Derwent Creek, Cowarie Waterhole
2012 Aquatic Ecosystem Condition Report
- Semi-permanent, isolated waterhole when sampled in spring 2012
- Moderately diverse macroinvertebrate community with no rare or sensitive species
- Water was fresh, moderately turbid and enriched with nutrients
- Riparian vegetation consisted of native trees and shrubs and only one type of aquatic plant was recorded from the waterhole
About the location
The Derwent Creek rises at an elevation of 59 m to the south of Mungeranie and flows for about 55 km in a north-north-westerly direction where it discharges into Warburton Creek, north of Cowarie. It is an ephemeral, tributary stream that only flows following heavy local rains and has few semi-permanent waterholes. The only permanent water in the catchment is the artificially maintained Mungeranie Waterhole that is sustained by pumping from an artesian bore.
The major land use in the Derwent Creek catchment is cattle grazing.
The monitoring site was located at Cowarie on the Cowarie-Mungeranie Track off the Birdsville Track, about 250 km north of Marree in northern South Australia.
The creek 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. Therewas evidence of human disturbance including nutrient enrichment and stock accessing the banks but the stream still provided habitat for a range of aquatic species.
A moderately diverse community of at least 11 species of macroinvertebrates was collected from the 10 m wide, non-flowing, isolated waterhole in spring 2012. The community was dominated by large numbers of waterbugs (Micronecta gracilis) and chironomids (Procladius and Polypedilum) but included smaller numbers of 3 types of beetles (Sternopriscus, Megaporus and Allodessus), 2 waterbugs (Agraptocorixa eurynome and Anisops), a species of freshwater prawn (Macrobrachium australiense) and a caddisfly (Ecnomus). Yabby holes and empty mussel shells were also seen at the site, indicating that the waterhole must regularly hold water for long enough for these long-lived species to be able to have colonised and persisted in the local stream network. The assemblage was dominated by generalist insect groups with generally good dispersal capabilities that frequently occur from other waterholes within the region. No rare or sensitive species were collected, although the caddisfly is an uncommonly collected genus that has not been extensively recorded from the region. No fish were seen or collected at the site.
The water was fresh (salinity of about 298 mg/L), moderately well oxygenated (54% saturated) and moderately turbid (secchi depth 17 cm), with high concentrations of nutrients such as nitrogen (5.19 mg/L) and phosphorus (0.5 mg/L).
The sediments were dominated by sand and bedrock, with smaller amounts of boulder, silt and detritus; samples taken from below the surface were sandy in appearance and showed no evidence that the sediments were sulfidic and anaerobic, or lacking in oxygen. No evidence of any bank erosion was noted but cattle faeces were recorded from the banks and within the channel in places.
A large amount of phytoplankton was present (chlorophyll a 31.1 Âµg/L) and included some blue-green algae or cyanobacteria (chlorophyll b 7 Âµg/L). No filamentous green alga was recorded but about 10% of the waterhole was covered by isolated patches of an aquatic plant called Water Buttons (Cotula). The riparian vegetation was dominated by coolabah gums and acacias on the moderately well vegetated banks (50-79% vegetative cover). The surrounding vegetation comprised low shrubland dominated by acacias and senna.
Special environmental features
Derwent Creek at Cowarie Waterhole provides a small refuge pool that is used by a range of birds (e.g. darter, ducks, herons, kites and corellas).
Pressures and management responses
|High nutrient concentrations causing excessive algal growth although the source(s) of all the nutrients is not known with certainty.||The EPA in collaboration with the Department for Environment, Water and Natural Resources is anticipating a study program to investigate the source(s) of nitrogen and phosphorus. This will provide a better understanding of nutrient dynamics with the aim of developing a management strategy (if appropriate).|
|Livestock have direct access at the site and upstream in the catchment exerting excessive grazing pressure on vegetation, causing sediment erosion and adding some nutrients to the watercourse.||The SA Arid Lands Natural Resources Management Board recognizes that both direct and diffuse impacts on aquatic ecosystem condition can occur through direct stock access and excessive grazing pressure from stock and feral herbivores. Technical advice and incentives are offered to land managers in the region, as funding permits, to address these impacts through appropriate activities suitable for the context. In addition, projects are underway across the region to identify, prioritise and address impacts at key aquatic sites.|