Drain L, near Robe
2009 Aquatic Ecosystem Condition Report
- Permanently wet with slow-flowing pool habitats present in autumn and spring.
- Moderately diverse macroinvertebrate community with one sensitive species and a few habitat specialists.
- Obvious signs of moderate nutrient enrichment.
- Riparian vegetation limited to weeds and introduced grasses.
- High levels of silt deposited 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 evel 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 near the coast at Sandy Lane, about four kilometres east of Robe.
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 evidence of human disturbance, including nutrient enrichment and poor habitat features, although the drain still provided a refuge for some notable native species.
A moderately diverse community of about 39 species of macroinvertebrates was collected from slow-flowing pool habitats within the drain, 14–17 metres wide and 52–72 cm deep, in autumn and spring 2009. The community was dominated by generalists and species tolerant to poor water quality such as corixid waterbugs (Micronecta), leptocerid caddisflies, chironomids, and snails that included native (Angrobia) and introduced species (Physa). The only sensitive species found was a leptophlebiid mayfly (Atalophlebia), although the presence of flow dependent blackfly larvae (Simulium ornatipes) and a rare beetle for the region (whirligig beetle Aulonogyrus strigosus) indicated that the site supported a significant range of species.
The water was moderately fresh (salinity ranged from 1,569–2,458 mg/L), well oxygenated (110–137% saturation) and clear, with moderate to high concentrations of nutrients such as nitrogen (1.11–1.63 mg/L) and phosphorus (0.02 mg/L).
The sediments were dominated by detritus, algae, fine sediments with smaller amounts of pebble and cobble; samples taken from below the surface were occasionally blackened, sulphidic and anaerobic, or lacking in oxygen. More than 10 cm of silt was deposited in the middle of the channel in spring.
A range of submerged (Stuckenia or Ruppia) and emergent aquatic plants (Phragmites, Schoenoplectus, Triglochin, Juncus and introduced Rorippa) were growing in the channel and on the water’s edge. The plants covered over 35% of the channel in autumn but this reduced to 10–35% of the drain in spring. Moderate amounts of phytoplankton and filamentous green algae were present during both sampling periods.
The narrow riparian zone lacked trees and shrubs and only consisted of weeds and introduced grasses. The surrounding vegetation was cleared agricultural land adjacent to and downstream of the site; further upstream, areas of native vegetation dominated the local landscape.
Special environmental features
The site sampled from Drain L provided habitat for at least one sensitive and rare type of mayfly (Atalophlebia) and one threatened fish species (Southern Pygmy Perch). The site also supported several other species of fish (hardyhead, galaxiid, mullet and bream) and provided habitat for at least one flow dependent species (blackfly larvae), which were rarely recorded in the region during 2009. Drain L is also a significant refuge for the threatened Australian Mudfish and Southern Bell-frog (S Slater, Department of 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 doesn’t impede access for management and maintenance.|