Middle River, N from Bangor
2013 Aquatic Ecosystem Condition Report

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 sparse community of at least 19 species of macroinvertebrates was collected or seen from the river (11 species in autumn and 15 in spring), 4-4.4 m wide and over 1 m deep, in autumn and spring 2013. The river consisted of a slow-flowing channel in autumn but was deeper and more extensive in spring when large pools where connected by small areas of faster flowing riffle habitats. The community was dominated by moderate numbers of amphipods (Austrochiltonia) and included low numbers of mites, yabbies, beetles, mosquitoes, craneflies, biting midges, blackflies, chironomids, mayflies, waterbugs and caddisflies. Introduced marron were also present at the site and were being caught by fishermen during the autumn survey. Most species collected were tolerant, generalist species that have a wide distribution throughout the island and wetter parts of the State. Two sensitive flow-dependent species were collected when the river was flowing in spring, including the blackfly (Austrosimulium furiosum) and mayfly (Thraulophlebia inconspicua).  

The water was fresh (salinity ranged from 129-794 mg/L), well oxygenated (90-91% saturation), acidic (pH 6.59-6.95), clear and slightly coloured, and with moderate to high nutrient concentrations for phosphorus (0.01-0.03 mg/L) and nitrogen (0.66-0.88 mg/L).

The sediments were dominated by detritus and sand, with smaller amounts of silt, clay, and algae in spring, also present. Samples taken from below the surface were grey silts and clays that were anaerobic in spring, indicating that the sediments were lacking in oxygen. A small amount of bank erosion extending over nearly 10% of the site was noted in spring, presumably caused by recent winter flood damage. The only animal droppings seen in the vicinity of the site were from kangaroos. 

There was a small amount of phytoplankton (chlorophyll a 2.1-2.7 μg/L) recorded from the river in 2013 but filamentous algae (Spirogyra) was only seen in spring, when it covered over 10% of the channel. Over 10% of the channel was covered by aquatic plants such as rushes (Juncus) and tall-spike rush (Eleocharis) in autumn but these plants were not evident in the subsequent spring survey. The riparian zone extended 5-10 m wide and was dominated by gums and wattles over bracken, sedges and rushes. The surrounding vegetation near the river comprised dense eucalypt woodland.

Middle River provides a permanently wet, freshwater habitat and supports a sparse assemblage of mostly commonly occurring aquatic macroinvertebrates in the middle reaches. It does, however, provide habitat for two sensitive, flow-dependent species when the river flows in spring.

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