Lake Frome Outlet Drain, Southend
2009 Aquatic Ecosystem Condition Report
- Dry in autumn and slightly flowing channel present in spring.
- Sparse macroinvertebrate community with no rare or sensitive species.
- Obvious signs of gross nutrient enrichment.
- Riparian vegetation limited to introduced grasses.
- Moderately eroded banks and large deposit of silt in the channel.
About the location
Lake Frome Outlet Drain is a moderately sized drain in the lower South East with a catchment area of about 215 km2. It rises north-east of Millicent, receives drainage from Hatherleigh Drain and several small drains around Millicent, and flows in a south-westerly direction through Lake Frome and discharges into Rivoli Bay, near Southend. The major land uses are grazing and native vegetation.
Lake Frome Outlet Drain 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 monitoring site was located two kilometres upstream of the mouth of Lake Frome Outlet Drain, at Southend.
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 and poor in-stream and riparian habitat.
A sparse community of about 20 species of macroinvertebrates was collected from the slightly flowing channel, 15 metres wide and over one metre deep, in spring 2009; the drain was dry in autumn. The community was dominated by generalists and organic feeders such as hydrobiid snails and amphipod crustaceans. No sensitive or rare species were found. The site was notable by the lack of worms, nematodes, a wider range of crustaceans and dipteran flies, mayflies, caddisflies or dragonflies.
The water was moderately fresh (salinity of 1,510 mg/L), well oxygenated (109% saturation) and clear, with moderate to high concentrations of nutrients such as nitrogen (1.81 mg/L) and phosphorus (0.04 mg/L).
The sediments were dominated by algae and detritus; samples taken from below the surface were sandy grey and aerobic. Given the organically enriched composition of the sediments, they would occasionally become anaerobic when flows are low and decomposition rates high enough to remove oxygen from the water and sediments. A moderate amount of bank erosion was evident and a large deposit of silt was present in the middle of the channel.
The only aquatic plant growing in the channel and on the water’s edge was a small cover of native rush (Juncus). However, more than 65% of the channel was covered by an extensive growth of filamentous green algae, which highlights the excessive nutrient enrichment of this drain. The site also supported a moderate growth of phytoplankton.
The riparian zone was dominated by rushland and introduced grasses. The surrounding vegetation at the site was grazing land.
Special environmental features
Lake Frome Outlet Drain provides habitat for at least two threatened species of fish (Southern Pygmy Perch and Congolli) and two common species (Western Blue-spot Goby and Common Galaxias) based on sampling conducted in 2004 and 2009 (includes unpublished records from M. Hammer, Native Fish Australia SA).
Pressures and management responses
|Drought||The Drainage Network in the region supports nearly 200 regulators for water conservation and adaptive flows management practices. The freshwater weir pools of some regulators in the Lower South East are now known to support colonies of threatened aquatic species. The South Eastern Water Conservation and Drainage Board has undertaken preliminary investigations to identify additional biological hot spots in the Lower South East, and further investigations may be undertaken. This may lead to the installation of additional regulators to retain water as drought refuge at these key drain locations.|
|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.|