (Galaxiella pusilla); both are also common inhabitants of downstream Lake Bonney SE.
The water was fresh (salinity ranged from 47-528 mg/L), well oxygenated (64-145% saturation), clear and slightly coloured, and with moderate to high concentrations of nutrients such as nitrogen (0.55-0.64 mg/L) and phosphorus (0.03-0.1 mg/L); the low salinity recorded in autumn is more like what would be expected from rainwater rather than a drain or creek, suggesting that localised heavy rainfall had contributed freshwater to the site via the Millicent stormwater system prior to sampling in autumn 2014.
The sediments were dominated by detritus, silt and filamentous algae, with smaller amounts of sand, gravel, cobble and boulder also present: samples taken from below the surface were black silts that released sulfide when tested, indicating that the sediments lacked oxygen and were a harsh environment for most burrowing species to live in. Over one centimetre of fine silt, filamentous algae and biofilm covered the middle of the drain but it originated within the channel because there was no evidence of any bank erosion or stock accessing the site during the year.
A moderate amount of phytoplankton was recorded from the site (chlorophyll a ranged from 3.7-4 Âµg/L) but filamentous algae (Cladophora and Spirogyra) was only seen in spring when it formed an extensive cover over more than 35% of the drain. A similar area was also covered by a range of submerged (Ruppia) and introduced emergent plants (Rorippa, Rumex, Plantago and Aponogeton).
The narrow (<5 metres wide) riparian zone was dominated by introduced grasses with a few isolated pine trees and the surrounding terrestrial vegetation comprised rural residential properties and urban parkland with associated walking trails.
Special environmental features
The most notable feature of this drain was the collection of rare, sensitive and threatened aquatic species in 2014, including the swamp crayfish, leptophlebiid mayfly, pygmy perch and Dwarf Galaxias.
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.|
|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.