Stunsail Boom River, Gauge Station
2008 Aquatic Ecosystem Condition Report
- High nutrient levels affecting animal and plant life.
- Macroinvertebrate community dominated by species tolerant of pollution.
- Large growths of aquatic plants despite heavy shading.
- Substantial riparian zone well vegetated with native plants.
About the location
Stunsail Boom River forms where two rivers merge on the south coast of Kangaroo Island. The North West River starts in Flinders Chase National Park and flows through cleared grazing land in the lower reaches of the catchment. The North East River flows mainly through grazing land, with only a few small tributaries bringing water from the national park.
The site selected for monitoring was located off South Coast Road, at Stunsail Boom.
The river was given a Fair rating at this site because the ecosystem showed evidence of moderate changes to animal and plant life, and some changes to the way the ecosystem functions. There was evidence of high nutrient levels and some erosion, and large growths of aquatic plants despite heavy shading of the stream by the surrounding vegetation.
Slow-flowing pools provided habitat for a moderately diverse community of 32 species of macroinvertebrates when samples were taken in December 2008. Beetles dominated the community, particularly adult Minute Rove Beetles from the family Hydraenidae (Ochthebius) and larval Marsh Beetles from the family Scirtidae, however most species were present in very low numbers. The only rare or sensitive species collected were small numbers of one mayfly (Koorrnonga inconspicua).
The water was moderately fresh (salinity of 1,001 mg/L) and well oxygenated (97% saturation). It was cloudy or turbid, strongly coloured with tannins, and carried moderate to high concentrations of nutrients such as nitrogen (0.8 mg/L), phosphorus (0.03 mg/L) and organic carbon (17.7 mg/L).
Sediments from the riverbed were mostly clay and detritus, with small amounts of fine silt; they were anaerobic and sulfidic in places. More than 10% of the riverbank showed evidence of erosion, most likely caused by occasional periods of high flow after heavy rainfall.
Dense shade over most of the river inhibited algal growth. However, aquatic plants such as Water Ribbons (Triglochin), rushes (Juncus) and Waterbuttons (Cotula) were common, covering up to 65% of the river’s edge. Most of the riparian and surrounding vegetation was native woodland, with little evidence of weeds.
Special environmental features
Stunsail Boom River provides habitat for at least one notable species of mayfly (Koorrnonga inconspicua), and an unexpectedly large number of hydraenid beetles (Ochthebius), which are normally only found in low numbers in the more southerly areas of South Australia. In the upstream catchment, the river’s riparian zones are well vegetated with mainly native plants, a rare occurrence in the region outside national or conservation parks.
Pressures and management responses
|Drought in the catchment, reducing natural water flows (reducing ecological integrity).||The Kangaroo Island NRM Board seeks to manage a sustainable water supply for the region so that there is enough water available for everyone (including the environment) even in drought conditions.|
|Livestock have direct access at the site and upstream, 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.|
|Large nutrient inputs from numerous diffuse sources in the catchment (leading to extensive growth of algae 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.|
|Altered flow regime resulting from catchment clearing or modification (reducing ecological integrity).||The Regional NRM Plan includes a target to address surface water flow management.|