Deep Creek, near Riddock Bay
2009 Aquatic Ecosystem Condition Report
- Permanently wet with flowing habitats present in autumn and spring.
- Sparse macroinvertebrate community that includes several rare and sensitive species.
- Obvious signs of moderate nutrient enrichment.
- Riparian vegetation limited to reeds and introduced grasses.
- Large amount of silt deposited in the channel.
About the location
Deep Creek is a small, 2.6-km long coastal stream in the lower South East with a catchment area of nearly 15 km2. It rises at an elevation about five metres above sea level and flows into the Southern Ocean at Riddock Bay.
Deep Creek 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 use is grazing by dairy cattle. The monitoring site was located near the mouth of the creek and gauge station on Eight Mile Creek Road, about seven kilometres east of Port MacDonnell.
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, poor riparian habitat and fine sediment deposition, although the creek still provided a refuge for some notable native species.
A low diversity of about 23 species of macroinvertebrates was collected from the slow-flowing channel, ranging from 9–12 metres wide and up to 35 cm deep, in autumn and spring 2009. The community was dominated by species that feed on plants and organic detritus, including small hydrobiid snails, amphipod crustaceans and chironomids.
A wide range of biological groups were collected and included a flatworm, snails, freshwater limpet, leech, two types of amphipods, freshwater shrimp, freshwater crab, three beetles, three chironomids, a mayfly, waterbug, two damselflies and three caddisflies. This list included two sensitive and rare species for the region; namely, a baetid mayfly (Offadens) and case-dwelling conoesucid caddisfly (Lingora). The endangered Glenelg Spiny Crayfish (Euastacus bispinosus) was also seen at the site.
The water was moderately fresh (salinity ranged from 1,435–1,688 mg/L), well oxygenated (77–105% saturation) and clear, with a low phosphorus concentration (0.01 mg/L) and very high nitrogen concentration (3.47–3.54 mg/L); the latter was due to the high nitrate levels in the shallow groundwater that discharges into the drain.
The sediments were dominated by detritus, algae, silt, sand and cobble; samples taken from below the surface were well aerated in autumn but were blackened, sulphidic and anaerobic, or lacking in oxygen in spring, when the large amount of organic matter in the sediments had presumably started to decay.
Several submerged (Chara, Vallisneria and Stuckenia or Ruppia) and emergent plants (Juncus, Phragmites and introduced Rorippa) were growing in the channel and on the water’s edge. These plants covered more than 35% of the creek and both phytoplankton and filamentous algal growths were also evident at moderate levels during 2009, demonstrating several obvious biological responses to nutrient enrichment in this permanently flowing drain.
The narrow riparian zone along the banks was comprised Common Reed (Phragmites) and introduced grasses, with no trees or shrubs lining the edge of the drain. The surrounding vegetation at the site was dairy farming pasture comprising introduced grasses.
Special environmental features
Deep Creek provides habitat for at least one sensitive baetid mayfly (Offadens), a rare and flow-dependent caddisfly (Lingora) typically found from sites with good water quality, and a submerged plant called Ribbonweed (Vallisneria) that does not commonly occur in creeks and drains in the region or State. The drain also provides habitat for the endangered Glenelg Spiny Crayfish (Euastacus bispinosus) and Southern Pygmy Perch (Nannoperca australis), a threatened fish species, that were observed at the site during the 2009 surveys.
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.|