Light River, Linwood
2016 Aquatic Ecosystem Condition Report
Permanently slow-flowing stream in autumn and spring 2016
Moderately diverse saline tolerant macroinvertebrate community with no rare or sensitive species recorded
Riparian vegetation consisted of mostly reeds and weedy species
No evidence that the 2015 Pinery fire caused any additional sediment or nutrient runoff to the river
About the location
The Light River is a large river in the Northern Mount Lofty Ranges that rises south from Waterloo in the Mid North and flows in a southerly direction past Kapunda, and then heads south-west where it eventually discharges into Gulf St Vincent near Middle Beach. The major land uses in the catchment include cereal cropping and grazing, with smaller areas used for legumes, remnant native vegetation, and various other agricultural and residential uses. The monitoring site was located near Linwood Bridge off Main North Road, about 1 km north from Linwood and 4 km south-east from Stockport. The site was selected from near the eastern edge of the 2015 Pinery fire zone to determine if any subsequent runoff caused additional sediment and nutrient enrichment of the Light River.
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, salinization and a weedy riparian zone. Only saline tolerant aquatic macroinvertebrates and plants were recorded from the site.
A moderately diverse community of at least 30 species of macroinvertebrates was collected from the river (14 species in autumn and 17 in spring), 3.8-5.5 m wide and up to 53 cm deep, in autumn and spring 2016. The river consisted of a slow-flowing channel in both seasons sampled. The community was dominated by moderate numbers of dytiscid beetles and mosquitoes in autumn and by large numbers of amphipods (Austrochiltonia and Eusiridae) in spring. It also included smaller numbers of snails, mites, yabbies, springtails, other beetles, blackflies, soldierflies, chironomids, waterbugs, damselflies, dragonflies and caddisflies. All were saline tolerant and generalist species and no rare or sensitive species were recorded. The only flow-dependent species collected was a blackfly (Simulium ornatipes), which commonly occurs in saline, flowing waters throughout South Australia.
The water was saline (salinity ranged from 6,108-7,148 mg/L), well oxygenated (74-90% saturation), clear, and with generally moderate to high concentrations of nutrients such as phosphorus (0.02-0.06 mg/L) and nitrogen (1.09-1.18 mg/L).
The sediments were dominated by detritus and silt; samples taken from below the surface were mostly grey clay and silt that did not release any sulfide when tested, but the presence of blackened rocks buried in the river indicates the sediments were at least occasionally anaerobic or lacking in oxygen. There was some evidence of bank erosion seen in spring, which was attributed to recent winter flood damage. No stock or other animal droppings were seen in the vicinity of the river.
A moderate to large amount of phytoplankton was recorded during the year (chlorophyll a ranged from 8-18.1 μg/L) but less than 10% of the river was covered by filamentous algae (Cladophora, Spirogyra and Enteromorpha). Over 35% of the channel was also covered by aquatic plants, including large stands of reeds (Phragmites) and patches of sedges (Bolboschoenus) and rushes (Juncus). The riparian zone extended over 30 m wide in places and comprised reeds, lignum, some scattered gums and significant amounts of weedy grasses, castor oil plants and wild roses. The surrounding landscape was mostly cleared cropping and grazing land with a few isolated gum trees.
Special environmental features
Pressures and management responses
Nutrient inputs to the creek from numerous diffuse sources (potentially leading to excess growth of algae and aquatic weeds)
The Adelaide and Mount Lofty Ranges NRM Board land management program encourages and promotes managing land to improve water quality. This includes working with industry and landholders to ensure efficient use of fertilisers and discuss ways to reduce runoff of nutrients into waterways.
Saline groundwater inflows to the creek
The Adelaide and Mount Lofty Ranges NRM Board has installed telemetered groundwater monitoring stations at key locations within the region. These are monitored for level and salinity; unusual results (such as high salinity influxes) are investigated.
Widespread introduced weeds in the riparian zone at the site and upstream
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.
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.