Drain L, east from Lake Hawdon North
2009 Aquatic Ecosystem Condition Report
- Permanently wet with shallow pool habitats in autumn and spring.
- Diverse macroinvertebrate community with only one rarely collected insect.
- Obvious signs of gross nutrient enrichment.
- Riparian vegetation limited to only introduced grasses.
- Large deposit of silt 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 at Barnett Road (also called Baxter Hill Road), about 22 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 poor riparian habitat and large plant growths in response to nutrient enrichment, although the drain still provided a refuge for many aquatic species.
A diverse community of about 42 species of macroinvertebrates was collected from pool habitats within the drain that ranged from 45–47 metres wide and up to 49 cm deep, during autumn and spring 2009. The community was dominated by low numbers of generalists and species tolerant to poor water quality such as worms, mites, chironomids, corixid waterbugs and amphipod crustaceans. The only rare species detected was a chironomid (Apsectrotanypus), however, no sensitive species were detected.
The water was moderately fresh (salinity ranged from 1,124–1,981 mg/L), well oxygenated (70–234% saturation) and clear, with moderate to high concentrations of nutrients such as nitrogen (0.79–1.26 mg/L) and phosphorus (0.02–0.03 mg/L).
The sediments were dominated by detritus, algae and silt; samples taken from below the surface were blackened, sulfidic and anaerobic, or lacking in oxygen. Deposits of over 10 cm of silt occurred in the channel.
A range of submerged (Chara and Stuckenia) and emergent plants (Bolboschoenus, Cotula, Isolepis, Juncus, Mimulus, Rumex, Schoenoplectus, Triglochin and the introduced Watercress Rorippa nasturtium-aquaticum) were growing in the channel and on the water’s edge. These plants covered more than 35% of the channel in autumn. A moderate amount of phytoplankton was present during both periods sampled and a large growth of filamentous algae covered more than 65% of the drain in spring.
The narrow riparian zone lacked any trees or shrubs and only consisted of introduced terrestrial grasses. The surrounding vegetation at the site was native woodland with an understorey dominated by bracken and introduced grasses.
Special environmental features
The site sampled from this drain in 2009 provided habitat for a rarely collected species of chironomid (eg midge), a wide range of generalist macroinvertebrate species and a regionally common native fish called a hardyhead. The drain also supports a wide range of threatened and common fish species including Australian Mudfish, Southern Pygmy Perch and Common Galaxias, and the threatened Southern Bell-frog is also found in the Drain L catchment (S Slater, Department for Environment and Natural Resources, 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 does not impede access for management and maintenance.|