Eight Mile Creek, Riddock Bay
2014 Aquatic Ecosystem Condition Report
- Permanently wet, slow-flowing channel in autumn and spring 2014
- Moderately diverse macroinvertebrate community with several rare and sensitive species present
- Little evidence of any nutrient enrichment responses despite the presence of high concentrations of dissolved nitrogen
- Riparian vegetation limited to reeds and introduced grasses
About the location
Eight Mile Creek is a very small drain in the lower South East with a catchment area of about 12 square kilometres. It rises in the Ewen Ponds Conservation Park, and flows in a southerly direction into the Southern Ocean.
Eight Mile 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 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 creek.
The drain was given a good rating because the site sampled showed evidence of relatively minor changes in ecosystem structure and function. There was some evidence of human disturbance, including the poor riparian habitat lining the creek and presence of fine sediment in the channel but the creek provided significant habitat for several notable native species for the region.
A moderately diverse community of about 21 species of macroinvertebrates (15 species in autumn and 13 in spring) was collected or seen from slow-flowing creek, 14 metres wide and over 1 metre deep, in autumn and spring 2014. The community was dominated by generalists and species tolerant to poor water quality such as snails (Angrobia, introduced Potamopyrgus, and Coxiella) and shrimp (Paratya). It also included smaller numbers of leeches, planorbid snails, amphipods (including Paracalliopidae), freshwater crabs, dytiscid beetles, chironomids, waterbugs and several caddisflies. Glenelg Spiny Crayfish (Euastacus bispinosus) were also seen in the shallows during the autumn survey. The community included several sensitive and rare species that are normally associated with flowing freshwater habitats, including baetid mayflies (Offadens sp. MV5), two types of caddisflies (Lingora aurata and Taschorema complex) and the spiny crayfish. The site also provided habitat for a newly identified amphipod family (Paracalliopidae) that appears to have recently colonised a few coastal drains and streams in the lower South East. Several native fish were also seen at the site, including pygmy perch, galaxiids, bream and salmon trout.
The water was fresh (salinity ranged from 657-760 mg/L), well oxygenated (82-128% saturation) and clear, with very high nitrogen concentrations (5.4-6.2 mg/L) but low concentrations of phosphorus (0.01-0.02 mg/L); the majority of the nitrogen originates as nitrate from the shallow groundwater that flows into the creek.
The sediments were dominated by sand, silt and detritus with smaller amounts of gravel and filamentous algae also present; samples taken from below the surface were anaerobic smelling grey sands, which indicates the sediments lacked oxygen and were probably a harsh environment for most burrowing organisms to survive in. Over 1 centimetre of silt was deposited in the deeper parts of the channel, despite the lack of any evidence of bank erosion or stock accessing the site in 2014.
A small amount of phytoplankton (chlorophyll a 0.8-1.4 Âµg/L) was recorded but filamentous algae (Cladophora) was only seen in spring when it covered less than 10% of the channel. Over 10% of the channel was covered by a small number of submerged (Ruppia) and emergent plants (Triglochin procerum, Hydrocotyle and introduced Rorippa and Rumex). None of these plant responses indicate that the stream was significantly affected by excessive amounts of nutrients during either survey period.
The narrow (<5 m wide) riparian zone lacked any trees or shrubs and was dominated by introduced grasses and reeds (Phragmites). The surrounding vegetation at the site was pasture land used for grazing by dairy cows.
Special environmental features
Eight Mile Creek is one of the most important biodiversity hotspots in the region. The presence of permanently flowing, freshwater habitats is probably the most critical factor that has allowed a rich assemblage of rare and sensitive species to consistently inhabit this stream, since sampling began in the region in the mid 1990’s. During the surveys conducted in 2014, sensitive species of mayflies and caddisflies were collected, a new family of amphipod was recorded, and endangered spiny crayfish were seen at the site. In the past, a wider range of rare and/or sensitive macroinvertebrates have also been collected from this creek including stoneflies (Dinotoperla brevipennis), mayflies (Atalophlebia species, Thraulophlebia conspicua and Nousia pilosa) and another caddisfly (Ulmerophlebia species). Flow-dependent blackfly larvae (Simulium ornatipes) were also recorded at the site in spring 2009. The creek is also a known habitat for rarely collected freshwater mussels (Hydridella narracanensis)) and several threatened 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.|