Tod River, Whites Flat
2010 Aquatic Ecosystem Condition Report
- Permanently wet, saline, slow-flowing stream in autumn and spring.
- Sparse macroinvertebrate community with at least one rare species.
- Obvious signs of moderate nutrient enrichment.
- Riparian vegetation largely consisted of introduced grasses and weeds.
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 stock grazing and 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 on Gawler Ponds Road, about one kilometre south of Whites Flat.
The river was given a Fair rating because the site sampled showed moderate changes in ecosystem structure and some changes to the way the ecosystem functions. There was evidence of human disturbance, including nutrient enrichment and poor riparian habitat, although the stream still provided an important refuge for at least one rare species of macroinvertebrate.
A sparse to moderately diverse community of about 29 species of macroinvertebrates was collected from the slow-flowing channel, four metres wide and 20 centimetres deep, in autumn and spring 2010. Sections of faster flowing riffle habitats were present in both seasons but were only extensive enough to sample in spring. The community was dominated by species tolerant to poor water quality and high salinity, including large numbers of amphipods, blackfly larvae, chironomids and caddisflies (Notalina). Smaller numbers of beetles, damselflies, dragonflies and introduced mosquitofish were also collected. A flow-dependent caddisfly (Cheumatopsyche), rare to Eyre Peninsula, was also collected from the site. A small school of native fish were seen at the site that may have been a threatened species called the Climbing Galaxias.
The water was saline (salinity of 7,294 mg/L in autumn and 5,377 in spring mg/L), well oxygenated (87–101% saturation) and clear, with moderate to high concentrations of nutrients such as nitrogen (0.84–1.25 mg/L) and phosphorus (0.04–0.1 mg/L).
The sediments were dominated by gravel and sand, with some algae and detritus also present in the non-flowing areas. There were no signs that the sediment was blackened or lacking in oxygen.
Large amounts of phytoplankton were present in autumn and filamentous algae (Cladophora and Spirogyra) covered more than 35% of the site in spring. Over 90% of the channel was covered by emergent aquatic plants (Bolboschoenus, Mimulus, Cotula, Crassula, Juncus and Rumex).
The riparian zone consisted of introduced grasses, weeds, rushes (Juncus), sedges (Baumea), samphire and dead trees and shrubs. The surrounding vegetation was cropping and grazing land.
Special environmental features
Tod River at Whites Flat provides habitat for a rare, flow-obligate caddisfly (Cheumatopsyche) and two other more commonly occurring flow-dependent macroinvertebrate species (blackfly Simulium ornatipes and dytiscid beetle Platynectes decempunctatus) for the region.
The site also supports a native galaxiid fish species that may have been the threatened Climbing Galaxias; further work will need to be conducted to confirm that this species occurs in the lower reaches of the catchment.
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).|
|Saline groundwater inflow (reducing ecological integrity).||The Eyre Peninsula NRM Board promotes the revegetation of recharge areas known to contribute to dryland salinity and encourages the adoption of perennial pastures as an alternative to annual cropping in these areas.|