Tod River, near North Shields
2010 Aquatic Ecosystem Condition Report
- Permanently wet, slow-flowing, saline stream in autumn and spring.
- Sparse macroinvertebrate community with no rare or sensitive species.
- Obvious signs of gross nutrient enrichment.
- Riparian vegetation consisting of native trees and introduced grasses.
About the location
Tod River is the major surface water supply catchment in Eyre Peninsula. It rises north of Yallunda Flat and flows in a southerly direction before discharging into Spencer Gulf at the southern end of Louth Bay. The major land uses are sheep and cattle grazing, cropping, and minor areas of native vegetation and urban settlement.
The Tod Reservoir, located between Koppio and Whites Flat, is a large off-stream dam that receives water via concrete aqueducts from Pillaworta Creek and the Tod River. The reservoir has not been used to supply water to the region for over a decade due to the high salinity of water in the river and reservoir.
The monitoring site was located in the lower reaches of the river near the Lincoln Highway, about four kilometres north of North Shields.
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 and limited vegetation in the riparian zone.
A sparse community of about 21 species of macroinvertebrates was collected from the slow-flowing channel, 6–10 metres wide and over one metre deep, in autumn and spring 2010. The community was dominated by large numbers of species tolerant to poor water quality such as amphipods and chironomids. Snails, mites, springtails, beetles, blackfly larvae, damselflies and leptocerid caddisflies were also present. The presence of two flow-dependent species (blackfly Simulium ornatipes and dytiscid beetle Platynectes decempunctatus) from the edge habitat in spring indicates that there was sufficient water movement to sustain these species despite the absence of their more preferred, fast-flowing riffle habitats. No sensitive or rare macroinvertebrate species were found but native fish were seen among introduced mosquitofish in both autumn and spring.
The water was saline (salinity of 6,860 mg/L in autumn and 4,390 mg/L in spring), well oxygenated (74–91% saturation), slightly turbid and coloured, with moderate to high concentrations of nutrients such as nitrogen (0.94–1.02 mg/L) and phosphorus (0.02–0.04 mg/L).
The sediments were dominated by detritus but included smaller amounts of gravel, silt and algae. There were no indications that the sediment was lacking in oxygen in autumn but sulfide was present in spring, indicating that the decomposition of organic matter was creating anaerobic sediments below the surface.
A moderate growth of phytoplankton was present in autumn and filamentous algae (Enteromorpha) covered more than 10% of the site in spring. Several types of emergent aquatic plants (Eleocharis, Schoenoplectus, Bolboschoenus, Cotula, Juncus, Phragmites, Isolepis and Rumex) covered over 65% of the channel and water’s edge in spring. These plant indicators highlight the wide range of biological responses that can occur in response to the obvious nutrient enrichment of the catchment.
The riparian zone consisted of gum trees, paper barks, acacias and tea-trees over an understorey of introduced grasses. The surrounding vegetation at the site was cropping land.
Special environmental features
The downstream extent of the Tod River provides habitat for a common species of native fish called the Common Galaxias (Galaxias maculatus). Further upstream, the threatened Climbing Galaxias (Galaxias brevipinnis) was possibly seen at Whites Flat in 2010, which suggests that the river is probably one of the most important habitats for native freshwater fish in the region.
Pressures and management responses
|Livestock have direct access at the site and upstream in the catchment, causing sediment erosion and adding excessive nutrients (which leads to habitat disturbance, algal growth and aquatic weeds).||
The Eyre Peninsula NRM Board promotes managing land to improve water quality. This includes incentives for:
|Limited natural riparian vegetation at the site and upstream in the catchment, providing minimal buffer protection from catchment landuses (reducing habitat quality).|