Torrens River, Bonython Park
2011 Aquatic Ecosystem Condition Report
- Permanently wet, slow-flowing, freshwater river in autumn and spring 2011
- Moderately diverse macroinvertebrate community with no rare or sensitive species
- Obvious signs of gross nutrient enrichment
- Riparian vegetation dominated by planted trees and introduced grasses.
About the location
The Torrens River is a large stream in the Southern Mount Lofty Ranges that rises a few kilometres north from Mount Pleasant and drains in a south-westerly direction where it discharges into Gulf St Vincent at Breakout Creek. The monitoring site was located downstream from Torrens Lake in Bonython Park, on the northern edge of Adelaide. The major land use in the 48,773 hectare catchment is stock grazing, with smaller areas used for remnant native vegetation, forestry, irrigated cropping, water supply, and rural and urban residential living.
The river was given a Poor rating because the site sampled showed evidence of major changes in ecosystem structure, and moderate changes to the way the ecosystem functions. There was considerable evidence of human disturbance including nutrient enrichment and degraded riparian vegetation for this site that was located in an urban parkland setting.
A moderately diverse community of at least 37 species of macroinvertebrates was collected from the river, 4.8-5.5 m wide and up to 35 cm deep, in autumn and spring 2011. The river consisted of still to slow-flowing pool habitats connected by smaller areas of faster-flowing riffle habitats. The community was dominated by generalists and species tolerant to poor water quality such as waterbugs (Micronecta), introduced snails (Physa), chironomids and blackflies. It also included smaller numbers of hydrozoans, flatworms, nematodes, limpets, bivalves, native snails, worms, freshwater shrimp, yabbies, beetles, craneflies, mayflies, odonates, caddisflies and several types of waterbugs. No rare or sensitive species were recorded but two uncommonly collected species, a waterbug (Ranatra) and caddisfly (Ecnomus turgidus), were present. A few flow-dependent species were collected, including a blackfly (Simulium ornatipes), chironomid (Rheotanytarsus) and caddisfly (Cheumatopsyche sp. 2). Two species of fish were recorded at the site, including a native carp gudgeon (probably Western Carp Gudgeon) and an introduced pest called Mosquitofish (Gambusia).
The water was fresh (salinity ranged from 439-679 mg/L), well oxygenated (102-115% saturated), clear, and with high concentrations of nutrients such as nitrogen (1.22-1.29 mg/L) and phosphorus (0.05-0.12 mg/L).
The sediments were dominated by detritus, sand and cobble. Samples taken from below the surface were sandy grey in appearance and showed no evidence that the sediments were anaerobic or lacked oxygen. A small amount of silt, less than 1 cm deep, covered the streambed and over 10 m of bank showed evidence of erosion caused by recent flood damage.
A large amount of phytoplankton occurred at the site and growths of filamentous algae (mostly Cladophora) covered over 10% of the channel in places. Aquatic plants extended over 35% of the site at times and included a wide range of emergent species (Cyperus, Juncus, Bolboschoenus, Persicaria, Schoenoplectus, Typha and Isolepis). The riparian vegetation consisted of several types of planted trees (wattles, gums and sheoaks) over introduced grasses and weeds. The surrounding vegetation comprised an urban parkland on the edge of the City of Adelaide.
Special environmental features
The urban section of the Torrens River provides habitat for several flow-dependent aquatic macroinvertebrates. The lower Torrens River also occasionally supports a range of threatened and common species of native fish, including Climbing Galaxias, Short-finned Eel, Pouched Lamprey, Short-headed Lamprey, Congolli and Freshwater Catfish (M. Hammer, S. Wedderburn & J. van Weenen, 2009, Action Plan for South Australian Freshwater Fishes 2007-2012. Native Fish Australia (SA) Inc.).
Pressures and management responses
|Insufficient natural water flows in the creek resulting from water extraction and climate variability (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.|
|Widespread introduced weeds in the riparian zone at the site and upstream (reducing habitat quality).||The Adelaide and Mount Lofty Ranges NRM Board has several pest plant (weed) mitigation and control programs. They work closely with landholders to control weeds on their property and to help stop the spread to other properties and waterways.|
|Limited riparian zone vegetation at the creek and upstream (reducing habitat quality, increasing sediment erosion).||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.|
|Stormwater runoff containing high nutrient and sediment loads discharging to the creek (causing habitat disturbance, algal growth and aquatic weeds).||The Adelaide and Mount Lofty Ranges NRM Board has a well developed stormwater quality improvement, harvesting and reuse program which has installed (and maintains) gross pollutant (and silt) traps in several watercourses across the region to catch litter, debris and silt in order to minimise impacts and damage to seagrass in the receiving marine environment. Stormwater captured is also treated through artificial wetlands across the region which act as suspended solid and nutrient filters; these wetlands also provide important habitat for many native species.|
This aquatic ecosystem condition report is based on monitoring data collected by the EPA. It was prepared with and co-funded by the Adelaide and Mount Lofty Ranges NRM Board.