Bray Drain, near Lake Hawdon South
2014 Aquatic Ecosystem Condition Report
- Permanently wet, non-flowing channel in autumn and spring 2014
- Moderately diverse macroinvertebrate community but no rare or sensitive species present
- Obvious signs of gross nutrient enrichment
- Riparian vegetation limited to narrow area of introduced grasses on the banks
About the location
Bray Drain is a moderately sized drain in the lower South East with a catchment area of over 260 square kilometres. It rises at an elevation of about 15 metres above sea-level near Biscuit Flat, receives drainage from the Biscuit Flat Drain, and flows in a westerly direction where it discharges into Lake Hawdon South providing the main source of hydration for this important large wetland area. The major land uses are cattle and sheep grazing. The monitoring site was located in the lower reaches near the gauge station on a track off Lake Hawdon Road, about 20 kilometres east-south-east from Robe.
Bray Drain is an artificially constructed drain where the primary function is to remove surface water 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 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, salinization, presence of stock droppings on the banks, lack of vegetative cover in the riparian zone, and presence of two introduced species of snail. Despite this the site supported a wide range of aquatic species, including a threatened fish for South Australia.
A moderately diverse community of at least 39 species of macroinvertebrates (26 species in autumn and 21 in spring) was collected from the 20-21 metre wide, non-flowing channel that extended up to 55 centimetres deep in autumn and spring 2014. The community was dominated by species that favour nutrient enriched pools with abundant vegetation such as hydrobiid snails (Angrobia and introduced Potamopyrgus), amphipods (Austrochiltonia), caddisflies (Notalina) and damselflies (Xanthagrion and Austrolestes). It also included flatworms, mites, another introduced snail (Austropeplea), leeches, springtails, beetles, waterbugs, chironomids, damselflies and caddisflies. No sensitive or rare species were collected. The only fish seen at the site in autumn was a single threatened Southern Pygmy Perch (Nannoperca australis) and a few introduced Mosquitofish (Gambusia) were seen at the site in spring.
The water was moderately fresh (salinity ranged from 1,676-1,871 mg/L), moderately well oxygenated (41-62% saturation), clear and slightly coloured, and with moderate to high concentrations of nutrients such as nitrogen (1.4-1.8 mg/L) and phosphorus (0.03-0.05 mg/L).
The sediments were dominated by filamentous algae, detritus and silt, with smaller amounts of cobble and boulder also present; samples taken from below the surface were blackened and anaerobic in both seasons sampled, indicating that the sediments lacked oxygen and were a harsh environment for most benthic species to live in. Over 5 centimetre of silt and filamentous algae covered the bottom of the channel, presumably caused by stock occasionally accessing the banks of the drain.
A range of submerged (Callitriche and Ruppia) and emergent plants (introduced watercress Rorippa, waterbuttons (Cotula), dock (Rumex), sedges (Schoenoplectus and Isolepis) extended over more than 10% of the channel. A more extensive growth of filamentous algae (Cladophora, Spirogyra and Enteromorpha) was present and covered over 65% of the drain in spring. A large amount of small algae called phytoplankton was recorded from the drain in autumn (chlorophyll a 12.7 Âµg/L) but only traces were noted in spring (chlorophyll a 0.8 Âµg/L).
The narrow riparian zone lacked any native trees, shrubs and understorey plants and consisted of introduced grasses and weeds. The surrounding vegetation at the site was cleared grazing land for sheep and cattle.
Special environmental features
The most significant feature of this site was the presence of a threatened native fish. Otherwise the drain supports a wide range of tolerant and generalist species of aquatic macroinvertebrates and plants, including a few introduced snails and a pest fish species.
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.|