Stunsail Boom River, Gauge Station
2013 Aquatic Ecosystem Condition Report
- Permanently wet stream in autumn and spring 2013
- Sparse macroinvertebrate community with one rare and one sensitive, flow-dependent species recorded
- Water was moderately fresh, clear and high in nitrogen
- Riparian vegetation comprised native trees over rushes and bracken
About the location
Stun’sail Boom River is a large stream that drains a large part of the south-western side of Kangaroo Island. It rises as the North-East and North-West rivers within Flinders Chase National Park, and flows southwards where the streams merge to form the Stun’sail Boom, which then flows for about 8 km before discharging into the Southern Ocean. The major land-uses in the 21,239 hectare catchment upstream from the site sampled included nature conservation (43%), grazing modified pastures (19%), plantation forestry (19%) and other minimal uses (13%), with minor areas also used for cropping, roads, wetlands, dams, intensive animal production, irrigated horticulture and residential living. The site sampled was located near the gauge station downstream from South Coast Road in the lower catchment, about 5.5 km south-east from Karatta.
The river was given a Fair rating because the site sampled showed evidence of moderate changes in ecosystem structure and some changes to the way the ecosystem functions. There was evidence of human disturbance due to nitrogen enrichment and the presence of marron.
A sparse community of at least 19 species of macroinvertebrates was collected or seen from the river (only 7 species in autumn and 14 in spring), 8-10 m wide and over 1 m deep, in autumn and spring 2013. The river consisted of a slow-flowing channel in both seasons sampled. The community was not dominated by any species in autumn but included low to moderate numbers of amphipods (Austrochiltonia), mosquitoes (Anopheles and Culex) and mayflies (Thraulophlebia inconspicua) in spring. The community also included low numbers of worms, mites (Hygrobatidae and Koenikea), beetles (Liodessus and Paracymus), biting midges (including Alluauodomyia), chironomids (Paralimnophyes pale and dark species, Tanytarsus and Cladopelma) and waterbugs (Microvelia, Micronecta annae and Anisops deani). Freshwater crayfish burrows were also seen at the site, which were assumed to be signs that introduced marron occurred in the river. These species are mostly tolerant, generalist macroinvertebrates that frequent fresh to brackish, nutrient enriched streams elsewhere on the island and from the southern part of South Australia. The only rarely collected species was the biting midge (Alluauodomyia) and the mayfly was the only sensitive macroinvertebrate recorded from the site in 2013 that also frequents flowing water habitats. The site was notable for the unexpected absence of many macroinvertebrates that commonly occur in other freshwater streams in the State, including snails, shrimp, stoneflies, odonates and caddisflies.
The water was moderately fresh (salinity ranged from 1,374-2,325 mg/L), well oxygenated (85-90% saturation), clear and slightly coloured, and with low to moderate concentrations of phosphorus (0.02-0.03 mg/L) and high nitrogen concentrations (0.86-0.96 mg/L).
The sediments were dominated by detritus, sand and clay, with a smaller amount of silt also present. Samples taken from below the surface were grey silts that showed no evidence to indicate the sediments had recently been anaerobic or lacking in oxygen. Over 1 cm of fine silt covered the bottom of the stream in spring; presumably deposited following higher winter flows in the river. There was no evidence of any significant areas of bank erosion noted at the site and the only animal droppings seen in the vicinity of the stream were from kangaroos.
There was a small amount of phytoplankton (chlorophyll a ranged from 0.9-1 μg/L) present but no filamentous algae was seen at the site during 2013. The only aquatic plants seen in the river were a rush (Juncus) and Water Ribbons (Triglochin), which covered over 10% of the channel. The riparian zone was dominated by gums and wattles over patches of rush and bracken. The surrounding vegetation near the river comprised dense eucalypt woodland.
Special environmental features
The only significant environmental values noted in 2013 were the records of the rarely collected biting midge and the sensitive mayfly during the spring sampling period.
Pressures and management responses
|Moderate to 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.