Myponga River, Myponga Pumpshed
2016 Aquatic Ecosystem Condition Report
Permanently flowing stream in autumn and spring 2016
Diverse macroinvertebrate community with many rare, sensitive and flow-dependent species recorded
Water was fresh, clear and showed signs of nutrient enrichment despite low to moderate concentrations recorded from the stream
Riparian vegetation consisted of mostly native reeds, sedges and gum trees and introduced grasses
About the location
The Myponga River is a relatively large stream on the Fleurieu Peninsula that rises south from Willunga at an elevation of about 350 m, and flows south-west where it flows into Myponga Reservoir, before eventually turning north-west and discharging into Gulf St Vincent at Myponga Beach. The major land uses in the 12,876 hectare catchment upstream from the site sampled were stock grazing (63%) and native vegetation (10%), with minor areas used for forestry, cropping, irrigated pasture, rural living, roads to the reservoir (ca 2%). The monitoring site was located downstream from the reservoir in the lower reaches of the river, off a track from Myponga Beach Road, about 500 m upstream from Myponga Beach.
The river was given a Good rating because the site sampled showed evidence of relatively minor changes in ecosystem structure and function. There was minor evidence of human disturbance due to the presence of indicators of nutrient enrichment (eg filamentous algae and extensive growths of aquatic plants) but the stream provides habitat for a large number of macroinvertebrates, including several significant species.
A diverse community of at least 51 species of macroinvertebrates was collected from the river (36 species in autumn and 27 in spring), 2.2-8.9 m wide and over 1 m deep, in autumn and spring 2016. The river consisted of mostly slow-flowing to still pools (80-90%) connected by smaller areas of faster-flowing riffles (10-20%). The community was dominated by amphipods, worms and chironomids. It also included smaller numbers of flatworms, snails (including introduced Potamopyrgus and Physiella), yabbies, beetles, craneflies, dixids, mosquitoes, mayflies, waterbugs, damselflies, dragonflies, stoneflies and caddisflies. Most were generalist and tolerant species but the site provided habitat for several rare, sensitive and flow-dependent species, including a chironomid (Riethia), dragonfly (Austrogomphus), stonefly (Dinotoperla), blackflies (Austrosimulium and Simulium), mayflies (Offadens, Thraulophlebia and Atalophlebia australasica) and caddisflies (Taschorema evansi, Taschorema complex, Cheumatopsyche, Lingora and Anisocentropus). The only fish seen at the site were some small native galaxiids, which were particularly common in spring.
The water was moderately fresh (salinity ranged from 1,203-1,319mg/L), well oxygenated (68-78% saturation), clear, and with generally low to moderate concentrations of nutrients such as phosphorus (0.014-0.015 mg/L) and nitrogen (0.27-0.42 mg/L).
The sediments were dominated by detritus, cobble and sand; samples taken from below the surface were mostly grey clay and silt that released sulfide when tested, indicating that the sediments were anaerobic and lacking in oxygen. There was no evidence of any significant bank erosion seen during either survey. The presence of sheep droppings within the channel and on the banks indicates that stock have regular access to the lower reaches of the river.
Only a small amount of phytoplankton was recorded from the river (chlorophyll a ranged from <0.1-1.84 μg/L) but large growths of filamentous algae (Cladophora and Spirogyra) covered more than 10% of the channel in spring. Over 35% of the river was also covered by aquatic plants, including sedges (Cyperus), reeds (Phragmites), cumbungi (Typha) and introduced watercress (Rorippa) and dock (Rumex). The riparian zone extended over 30 m wide in places and comprised reeds, sedges and introduced grasses under a few scattered gum trees. The surrounding landscape was largely cleared sheep grazing land with a few isolated gums and deciduous trees.
Special environmental features
The site provided habitat for a rich assemblage of macroinvertebrates, including a number of rare and sensitive species, and at least one native fish species.
Pressures and management responses
Nutrient inputs to the creek from numerous diffuse sources (potentially leading to 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.
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.