Marne River, near Walker Flat
2010 Aquatic Ecosystem Condition Report
- Dry in autumn and slow-flowing, saline stream in spring.
- Sparse macroinvertebrate community with no rare or sensitive species.
- Obvious signs of gross nutrient enrichment.
- Riparian vegetation mostly reeds and weeds.
About the location
Marne River is a large stream in the eastern Mount Lofty Ranges that rises west of Springton and flows in an easterly direction where it eventually discharges into the River Murray north of Walker Flat. The major land uses are sheep grazing and areas of native vegetation.
The monitoring site was located on Mannum Road, about six kilometres north-west of Walker Flat.
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, fine sediment deposition and poor vegetation cover in the riparian zone.
A sparse community of only about 10 species of macroinvertebrates was collected from the slow-flowing river, seven metres wide and 40 centimetres deep, in spring 2010; the site was dry in autumn. The community was dominated by large numbers of mosquito larvae and included smaller numbers of mites, beetles, chironomids and waterbugs. The only non-insect collected was a mite from the Family Hydryphantidae. The insects included many early colonising species that frequent temporary, saline habitats such as culicid mosquitoes (Aedes), chironomids (Tanytarsus and Dicrotendipes) and beetles (Eretes, Laccobius and Limnoxenus). No sensitive or rare species were found.
The water was saline (salinity of 4,482 mg/L), well oxygenated (70% saturation) and slightly coloured, with high concentrations of nutrients such as nitrogen (1.91 mg/L) and phosphorus (0.12 mg/L).
The sediments were dominated by algae, silt and detritus, with smaller amounts of gravel and sand also present; samples taken from below the surface were blackened and anaerobic, indicating that too much organic matter had entered the river in the past. Silt deposits between 1–5 centimetres thick were recorded from the riverbed in places.
More than 65% of the site was covered by filamentous algae (Cladophora) and emergent aquatic plants (Juncus, Phragmites, Rumex and Typha).
The riparian zone consisted of extensive growths of an introduced weed called the Sharp Rush (Juncus acutus) and Common Reed (Phragmites australis), with some thistles, gum trees and acacias also present. The surrounding vegetation was mostly native woodland dominated by gums and acacias.
Special environmental features
None detected, however, recent fish surveys in the catchment have recorded the presence of the two threatened fish species (Mountain Galaxias and River Blackfish) in stream-fed, freshwater refuge pools further upstream (M Hammer, Aquasave Consultants, 2009).
Pressures and management responses
|Livestock have direct access at the site and upstream, causing sediment erosion and adding excessive nutrients (which leads to habitat disturbance, algal growth and aquatic weeds).||The SA Murray–Darling Basin NRM Board acknowledges the significant impacts that livestock have on aquatic environments and seeks to provide free technical advice and incentives to land managers for fencing and other works as funding permits. Funding incentives are limited in value and extent and require land managers to volunteer to be involved.|
|Limited riparian vegetation at the site and upstream, providing minimal buffer protection from catchment landuses (reducing habitat quality).||The SA Murray–Darling Basin NRM Board recognises that the management of riparian vegetation requires a long-term, integrated approach to achieve ecosystem benefits. The board therefore provides free technical advice on a range of topics for land managers and various incentives for works as funding permits.|
|Insufficient natural water flows resulting from water extraction and climate variability (reducing ecological integrity.||The SA Murray–Darling Basin NRM Board has developed a water allocation plan and licensing system that aims to balance social, economic and environmental needs for water. The objective for providing water to the environment is to maintain and/or restore self-sustaining water-dependent ecosystems.|
|Saline groundwater inflow (reducing ecological integrity).||Saline groundwater inflows may be exacerbated by two things; vegetation clearing and resultant increase in rainfall recharge, or the extraction of surface water reducing the dilution factor in natural saline discharge zones. The SA Murray–Darling Basin NRM Board’s Land Management Program strategically invests in salinity ‘hotspots’ by providing incentives to land managers to plant perennial pasture/fodder crops/ or revegetation to reduce recharge. The NRM Board works with various agencies to minimise any further vegetation clearing which may impact on the catchment’s water balance. The NRM Board seeks to manage and provide for environmental flows to allow natural dilution of saline waters through the development of Water Allocation Plans and Water Affecting Activity policies across the region.|