Gawler River, Virginia Park
2008 Aquatic Ecosystem Condition Report
- Significantly affected by nutrient enrichment and fine sediment.
- Macroinvertebrate community dominated by species tolerant of pollution.
- Excessive growths of algae and aquatic plants.
- Riverbanks extensively eroded, and the riparian zone invaded by weeds.
About the location
Gawler River forms at the junction of the North and South Para rivers in Gawler, and flows west before discharging into Gulf St Vincent in the Port Gawler Conservation Park. Vineyards, livestock grazing and urban development are the major land uses in the catchment.
The site selected for sampling was located off Broster Road at Virginia Park, in the lower reaches of the river.
The river was given a Poor rating at this site because the ecosystem showed evidence of major changes in the animal community and plant life, and moderate changes to the way the ecosystem functions. The stream was significantly affected by high loads of nutrients and fine sediment. The creekbanks were heavily eroded and the riparian zone was limited and severely disturbed by the invasion of weeds.
Connected pools of still water with variable depths formed the creek at this site when it was sampled in October 2008.
A moderately diverse community of 30 macroinvertebrate species was collected. The community was dominated by species tolerant of high nutrient levels and poor water quality, such as worms and chironomids (Corynoneura and Dicrotendipes). No sensitive species were detected, and the only mayfly collected was an immature baetid (probably a juvenile of the tolerant genus Cloeon). Schools of introduced mosquitofish were the only fish observed.
The water was moderately fresh (salinity of 1,100 mg/L), moderately well oxygenated (47% saturation) and very cloudy, or turbid. It contained high concentrations of nutrients such as nitrogen (2 mg/L), phosphorus (0.4 mg/L) and organic carbon (25 mg/L).
Considerable amounts of phytoplankton were found in the stream, as well as large patches of aquatic plants such as Sea Clubrush (Bolboschoenus caldwellii), dock (Rumex), knotweed (Persicaria) and Common Reed (Phragmites australis).
The sediments were dominated by detritus and fine clay particles, and were anaerobic. More than 50% of the streambanks showed evidence of erosion due to flood damage, with many tree roots exposed throughout the site.
River Red Gums and introduced deciduous trees (maples) lined the stream; patches of wattles were also found in the riparian zone, but the understorey was completely dominated by introduced woody and herbaceous weeds such as wheel cactus, fennel, kikuyu, paspalum, olives, California Burr, Castor Oil plants, boxthorn, Tobacco Tree and various thistles. Vineyards, cropping and irrigated vegetable growing occurred in the surrounding area.
Special environmental features
Pressures and management responses
|Extensive weed growth in the riparian zone at the site and upstream (causing habitat disturbance).||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 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.|
|Stormwater runoff causing high water velocities, containing nutrients and sediments (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 and prepared in conjunction with the Adelaide and Mount Lofty Ranges NRM Board.