Eight Mile Creek, Riddock Bay
2009 Aquatic Ecosystem Condition Report
- Permanently wet, slow-flowing channel in autumn and spring.
- Sparse macroinvertebrate community with many rare and sensitive species.
- Obvious signs of moderate nutrient enrichment.
- Riparian vegetation limited to reeds and introduced grasses.
- Large deposit of silt in the middle of the drain.
About the location
Eight Mile Creek is a very small drain in the lower South East with a catchment area of about 12 km2. It rises in the Ewen Ponds Conservation Park, and flows in a southerly direction into the Southern Ocean.
The primary function of Eight Mile Creek is to remove surface water and drain saline groundwater to improve agricultural productivity in the region (Department for Water 2010). 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 South Eastern Water Conservation and Drainage Board takes a very active role in its management.
The major land uses are dairy grazing and irrigated pastures and cropping, and a few rural residential properties also occur in the catchment. The monitoring site was located on Eight Mile Creek Road, about 300 metres from the mouth of the drain.
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 considerable evidence of human disturbance, including nutrient enrichment, poor riparian habitat and fine sediment deposition although the drain still provided a refuge for several notable native species.
A sparse community of about 25 species of macroinvertebrates was collected from the slowing flowing pool habitats found in the 11–15 metres wide drain in autumn and spring 2009. The community was dominated by generalists and species tolerant to poor water quality such as hydrobiid snails, introduced snails (Potamopyrgus) and amphipod crustaceans. Other species found included nematodes, leeches, a shrimp, a crab, beetles, flies, mayflies, waterbugs, caddisflies and a dragonfly. Several sensitive and rare species were recorded, including a mayfly (Offadens) and two caddisflies (Taschorema and Lingora) that prefer flowing, freshwater habitats. Flow dependent blackfly larvae (Simulium ornatipes) were also recorded in spring.
The water was fresh (salinity ranged from 729–736 mg/L), well oxygenated (74–122% saturation) and clear, with very high nitrogen concentrations (5.9-6.1 mg/L) but very low concentrations of phosphorus (0.009–0.01 mg/L); the majority of the nitrogen originates as nitrate from the shallow groundwater that flows into the drain.
The sediments were dominated by detritus, sand, algae and silt, with small amounts of bedrock, cobble, pebble and gravel also present; samples taken from below the surface were aerated sandy grey sediments. More than 10 cm of silt was deposited in the deeper parts of the channel in spring.
A wide range of submerged (Stuckenia and possibly Ruppia) and emergent plants (Cyperus, Phragmites, introduced Rorippa, Rumex, Triglochin and Berula) were growing in the channel and on the water’s edge. These plants as well as filamentous algae each covered more than 35% of the channel in spring. A moderate amount of phytoplankton was also recorded in spring, highlighting the large nutrient load that was available for extensive plant growths in the drain.
The narrow riparian zone consisted of introduced grasses and the Common Reed (Phragmites). The surrounding vegetation at the site was pasture land used for grazing by dairy cows.
Special environmental features
Eight Mile Creek is an important biodiversity hotspot in the region. The presence of permanently flowing, freshwater habitats over a range of sediment types is probably the most critical factor that has allowed such a rich assemblage of rare and sensitive species to inhabit this drain. During the surveys conducted in 2009, a regionally rare mayfly and two sensitive caddisflies were collected.
In the past, a wider range of rarely collected macroinvertebrates has been collected from this drain including stoneflies (Dinotoperla brevipennis), mayflies (Atalophlebia, Koorrnonga conspicua and Nousia pilosa) and a caddisfly (Ulmerophlebia).
The drain also provides crucial habitat for two macroinvertebrates with very limited distributions in South Australia (eg the threatened Glenelg River Crayfish (Euastacus bispinosus) and a type of freshwater mussel (Hydridella narracanensis)) and several threatened species of fish (eg Ewens or Variegated Pygmy Perch, Yarra Pygmy Perch, Southern Pygmy Perch, River Blackfish, Spotted Galaxias, Australian Grayling and Congolli).
Pressures and management responses
|Intensive livestock grazing in the catchment (adding excessive nutrients).||The South Eastern Water Conservation and Drainage Board is supporting a DENR program to undertake revegetation and creation of constructed refuge pools for the resident Glenelg River Crayfish.|
|Limited riparian zone vegetation (reducing habitat quality, increasing sediment erosion).||The South Eastern Water Conservation and Drainage Board has undertaken a limited revegetation program along Eight Mile Creek in the past, and is supporting further revegetation proposed by DENR. A change in flow path clearance methods from excavation to reed cutting should result in the retention of riparian vegetation which will reduce the possibility of erosion and discharge of nutrients. This change in practice is intended to reduce disturbance to the substrate of the creek and ensure the retention of strips of aquatic vegetation that will provide refuge for native fish and other aquatic biota.|