Drain L, north from Biscuit Flat
2009 Aquatic Ecosystem Condition Report
- Permanently wet with slow-flowing pool habitats in autumn and spring.
- Moderately diverse macroinvertebrate community with no rare or sensitive species and only one habitat specialist.
- Obvious signs of gross nutrient enrichment.
- Riparian vegetation limited to weeds and introduced grasses.
- Large silt deposit in the channel.
About the location
Drain L is a large drain in the South East with a catchment area of about 780 km2. It rises at an elevation about 40 metres above sea level near Lucindale, receives drainage from Drain K, Avenue Flat-K Drain, Reedy Creek Drain and Wilmot Drain, and flows in a westerly direction into Lake Hawdon North and ultimately discharges into Guichen Bay, Robe.
Drain L 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 sheep and cattle grazing, cropping and some areas of native vegetation. The monitoring site was located upstream of Princes Highway, about 32 km east of Robe.
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 nutrient enrichment, sediment deposition and a lack of vegetative cover in the riparian zone.
A moderately diverse community of about 28 species of macroinvertebrates was collected from slow-flowing pool habitats within the drain that ranged from 6–8 metres wide and up to 80 cm deep, in autumn and spring 2009. Only a few individuals and species were collected in autumn. However, a well developed community dominated by generalists and species tolerant to poor water quality were collected in spring, and included introduced snails (Physa), a caddisfly (Notalina), hydrobiid snails and biting midges. No sensitive or rare species were found, although flow-dependent blackfly larvae (Simulium ornatipes) were collected during both seasons.
The water was fresh to moderately fresh (salinity ranged from 668–1,813 mg/L), well oxygenated (137–162% saturation) and clear, with moderate to high concentrations of nutrients such as nitrogen (0.87–1.37 mg/L) and phosphorus (0.02–0.03 mg/L).
The sediments were dominated by detritus, silt, algae and clay; samples taken from below the surface were occasionally blackened, sulphidic and anaerobic, or lacking in oxygen. More than 10 centimetres of silt was deposited in the middle of the channel.
A wide range of submerged (Chara, Crassula, Vallisneria and Ruppia or Stuckenia) and emergent plants (Cyperus, Juncus, Persicaria, introduced Rorippa, and Triglochin) were growing in the channel and on the water’s edge. These plants covered more than 35% and 65% of the channel in autumn and spring, respectively. A moderate amount of phytoplankton was recorded during both seasons and a large growth of filamentous algae extended over more than 35% of the channel in spring. These plant responses highlight the high nutrient status of this drain.
The narrow riparian zone lacked any trees or shrubs and consisted of several types of weeds and introduced grasses. The surrounding vegetation at the site was grazed pasture with no trees evident in the local landscape.
Special environmental features
Drain L maintains permanent or at least nearly permanent, flowing habitat and supported at least one flow dependent species, features which were not commonly recorded from the region during 2009. The site sampled also provided habitat for Ribbonweed (Vallisneria), a rare aquatic plant in the region, and a saline tolerant species of hardyhead fish. Further downstream, the drain also supports several threatened species (including fish and frog species) which highlights its significance as a biodiversity hot-spot in the region.
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.|