Middle River, NNW from Gosse
2013 Aquatic Ecosystem Condition Report
- Permanently wet with isolated pools in autumn and slightly flowing channel in spring 2013
- Moderately diverse macroinvertebrate community with one sensitive, flow-dependent species present
- Water was fresh, turbid in autumn but clear in spring and high in nutrients
- Riparian vegetation comprised native trees and understorey species
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 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 1,412 hectare catchment upstream from the site sampled were grazing modified pasture (39%), natural conservation (27%) and plantation forestry (17%), with minor areas also used for other minimal uses, roads, wetlands, intensive animal production, land in transition and dams. The site sampled was located in the upper catchment near a gauge station off the North Coast Road, about 4 km NNW from Gosse.
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 nutrient enrichment and the presence of introduced marron in 2013.
A moderately diverse community of at least 26 species of macroinvertebrates was collected or seen from the river (13 species in autumn and 15 in spring), 8.3-10 m wide and over 1 m deep, in autumn and spring 2013. The river consisted of isolated pools in autumn but was a connected, slow-flowing channel in spring. The community was dominated by moderate numbers of waterbugs (Micronecta) in autumn and mites (Piona) in spring, and included low numbers of worms, mites (Koenikea), amphipods, yabbies, introduced marron, 8 species of beetles from four families, mosquitoes (Anopheles), 5 types of chironomids, mayflies (Cloeon and Thraulophlebia inconspicua), waterbugs (Ranatra and Anisops) and caddisflies (Lectrides varians). Most species were tolerant generalists that are frequently found from similar pool and slow-flowing habitats elsewhere on the island and from the wetter parts of South Australia. The presence of yabbies and marron in the same reach was unusual because normally the larger marron out-compete the smaller yabbies. One of the waterbugs collected was a nepid (Ranatra), an uncommonly collected species that usually occurs in dense emergent macrophytes on the edge of streams. The leptophlebiid mayfly (Thraulophlebia) was the only sensitive species collected, which is usually found in flowing freshwater streams from the southern part of the State.
The water was fresh (salinity ranged from 189-338 mg/L), moderately well oxygenated (57-66% saturation), turbid, alkaline and slightly coloured in autumn but clear and acidic (pH 6.29) in spring, and with generally high nutrient concentrations such as phosphorus (0.02-0.04 mg/L) and nitrogen (0.67-1.4 mg/L).
The sediments were dominated by detritus with smaller amounts of silt, sand, gravel, clay, and algae in spring, also present. Samples taken from below the surface were grey silts and sands but showed no evidence that the sediments were anaerobic and lacked oxygen. Over 1 cm of fine silt was deposited over the channel, which caused the water to become highly turbid when disturbed. There was no sign of any significant bank erosion at the site in 2013. The only animal droppings seen in the vicinity of the river were from kangaroos.
There was a large amount of phytoplankton (chlorophyll a 38 μg/L) and a smaller growth of cyanobacteria or blue-green algae (chlorophyll b 6.1 μg/L) present in autumn; the phytoplankton and cyanobacteria would have contributed towards the high turbidity of the river. In spring only a moderate amount of phytoplankton was recorded (chlorophyll a 3.3 μg/L). Over 10% of the channel was covered by filamentous algae (Cladophora and Spirogyra) in spring, when the only aquatic plant, a pennywort (Hydrocotyle), was recorded from the river; they covered less than 10% of the stream. The riparian zone was dominated by gums and wattles over bracken, sedges and rushes. The surrounding vegetation near the river comprised dense eucalypt woodland.
Special environmental features
Middle River provides a permanently wet, freshwater habitat and supports a moderately diverse assemblage of commonly occurring aquatic macroinvertebrates at the site sampled in 2013. It also supports at least one sensitive species that is frequently associated with flowing water.
Pressures and management responses
|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.|