Sixth Creek, near Castambul
2016 Aquatic Ecosystem Condition Report

The creek was given a Good rating because the site sampled showed evidence of relatively minor changes in ecosystem structure and function. There was evidence of human disturbance due to nutrient enrichment and degraded riparian habitat but the stream supports a large number of rare, sensitive and flow-dependent macroinvertebrate species.

A diverse community of at least 52 species of macroinvertebrates was collected from the creek (38 species in autumn and 27 in spring), 5.1 m wide and up to 44 cm deep, in autumn and spring 2016. The creek consisted of slow-flowing pools connected by shallower, faster flowing riffle habitats in both seasons sampled. The community was dominated by large numbers of hydrobiid snails, including the introduced snail Potamopyrgus, amphipods (Austrochiltonia australis), chironomids, blackflies (Austrosimulium furiosum, Simulium melatum and Simulium ornatipes) and water boatmen. It also included low to moderate numbers of limpets, worms, isopods, shrimp, beetles, mosquitoes, mayflies, waterbugs, stoneflies and caddisflies. Many of these were rare, sensitive and/or flow-dependent species, including the above-listed blackflies, the beetles Simsonia and Platynectes, mayflies (Offadens sp. 5 and Atalophlebia australasica, Atalophlebia australis, Thraulophlebia inconspicua), a stonefly (Dinotoperla evansi), and caddisflies (Ulmerochorema membrum, Taschorema evansi, Lingora aurata, Cheumatopsyche and Notalina fulva). The only fish seen at the site were a few introduced trout in autumn.

The water was fresh (salinity ranged from 286-352 mg/L), well oxygenated (83-93% saturation) and clear but slightly coloured in autumn, and with moderate to high nutrient concentrations such as phosphorus (0.01-0.02 mg/L) and nitrogen (0.44-0.90 mg/L).

The sediments were dominated by cobbles, detritus, pebbles and silt with other rocky substrate and fine substrate also present in small amounts. Samples taken from below the surface were grey sands that released sulfide when tested in spring, indicating that the sediments were anaerobic or lacking oxygen for at least part of the year. Between the autumn and the spring sampling rounds a series of heavy rain events led to considerable alteration of this part of the creek, with the deeper pools becoming infilled with cobbles and smaller sized substrate and considerable bank erosion (10-50 m) noticeable at the site in spring. There was no sign of any stock accessing the creek at the site sampled in 2016.

There was only a small amount of phytoplankton present during the year (chlorophyll a ranged from 0.23-0.81 μg/L) and small growths of a filamentous alga (Cladophora) covered less than 10% of the creek in spring. More than 35% of the site was covered by aquatic plants, comprising introduced watercress (Rorippa) and dock (Rumex), and native macrophytes such as sedges (Cyperus), reeds (Phragmites), and buttercups (Ranunculus). The narrow, riparian zone was generally less than 5 m wide at the site and comprised introduced plants (mostly willow and bamboo), over introduced grasses. The surrounding vegetation comprised dense native woodland on one bank and introduced trees on cleared land on the other bank.

Sixth Creek provides a permanently flowing, freshwater stream that has consistently supported a large number of rare, sensitive and flow-dependent species since the mid 1990’s. It is among the most ecologically significant and best condition streams in the Southern Mount Lofty Ranges and the State. The lower reaches of Sixth Creek is also one of the few streams in the region that supports a rare, flow-dependent beetle called a waterpenny (Sclerocyphon), although it was not recorded at the site during 2016.

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