Narrow Neck Drain, near Rendelsham
2014 Aquatic Ecosystem Condition Report
- Permanently wet, slow-flowing channel in autumn and spring 2014
- Diverse macroinvertebrate community with a few sensitive and widely distributed species recorded
- Obvious signs of gross nutrient enrichment
- Riparian vegetation dominated by introduced grasses
- Large amount of silt, biofilm and benthic algae in the channel
About the location
Narrow Neck Drain is a moderately sized drain in the Lower South East that lies at the bottom of Drain 1B and receives drainage from the north of Millicent. It flows into Lake Frome and then out into Rivoli Bay and the Southern Ocean at Southend during very wet periods.
Narrow Neck 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 major land uses are grazing and cropping, with areas of remnant native vegetation also in the catchment. The monitoring site was located upstream of the Main South Eastern Road off Buhlmans Road, about three kilometres west from Rendelsham.
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 riparian habitat but the site still supported a wide range of aquatic species.
A diverse community of about 46 species of macroinvertebrates (35 in autumn and 30 in spring) was collected from the slow to non-flowing drain, up to 4.25 metres wide and 66 centimetres deep, in autumn and spring 2014. The community was dominated by large numbers of hydrobiid snails (Angrobia) in autumn but no macroinvertebrate was particularly abundant in spring. The community included smaller numbers of worms, leeches, water mites (Diplodontus and Eylais), introduced (Physiella and Potamopyrgus) and native snails (Austropeplea and Glyptophysa), amphipods (Austrochiltonia and Austrogammarus), isopods, springtails, several beetles, biting midges, chironomids, soldierflies, waterbugs, damselflies, dragonflies and caddisflies. The presence of holes in the banks also indicated that yabbies occurred at the site. The large number of organic feeding snails, amphipods and caddisflies, along with the presence of many predators (eg mites, dytiscid beetles, waterbugs (including the Giant Water Bug Diplonychus) and odonates), indicates that the drain is structurally diverse and highly productive. No rare species were recorded but a few sensitive and widely distributed macroinvertebrates were present eg (Angrobia, Austrogammarus and Diplonychus). The only fish seen at the site were large numbers of galaxiids (probably mostly Common Galaxias).
The water was fresh to moderately fresh (salinity ranged from 700-883 mg/L), well oxygenated (60-105% saturation), clear, and with variable concentrations of nutrients including low amounts of phosphorus (0.02 mg/L) but high levels of nitrogen (1.09-1.50 mg/L).
The sediments were dominated by detritus, filamentous algae, sand and silt, with smaller amounts of boulder, cobbles, pebbles and gravel also present; samples taken from below the surface were blackened or grey sands that released sulfide when tested, indicating that the anaerobic sediments were a harsh environment for most burrowing species to live in due to the lack of oxygen. Large deposits of 5-10 centimetres of silt, biofilm and algae were recorded from the middle of the channel in places. No evidence of any bank erosion or stock accessing the local area were noted during either survey.
A small to moderate amount of phytoplankton was recorded during the year (chlorophyll a ranged from 1.8-3 Âµg/L) but extensive growths of filamentous algae (Cladophora and Spirogyra) covered more than 65% of the drain in spring. About 35% of the drain was also covered by several types of aquatic plants, including submerged (Chara) and emergent species (Schoenoplectus, Ranunculus, Juncus, Rumex, Typha, Phragmites and introduced Rorippa and Rumex). These plant responses are typically seen from among the most nutrient enriched waterways in the region.
The narrow (<5 metres wide) riparian zone was dominated by introduced grasses and bare soil. The surrounding vegetation was mostly cleared cattle grazing land with introduced grasses and a row of planted wattles present near one bank.
Special environmental features
Narrow Neck Drain provides habitat for a wide range of commonly occurring generalist and opportunistic aquatic macroinvertebrates and plants, and at least one species of native fish. It also supports a few sensitive but widely distributed macroinvertebrates in the region. In 2009, the same site provided habitat for a sensitive, flow-dependent mayfly (Atalophlebia), a threatened species of fish called Dwarf Galaxias, as well as an introduced pest fish called mosquitofish (Gambusia). Past fish surveys in the catchment have also detected the occasional presence of threatened Southern Pygmy Perch and Congolli, and Common Galaxias, highlighting the obvious biological significance of this drain in the region.
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 doesn’t impede access for management and maintenance.|