Drain L, near Robe
2014 Aquatic Ecosystem Condition Report
- Permanently wet with slow-flowing pools connected by small areas of fast-flowing riffles in autumn and spring 2014
- Moderately diverse macroinvertebrate community with no sensitive or rare species
- Obvious signs of moderate to gross nutrient enrichment
- Riparian vegetation dominated by introduced grasses and weeds
About the location
Drain L is a large drain in the South East with a catchment area of about 780 square kilometres. It rises at an elevation of 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, ultimately discharging into Guichen Bay, Robe.
Drain L 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 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 from 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 a number of aquatic macroinvertebrates and fish.
A moderately diverse community of about 39 species of macroinvertebrates (23 in autumn and 25 in spring) was collected from the 9-15 metres wide and up to 53 centimetres deep channel in autumn and spring 2014. Most of the drain comprised a slow to non-flowing channel connected by small areas of shallow, faster-flowing riffle habitats. The community was dominated by generalists and species tolerant to poor water quality such as corixid waterbugs (Micronecta) and chironomids (Cricotopus, Chironomus, Tanytarsus, Dicrotendipes, Procladius and Paratanytarsus). It also included smaller numbers of leeches, native and introduced snails, pea mussels, water mites, amphipods, freshwater crabs, beetles, biting midges, waterbugs, damselflies, mayflies and caddisflies. The most notable records for the site related to the rich caddisfly assemblage (Notalina spira, Hellyethira simplex, Triplectides australis and Ecnomus cygnitus); all have a widespread distribution in the region and their presence together at the same site is probably due to the diverse range of habitat types present in 2014. The presence of two species of mayflies (Cloeon fluviatile and Tasmanocoenis tillyardi) was an indication that the drain provided a diversity of habitat types but was otherwise unremarkable since they are the most tolerant and widely distributed mayflies in South Australia. No rare or sensitive species were recorded and surprisingly no flow-dependent species were detected despite the presence of riffles in both seasons sampled. A number of fish were seen at the site in 2014 including large numbers of juvenile hardyheads and gobies in autumn and introduced mosquitofish and threatened Southern Pygmy Perch (Nannoperca australis) in spring.
The water was moderately fresh (salinity ranged from 1,229-2,130 mg/L), generally well oxygenated (60-67% saturation), clear, and with moderate to high concentrations of nutrients such as nitrogen (0.73-0.75 mg/L) and phosphorus (0.02 mg/L).
The sediments were dominated by sand, filamentous algae, silt and detritus, with small amounts of boulder and cobble also present; samples taken from below the surface were grey sands that had turned anaerobic in spring, indicating that the sediments lacked oxygen for at least part of the year. Over 1 centimetre of sand and algae covered rocky sediments in the middle of the channel.
A moderate amount of phytoplankton was present (chlorophyll a ranged from 2-2.8 Âµg/L) and over 65% of the channel was covered by filamentous algae (Cladophora, Spirogyra and Enteromorpha) during the warmer spring survey. Aquatic plants extended over more than 35% of the drain in both seasons sampled and included both submerged (Ruppia) and emergent species (Schoenoplectus, Typha, introduced Rorippa and Rumex, and Isolepis only in autumn). These plant responses all highlight the excessive nutrient loads that regularly pass down in this drain and allow the many different types of algae and aquatic plants to thrive in.
The narrow riparian zone lacked trees and shrubs and consisted of introduced grasses weeds and a few small sedges. The surrounding vegetation was cleared agricultural land adjacent to and downstream of the site; further upstream, areas of native woodland and shrubland dominated the local landscape.
Special environmental features
The site sampled on Drain L provided habitat for a moderately diverse range of freshwater macroinvertebrates that included many of the caddisflies that occur in the region. It also supported a threatened fish (Southern Pygmy Perch) and at least another two common native species that frequent the lower and estuarine reaches of drains and creeks in the region. Previous sampling of this site in 2009 showed that the drain has supported flow-dependent and/or sensitive macroinvertebrates in the past (eg mayfly Atalophlebia and blackfly Simulium). Drain L is also a known refuge for threatened Australian Mudfish and Southern Bell-frog (S. Slater, Department for Environment and Heritage, 2009), so its biodiversity value for the region is significant.
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.|