Harriet River, NE from Mt Taylor
2013 Aquatic Ecosystem Condition Report
- Permanently wet stream in autumn and spring 2013
- Moderately diverse macroinvertebrate community with two sensitive, flow-dependent species present
- Water was fresh, turbid in autumn but clear in spring, and enriched with nitrogen
- Riparian vegetation comprised native trees over rushes and sedges
About the location
Harriet River is a large stream that drains the south-central part of Kangaroo Island. The river rises about 15 km west from Parndana in the centre of the island and flows in a southerly direction until it discharges into the Southern Ocean at Vivonne Bay. Major land uses in the 6,479 hectares upstream from the site included grazing modified pastures (45%), plantation forestry (26%) and other minimal uses (21%), and included smaller areas of cropping, irrigated seasonal horticulture, roads, dams and intensive animal production. The site sampled from the middle of the catchment was located downstream from East West Two Highway, about 4.5 km north-east from Mount Taylor and 14 km north-west from Vivonne Bay.
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 deposition of fine sediment in the channel.
A moderately diverse community of at least 24 species of macroinvertebrates was collected or seen from the river (13 species in autumn and 18 in spring), 6.7-7.8 m wide and up to 59 cm deep, in autumn and spring 2013. The river consisted of a non-flowing channel in both seasons sampled. The community was not dominated by any species but included low to moderate numbers of introduced snails (Potamopyrgus), amphipods (Austrochiltonia australis), beetles (including Copelatus and Antiporus), mosquitoes (Aedes), chironomids (including Tanytarsus, Procladius and Larsia), mayflies (Atalophlebia australasica, Thraulophlebia inconspicua and Tasmanocoenis tillyardi), waterbugs (including Hydrometra) and caddisflies (Notalina spira). Yabby holes were present at the site, indicating that they also occurred in the river despite not being collected. Most of the species collected were tolerant, generalist insects, crustaceans and snails that frequent fresh to brackish, nutrient enriched streams elsewhere in South Australia. The only sensitive species collected were two mayflies (Atalophlebia and Thraulophlebia) which are generally found in riffle habitats from clear, freshwater streams in the State.
The water was fresh (salinity ranged from 589-995 mg/L), well oxygenated (88-102% saturation), slightly coloured and highly turbid in autumn but clear in spring, and with low to moderate concentrations of phosphorus (0.02-0.03 mg/L) and high nitrogen concentrations (0.85-1.27 mg/L); the presence of a significant amount of oxidised nitrogen (0.3 mg/L) in autumn indicates that groundwater baseflows were probably entering the river near the site.
The sediments were dominated by detritus, clay and sand, with smaller amounts of silt, gravel, boulder, cobble and pebble also present. Samples taken from below the surface were well-aerated gravels in autumn, whereas in spring the sediments were slightly blackened sands and gravels that released sulphide when tested, indicating that they were anaerobic and lacked oxygen. Over 1 cm of silt covered the bed of the river in spring, suggesting that winter rains had mobilised and deposited large loads of fine sediment during recent winter rains. There were no signs of any significant areas of bank erosion and the only animal droppings seen in the vicinity of the site were from kangaroos.
There was a small amount of phytoplankton recorded (chlorophyll a ranged from 1.8-2.5 μg/L) but no filamentous algae was seen at the site during 2013. The only aquatic plant seen in the river was a rush (Juncus) which covered less than 10% of the channel. The riparian zone was dominated by gums and wattles over patches of sedges (Carex) and rush. The surrounding vegetation comprised open woodland with a native understorey near the river, and cleared sheep grazing paddocks elsewhere in the local landscape.
Special environmental features
The major environmental value of the site was the presence of the two sensitive species of mayflies from flowing rocky pool habitats in spring 2013.
Pressures and management responses
|Moderate 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.