Middle River, Upstream Lagoon Flat
2013 Aquatic Ecosystem Condition Report
- Permanently flowing stream in autumn and spring 2013
- Sparse macroinvertebrate community with a few sensitive and flow-dependent species
- Water was fresh, clear and high in nitrogen
- Riparian vegetation comprised native vegetation with few weeds
About the location
Middle River is one of the largest streams on Kangaroo Island, located from around the middle to the northern part of the island. It rises at an elevation of about 275 m to the north from the Playford Highway near Gosse, and flows in a north-easterly direction for over 33 km before discharging into Investigator Strait at Snelling Beach. The major land uses in the 14,424 hectare catchment upstream from the site sampled were grazing modified pasture, native vegetation and nature conservation, with minor areas also used for plantation forestry, cropping, roads, wetlands, reservoir, dams and intensive animal production. The site sampled was located in the lower catchment off a track from the North Coast Road, about 1.5 km south-east from Snellings Beach.
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, fine sediment deposited in the stream and the presence of introduced marron.
A sparse community of at least 14 species of macroinvertebrates was collected or seen from the river (only 1 species in autumn and 14 in spring), 4.3-7.2 m wide and up to 70 cm deep, in autumn and spring 2013. The river consisted of a still to slow-flowing channel in both seasons sampled, although small areas of faster-flowing riffle habitats were also present in spring. No macroinvertebrates were caught in autumn and the only sign of any species present at the site were from the many crayfish holes in the sides of the bank; they were probably marron (Cherax tenuimanus) which have been seen at the site on previous occasions. The only other aquatic organisms seen in the water were hundreds of tiny zooplankton, which included copepods and ostracods. In spring, the community was more diverse and included low numbers of amphipods, beetles (3 species), mosquitoes, blackflies, chironomids (4 species), mayflies, stoneflies and caddisflies. The community included a number of commonly occurring generalist and tolerant species and several sensitive and flow-dependent species; the latter included the blackfly (Austrosimulium furiosum), chironomid (Rheotanytarsus), mayfly (Thraulophlebia inconspicua), stonefly (Dinotoperla evansi) and caddisfly (Ulmerochorema membrum).
The water was fresh to moderately fresh (salinity ranged from 899-1,466 mg/L), well oxygenated (66-105% saturation), clear and slightly coloured, and with low concentrations of phosphorus (0.01-0.02 mg/L) but high nitrogen concentrations (0.55-0.66 mg/L).
The sediments were dominated by detritus, sand, silt and gravel, with smaller amounts of clay, pebble, cobble and boulder also present. Samples taken from below the surface were black silts and clays in autumn but had a grey appearance in spring; the sediments were not sulphidic and appeared to be well aerated when sampled but the blackened silts indicate that the sediments had been anaerobic or lacking in oxygen in the past. Over 1 cm of fine silt was deposited over the channel, which caused the water to become highly turbid when disturbed. There was a large amount of bank erosion affecting over 50% of the site as a result of recent flood damage; the extensive network of marron holes in the soft banks may have contributed towards the bank collapse in the lower reach of this river. The only animal droppings seen in the vicinity of the river were from kangaroos.
There was a moderate growth of phytoplankton in autumn (chlorophyll a 4.6 μg/L) but the river did not support any significant growths of filamentous algae in 2013. About 10% of the channel was covered by rushes (Juncus), sedges (Cyperus), cumbungi (Typha) and water ribbons (Triglochin). The narrow riparian zone (<5 m wide) was dominated by gums over bracken, sedges and rushes; few weeds were noted at the site in 2013. The surrounding vegetation near the river comprised areas of sheep and cattle grazing and cropping land, with a few scattered gums in the local landscape.
Special environmental features
Middle River provides a permanently wet, freshwater habitat on the island and supports a relatively rich assemblage of aquatic life upstream from the reservoir. The downstream reach supports a less significant biota that may be degraded by the numerous marron that occur in the slow-flowing pools just upstream from the estuary.
Pressures and management responses
|Moderate nutrient and sediment inputs from diffuse sources in the catchment (leading to increased 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.|
|Introduced crayfish (marron) that have the potential to impact on aquatic biodiversity||This information is not available at the moment but it will be updated as soon as possible.|