Breakneck River, Kangaroo Island
2008 Aquatic Ecosystem Condition Report
- Stream and riparian zone affected by bushfire.
- High levels of nutrient and fine sediment, most likely due to ash from the fire.
- Lower than expected diversity of macroinvertebrates.
- Environmentally significant river which provides habitat to rare and sensitive species.
About the location
The Breakneck River catchment lies entirely within Flinders Chase National Park at the western end of Kangaroo Island. The area is relatively undisturbed, apart from an unsealed track which crosses the creek, and the occasional activities of feral pigs. Most of the catchment was burnt out by a bushfire in December 2007. The site selected for monitoring lies off West Bay Road.
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. Evidence confirmed moderately high nutrient levels, most likely as a result of the bushfire. As the riparian zone recovers and accumulated ash deposits in the river are washed away with winter floods, the ecological condition of the stream should markedly improve over the next few years.
Moderately fast-flowing riffles connected shallow, non-flowing pools at the site when it was sampled almost a year after the fire.
A moderately diverse community of at least 39 species of macroinvertebrates was collected from the edge of the pools, and 25 species from the flowing riffles. Species tolerant to high nutrient levels dominated the community in both habitats, particularly chironomids (Tanytarsus and Chironomus) and blackfly larvae (Austrosimulium). Small numbers of more sensitive and rare species were also collected, including fly larvae from the Empididae family, mayflies and caddisflies.
No crustaceans, roundworms, molluscs, mites, stoneflies or dragonflies were found although this type of stream would normally support them. A wider range of these species should recolonise the stream as the area recovers from the bushfire and the environmental conditions improve.
The water was fresh (salinity of 445 mg/L) and well oxygenated (92% saturated). It was slightly cloudy or turbid, strongly coloured from natural tannins, and had moderate to high concentrations of nutrients such as nitrogen (0.9 mg/L), phosphorus (0.03 mg/L) and organic carbon (16 mg/L).
Up to 35% of the river was covered with aquatic plants such as stonewort (Nitella) and Water Ribbons (Triglochin procerum). A few watercress plants (Rorippa nasturtium-aquaticum) were the only evidence of any introduced species. Most vegetation in the surrounding area was either dead because of the bushfire, or regenerating native species.
Sand, silt and detritus dominated sediments found on the riverbed at the water’s edge; gravel, sand and detritus were found in the riffles. The detritus was largely fine brown, organic ash from the bushfire. The sediments were not lacking in oxygen or sulfidic despite the large amount of organic matter present in the channel.
Special environmental features
Breakneck River provides habitat to several rare and sensitive species which highlight its potential environment significance. They included a mayfly (Centroptilum elongatum) and several types of caddisflies (Taschorema evansi, Oxyethira columba, Lectrides varians and Hellyethira species) typically found together in wetter, more biodiverse streams in the southern part of South Australia.
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.|
|Bushfire in the catchment, causing sediment erosion (habitat disturbance).||Bushfires are disturbances that can occur naturally. The affected area can take approximately five years or more to naturally recover.|
|Feral pigs in the catchment causing sediment erosion and adding excessive nutrients (leading to habitat disturbance, algal growth and aquatic weeds).||The Kangaroo Island NRM Board continues to work with landowners to reduce the impact of pigs on riparian vegetation by trialling coordinated pig management. They have trialled a number of new control options including Pigout baits and bait delivery systems such as the Boerbuffets and Hog hoppers developed by the Invasive Animal CRC but these were not reliably effective on Kangaroo Island.|