Chapman River, SW Lashmar Lagoon
2013 Aquatic Ecosystem Condition Report
- Permanently wet stream in autumn and spring 2013
- Moderately diverse macroinvertebrate community with no rare, significant or flow-dependent species
- Water was saline, clear and high in nitrogen
- Riparian vegetation comprised a few native trees over rushes and introduced weeds and grasses
About the location
Chapman River is a moderately sized stream on the Dudley Peninsula at the eastern end of Kangaroo Island. The river rises near Woodleigh, north from Willson River Road, and flows in an east to north-easterly direction for nearly 10 km until it discharges into the Southern Ocean at Antechamber Bay. The major land uses in the 5,917 hectare catchment upstream from the site sampled included grazing modified pastures (39%), other minimal uses (31%) and nature conservation (26%), with smaller areas also used for cropping, roads and irrigated horticulture. The site was located in the middle reaches off a track from Willson River Road, about 3 km south-west from Lashmar Lagoon.
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 evidence of human disturbance due to high salinity, nitrogen enrichment, fine sediment deposition and the presence of a weedy riparian zone.
A moderately diverse community of at least 24 species of macroinvertebrates was collected or seen from the river (16 species in autumn and 18 in spring), 1.3-7.4 m wide and over 1 m deep in places, in autumn and spring 2013. The river consisted of shallow slow-flowing pool habitats in both seasons sampled; small areas of faster-flowing riffle were also present in autumn. The community was not dominated by any species but included low to moderate numbers of worms, water mites (Arrenurus and Koenikea), amphipods, yabbies, springtails, beetles, mosquitoes, soldierflies, chironomids, waterbugs, damselflies and caddisflies (including Oecetis and Triplectides australis). The presence of crayfish holes in the banks indicated that yabbies commonly occurred at the site. These macroinvertebrates are all tolerant or generalist species that are frequently found in other brackish to saline, nutrient enriched streams elsewhere on the island and in South Australia. No rare, sensitive or flow-dependent species were collected and the site also lacked many groups that commonly occur in the wetter parts of the State, including snails, shrimp, mayflies, stoneflies and dragonflies. The only fish recorded from the site was a single Blue-spotted Goby collected in autumn; this is an estuarine species that frequents well-connected coastal streams and lakes throughout the southern part of the State.
The water was saline (salinity ranged from 6,901-7,340 mg/L), well oxygenated (79-88% saturation), clear and slightly coloured, and with low concentrations of phosphorus (0.01-0.02 mg/L) and high nitrogen concentrations (0.59-0.86 mg/L).
The sediments were dominated by detritus and sand, with smaller amounts of gravel, silt and clay also present. Samples taken from below the surface were grey sands that were black in places and released sulphide when tested, indicating that the sediments were, at least occasionally, anaerobic or lacking in oxygen. Over 5 cm of silt covered the stream bed in spring, indicating that large loads of fine sediment had been deposited at the site following recent winter flows in the river. There was a moderate amount of bank erosion noted over more than 10% of the site, which appeared to have been caused by cattle accessing and damaging the poorly vegetated banks. The only animal droppings seen in the channel and on the banks were from cattle.
There was a small to moderate amount of phytoplankton recorded (chlorophyll a 3.2-6.5 μg/L) but no filamentous algae was seen at the site during 2013. Aquatic plants covered over 10% of the channel and included a sedge (Bolboschoenus) and rush (Juncus). The riparian zone extended over more than 5 m in width and was dominated by a few gums and wattles over rushes, introduced grasses and weeds. The surrounding vegetation near the river comprised cleared cattle grazing country with a few patches of remnant native woodland remaining in the local landscape.
Special environmental features
The only significant environmental value of this site was the presence of a moderately diverse community of commonly found, generalist, saline-tolerant species.
Pressures and management responses
|Widespread introduced weeds in the riparian zone at the site and upstream (reducing habitat quality).|
|Saline groundwater inflows to the creek (reducing ecological integrity).|
|Large nutrient and sediment inputs from diffuse sources in the catchment (leading to extensive growth of algae and aquatic weeds as well as increased turbidity and smothering of habitat).||The Kangaroo Island NRM Board has funded the fencing of significant areas of riparian vegetation in the catchment and continues to work with landowners to increase the fencing of watercourses.|
|Livestock have direct access to the creek, causing sediment erosion and adding excessive nutrients (which leads to habitat disturbance, algal growth and aquatic weeds).||The Kangaroo Island NRM Board has funded the fencing of significant areas of riparian vegetation in the catchment and continues to work with landowners to increase the fencing of watercourses.|