Mosquito Creek, Struan
2014 Aquatic Ecosystem Condition Report
- Permanently wet with isolated pools present in autumn and spring 2014
- Moderately diverse macroinvertebrate community with no rare or sensitive species
- Obvious signs of gross nutrient enrichment
- Riparian vegetation dominated by reeds and introduced grasses
About the location
Mosquito Creek is a large stream in the lower South East with a catchment area over 1100 square kilometres. It rises east of Edenhope in south-western Victoria as two major branches, with the smaller northern branch called Yelloch Creek and the larger southern branch Mosquito Creek. Both flow in a westerly direction and join into the one channel about nine kilometres north-east of Struan. The creek flows into Bool Lagoon further downstream and then flows out of the game reserve as Drain M, which flows west before eventually discharging into Lake George near Beachport. The major land uses are grazing and cropping, and the creek flows through several kilometres of the Naracoorte Caves Conservation Park just upstream from the sampling location. The monitoring site was located downstream from the Riddoch Highway, near the gauge station at Struan, about 15 kilometres south from Naracoorte.
The creek 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 poor aquatic and riparian habitats.
There was too little water present to sample macroinvertebrates effectively in autumn 2014 but the small pool present provided habitat for an introduced snail (Physiella), yabbies and several types of beetles (including Necterosoma and Lancetes). In spring 2014, isolated pools up to 5 metres wide and 31 centimetres deep were present and they provided habitat for a moderately diverse community of about 26 types of aquatic macroinvertebrates. The community was dominated by moderate numbers of generalists and species tolerant of poor water quality, including snails (Physiella) and waterbugs (Micronecta, Agraptocorixa and Sigara). It also included many other species commonly found in waterholes and drying pool habitats, such as worms, water mites (Eylais), yabbies, biting midges, waterbugs, caddisflies (Triplectides australis) and a rich assemblage of 8 species of beetles and 6 chironomids. No rare or sensitive species were collected and the only fish seen at the site were some introduced mosquitofish (Gambusia).
The water was fresh (salinity ranged from 569-603 mg/L), only moderately well oxygenated (50-53% saturation), clear and strongly coloured when water levels were low in spring, and with high concentrations of nutrients such as nitrogen (3.65-6.96 mg/L) and phosphorus (0.33-0.70 mg/L).
The sediments were dominated by detritus, sand and clay, with smaller amounts of cobble, bedrock, pebble, gravel and silt also present; samples taken from below the surface were grey sands and released sulfide when tested in spring, indicating that the sediments lacked oxygen and were a harsh environment for most burrowing species to live in for at least part of the year. No signs of any significant areas of bank erosion or animal droppings were noted near the site in 2014.
A few submerged (Callitriche) and emergent plants (Phragmites, Eleocharis, Triglochin, Typha, Juncus and introduced Rumex) covered over 65% of the pools. A large phytoplankton bloom occurred at the site particularly in autumn when the shallow pool was nearly dry (chlorophyll a 43-585 Âµg/L). No filamentous algae was seen during either survey carried out in 2014.
The narrow riparian zone consisted of mostly reeds (Phragmites) and introduced grasses and dock, with a few small gum trees and rushes also present. The surrounding vegetation at the site was sheep grazing land and pasture with the occasional gum tree present in the general landscape.
Special environmental features
While no native fish were seen in 2014, further upstream the threatened Southern Pygmy Perch has been recorded from this creek in the past. The threatened Glenelg Spiny Crayfish (Euastacus bispinosus) was also collected just upstream adjacent to the Naracoorte Caves Conservation Park during the mid-2000s, although its presence in Mosquito Creek was probably due to the release of translocated crayfish from its natural range from the Glenelg River catchment in Victoria or Eight Mile Creek/Deep Creek in South Australia; this species is normally found in flowing streams so it will not be able to persist in the ephemeral waters of Mosquito Creek.
Pressures and management responses
|Limited water flow||Through ground and surface water allocation planning and the South East Regional NRM Plan water affecting activity permit process the NRM Board seeks to manage water for environmental, social and economic purposes in a range of climatic scenarios.|
|Livestock have direct access at the site and upstream, causing sediment erosion and adding excessive nutrients (which leads to habitat disturbance, algal growth and aquatic weeds).||The South East NRM Board supports targeted projects that provide opportunities for landholders to access grants for fencing for stock exclusion from time to time for priority catchments.|
|Limited riparian zone vegetation at the site and upstream, providing minimal buffer protection from catchment landuses (reducing habitat quality).||The South East NRM Board assists landholders to access targeted grant opportunities for revegetation and ecosystem protection when funding is available. The Board also works closely with landholders consistent with the Board’s Regional Pest Management Plan to control weeds on their property and to assist in halting their spread to other properties.|