Middle River, u/s from Middle River Reservoir
2013 Aquatic Ecosystem Condition Report
- Permanently wet, slow-flowing stream in autumn and spring 2013
- Moderately diverse macroinvertebrate community with at least two sensitive, flow-dependent species noted from riffle habitats in spring
- Water was fresh, clear 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 3,157 hectare catchment upstream from the site sampled were grazing modified pasture (38%), plantation forestry (25%), other minimal uses (22%) and nature conservation (11%), with minor areas also used for roads, cropping, intensive animal production, dams and residential living. The site sampled was located in the middle of the catchment off an ETSA track from Johncock Road, about 1 km upstream from the Middle River Reservoir.
Kangaroo Island NRM Regional Summary 2013
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.
A moderately diverse community of at least 20 species of macroinvertebrates was collected or seen from the river (12 species in autumn and 14 in spring), 2.7-3.7 m wide and up to 55 cm deep, in autumn and spring 2013. The river consisted of slow-flowing pools connected by smaller areas of faster-flowing riffle habitats in both seasons sampled. The community was dominated by low numbers of waterbugs (Micronecta) and included smaller numbers of amphipods, yabbies, beetles, biting midges, chironomids, other waterbugs and caddisflies. Numerous yabby and marron holes were recorded from the banks in autumn. The small riffles present in spring were notable because they supported flow-dependent species of stoneflies (Family Gripopterygidae) and blackflies. The other members of the community were tolerant, generalist species that are frequently found from similar pool and channel habitats elsewhere on the island and from the wetter parts of South Australia.
The water was fresh (salinity ranged from 355-793 mg/L), well oxygenated (111-114% saturation), clear and slightly coloured, and with moderate to high nutrient concentrations which included phosphorus (0.03-0.04 mg/L) and nitrogen (0.81-1.19 mg/L).
The sediments were dominated by detritus, clay and sand, with smaller amounts of boulder, cobble, gravel and silt also present. Samples taken from below the surface were grey silts and clays that were anaerobic in spring, which indicated that the sediments occasionally lacked oxygen. Nearly 10% of the banks showed evidence of erosion due to recent flood damage, which deposited large boulders in the channel near the site. The only animal droppings seen in the vicinity of the site were from kangaroos.
There was a large amount of phytoplankton was present in autumn (chlorophyll a ranged from 1.6-12 μg/L) but no sign of any filamentous algae was seen at the site in 2013. Over 10% of the channel was covered by aquatic plants, including rushes (Juncus) and sedges (Carex). The riparian zone extended over 30 m wide in places and was dominated by gums and wattles over yaccas, sedges and rushes. The surrounding vegetation near the river comprised dense eucalypt woodland.
Special environmental features
Middle River provides a permanently wet, freshwater habitat that supports a moderately diverse assemblage of mostly commonly occurring aquatic macroinvertebrates in its’ middle and upper reaches. The site upstream from the reservoir also supports at least two sensitive, flow-dependent species from flowing riffle habitats present in spring.
Pressures and management responses
|Moderate to large nutrient and sediment inputs from diffuse sources in the catchment (leading to extensive growth of algae and aquatic weeds; potentially 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.|
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