Inman River, 2 km upstream from Victor Harbor Golf Club
2011 Aquatic Ecosystem Condition Report
- Permanently wet, moderately fresh, still to slow-flowing stream in autumn and spring 2011
- Sparse macroinvertebrate community with a few rare, sensitive and flow-dependent species
- Obvious signs of gross nutrient enrichment
- Riparian vegetation dominated by introduced grasses and a few gum trees.
About the location
Inman River is a large stream in the Southern Mount Lofty Ranges that rises near Inman Valley and flows in a south-easterly direction before discharging into Encounter Bay at Victor Harbor. The monitoring site was located off the Inman Valley Road, just upstream from the junction with Swain’s Crossing Road and about 2 km west from Victor Harbor on the Fleurieu Peninsula. The major land uses in the 17,631 hectare catchment are stock grazing and remnant native vegetation, with smaller areas used for conservation, roads, cropping, rural residential living and minor irrigated horticultural activities.
The river 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 nutrient enrichment, degraded riparian vegetation and fine sediment deposition in the channel.
A sparse community of at least 26 species of macroinvertebrates was collected from the river, 4.8-7.9 m wide and up to 34 cm deep, in autumn and spring 2011. The river consisted of a still to slightly flowing channel and lacked any significant areas of faster-flowing riffle habitats in the two seasons sampled. The community was dominated by generalists and species tolerant to poor water quality such as chironomids, waterbugs (Micronecta) and blackflies. It also included amphipods, freshwater shrimp, springtails, beetles, biting midges, soldierflies, mayflies, odonates, stoneflies and caddisflies. Two rare or sensitive species were present in very low numbers, including a blackfly (Austrosimulium furiosum) and stonefly (Dinotoperla evansi). Several species normally associated with flowing water were collected, including a blackfly (Simulium ornatipes), chironomid (Rheotanytarsus), caddisfly (Cheumatopsyche sp. 2), and the above-listed rare and sensitive species. The only fish seen at the site included an introduced freshwater pest (Mosquitofish) and an estuarine native species (Western Blue-spot Goby).
The water was fresh to moderately fresh (salinity ranged from 919 mg/L in autumn to 1,413 mg/L in spring), well oxygenated (81-118% saturated), slightly turbid and coloured, and with high concentrations of nutrients such as nitrogen (1.41-1.43 mg/L) and phosphorus (0.15-0.18 mg/L).
The sediments were dominated by detritus, silt, sand, gravel and algae. Samples taken from below the surface were silt grey in appearance and sulfidic, indicating that the sediments were anaerobic or lacked oxygen. A large deposit of silt, 1-5 cm deep in places, covered the streambed and over 10 m of bank showed evidence of erosion due to cattle damage.
A large amount of phytoplankton was present in spring, at a time when nearly 10% of the channel was covered by filamentous algae (mostly Cladophora). Several types of aquatic plants covered over 10% of the site and included sedges (Cyperus and Isolepis), rushes (Juncus), reeds (Phragmites), dock (Rumex) and water ribbons (Triglochin). The narrow riparian zone was dominated by introduced grasses and weeds under a few scattered River Red Gums. The surrounding vegetation was cleared grazing land with only a few isolated gum trees remaining in the local landscape.
Special environmental features
The Inman River upstream from Victor Harbor supports low numbers of several rare, sensitive and flow-dependent species of macroinvertebrates. The mid to lower reaches of the Inman River and Back Valley Creek provides habitat for several threatened and common native fish species, including Southern Pygmy Perch, Climbing Galaxias, Murray-Darling Carp Gudgeon, Common Galaxias and Western Blue-spot Goby. However, recent sampling during drought years in 2007-09 failed to detect some of these species in the catchment (M. Hammer, Aquasave Consultants, 2009).
Pressures and management responses
|Widespread introduced weeds in the riparian zone at the site and upstream (reducing habitat quality).||The Adelaide and Mount Lofty Ranges NRM Board has several pest plant (weed) mitigation and control programs. They work closely with landholders to control weeds on their property and to help stop the spread to other properties and waterways.|
|Livestock having direct access at the site and upstream (causing sediment erosion and adding excessive nutrients).||The Adelaide and Mount Lofty Ranges NRM Board’s land management program encourages and promotes managing land to improve water quality. This includes incentives for waterway and wetland fencing to exclude or limit stock from entering riparian zones.|
|Limited riparian zone vegetation at the creek and upstream (reducing habitat quality, increasing sediment erosion).||The Adelaide and Mount Lofty Ranges NRM Board’s land management program encourages and promotes managing land to improve water quality. This includes incentives for revegetation programs around waterways and wetlands and stock exclusion as well as educating landholders about the importance of riparian vegetation in managing soil erosion.|
This aquatic ecosystem condition report is based on monitoring data collected by the EPA. It was prepared with and co-funded by the Adelaide and Mount Lofty Ranges NRM Board.