Brownhill Creek, Mitcham
2008 Aquatic Ecosystem Condition Report
- Moderately affected by nutrient enrichment and fine sediments.
- Diverse macroinvertebrate community dominated by species tolerant of pollution.
- Riparian zone heavily invaded by exotic trees and weeds.
- Important habitat for threatened fish species and notable insect species.
About the location
Brownhill Creek is made up of a network of three small streams flowing through steeply-sided valleys in the foothills of the southern Mount Lofty Ranges at Mitcham. Much of the native vegetation in the flatter areas of the catchment has been removed for cropping, sheep grazing and rural living.
The site selected for monitoring was located on the southern branch of the creek within the Brownhill Creek Recreation Reserve, off Brownhill Creek Road.
The creek was given a good rating at this site because the ecosystem showed evidence of relatively minor changes to its animal and plant life. However, there were some clear, emerging signs of occasional high nutrient loads degrading the creek, which could lead to further decline.
The creek was only three to four metres wide at the time of sampling in October 2008, and consisted of large pools connected by narrow areas of riffles flowing slowly through willow tree-roots. These riffles were not extensive enough to sample.
A diverse community of about 49 macroinvertebrate species was collected. They covered all the types usually found in streams in the region, including several species sensitive to pollution. However, the community was dominated by species tolerant to poor water quality that feed on organic material, such as introduced snails (Physa acuta, Potamopyrgus antipodarum and juvenile Hydrobiidae), chironomids and worms.
The water was very fresh (salinity of 410 mg/L) and moderately well oxygenated (58% saturation). It was clear although slightly coloured, with moderate to high concentrations of nutrients such as nitrogen (0.46 mg/L) and phosphorus (0.12 mg/L).
The sediments were mostly detritus, silt and clay. They were blackened and anaerobic below the surface layer, indicating large amounts of organic matter had created a harsh environment for macroinvertebrates to inhabit.
Up to 10% of the bottom of the creek was covered in green filamentous algae (Cladophora and Spirogyra) despite the dense shading provided by vegetation along the banks. The only other plants growing along the edge of the water were introduced Watercress (Rorippa), buttercups (Ranunculus) and dock (Rumex), and native sedges (Cyperus and Isolepis) and knotweed (Persicaria).
The riparian zone was heavily invaded by introduced trees, shrubs and other weeds, and included willow, olive, hawthorn, desert ash, onion weed, soursob, Arum Lily and nasturtium. Gum woodland over introduced grasses occurred further away from the creek.
Special environmental features
Brownhill Creek provides habitat to a range of sensitive species, including stoneflies (Riekoperla naso and Austrocerca tasmanica), mayflies (Atalophlebia, Koorrnonga inconspicua and Tasmanocoenis tillyardi) and fly larvae from the family Dixidae. Small numbers of the state-listed threatened native fish Mountain Galaxias (Galaxias olidus) were also collected at the site.
Pressures and management responses
|Limited riparian zone vegetation at the site and upstream, providing minimal buffer protection from catchment landuses (reducing habitat quality).||The Adelaide and Mount Lofty Ranges NRM Board's land management program encourages and promotes managing land to improve water quality. This includes incentives for revegetation programs around waterways and wetlands and stock exclusion as well as educating landholders about the importance of riparian vegetation in managing soil erosion.|
|Extensive deciduous tree growth in the riparian zone at the site and upstream (reducing habitat quality).|
|Large decrease in natural water flows (reducing ecological integrity).||Through water allocation planning the Adelaide and Mount Lofty Ranges NRM Board seeks to manage a sustainable water supply for the region so that there is enough water available for everyone (including the environment) even in drought conditions.|
This aquatic ecosystem condition report is based on monitoring data collected by the EPA and prepared in conjunction with the Adelaide and Mount Lofty Ranges NRM Board.