Butchers Gap Drain is a small coastal drain in the lower South East with a length of about eight kilometres. It drains in a north-easterly direction into wetlands in the Butchers Gap Conservation Park and ultimately discharges into the Southern Ocean on Pinks Beach.
Butchers Gap 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 ecosystemcondition, although it does provide significant habitat for many aquatic species in the region.
The major land uses are grazing and areas of remnant native vegetation among the adjoining swamps and conservation park at the downstream extent of the drain. The monitoring site was located in the mid reaches off Cape Jaffa to Kingston Road, about eight kilometres south-south-west from Kingston SE.
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 significant nutrient enrichment and poor riparian habitat.
Findings
A diverse community of about 42 species of macroinvertebrates (19 in autumn and 32 in spring) was collected from the slow to non-flowing drain, up to 9 metres wide and 25 centimetres deep, in autumn and spring 2014. The community was dominated by amphipods (Austrochiltonia) and included smaller numbers of water mites (Diplodontus and Eylais), introduced (Physiella) and native snails (Angrobia, Gyraulus, Glyptophysa and Coxiella), beetles, chironomids, brineflies, mosquitoes, waterbugs, damselflies, dragonflies and caddisflies (Notalina and Triplectides). All were generalist, opportunistic and species tolerant to poor water quality, typically found in organically enriched waters. No sensitive or rare species were recorded. The only fish seen or collected were a few juvenile hardyheads.
The water was moderately fresh (salinity ranged from 2,118-4,966 mg/L), well oxygenated (111-152% saturation), highly alkaline (pH ranged from 9.14-9.21), clear and slightly coloured, and with high concentrations of nutrients such as nitrogen (2.91-3.76 mg/L) and phosphorus (0.06-0.18 mg/L).
The sediments were dominated by detritus, silt and filamentous algae; samples taken from below the surface were anaerobic grey silts that released sulfide when tested, indicating that the sediments lacked oxygen and were a harsh environment for most burrowing species to be able to live in. Over 10 centimetres of fine silt and algae covered the middle of the drain, presumably caused by the decomposition of plants living in the drain. No significant areas of bank erosion were noted and there was no sign of any stock accessing or approaching the edge of the drain in 2014.
A large phytoplankton bloom occurred in the drain in autumn (chlorophyll a ranged from 2-54 µg/L) and over 10% of the channel was covered by filamentous algae (mostly Cladophora with some Spirogyra). A range of submerged (Chara and Ruppia) and emergent plants (Baumea, Juncus, Rumex, Cotula and Mimulus) covered over 35% of the drain, in response to the large amount of available nutrients flowing down this watercourse.
The narrow (<5 metres wide) riparian zone consisted of introduced terrestrial grasses, irises and native sedges (Baumea) and rushes (Juncus). The surrounding vegetation was mostly low coastal heath but further upstream the drain was largely surrounded by cleared grazing land.
Special environmental features
None detected apart from the presence of a large number of pollution tolerant macroinvertebrates and a common native species of estuarine fish.
Pressures and management responses
Pressures
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.
Observed Condition
Expected Condition
