De Mole River, u/s estuary
2013 Aquatic Ecosystem Condition Report
- Permanently slow-flowing stream in autumn and spring 2013
- Sparse macroinvertebrate community with two sensitive species present
- Water was fresh, clear but highly coloured, and high in nitrogen
- Riparian vegetation comprised native shrubs over a native understorey
About the location
De Mole River is located on the north-western end of Kangaroo Island. The river rises at an elevation of about 300 m just north of the Ravine Des Casoars Wilderness Protection Area, and flows in a west-north-westerly direction for about 11 km until it discharges into Investigator Strait near Cape Forbin. The major land uses in the 10,338 hectare catchment upstream from the site included minimal uses (41%), nature conservation (19%), plantation forestry (16%) and grazing modified pastures (16%), and included smaller areas of cropping, roads, intensive animal production and dams. The site sampled was located in the lower reaches at the end of Investigator Road, about 1.5 km south from Cape Forbin.
The river was given a Good rating because the site sampled showed evidence of relatively minor changes in ecosystem structure and function. There was some evidence of human disturbance due to siltation and nitrogen enrichment of the river, and the presence of introduced marron. Despite this, the river supported a rich mayfly community and represents one of the most well vegetated catchments on the island.
A sparse community of at least 17 species of macroinvertebrates was collected or seen from the river (9 species in autumn and 10 in spring), 11-20 m wide and over 1m deep, in autumn and spring 2013. The river consisted of deep, slow-flowing to still pools, with small areas of faster-flowing riffle habitats only present in autumn. The community was not dominated by any macroinvertebrate but included low to moderate numbers of amphipods (Austrochiltonia), sminthurid springtails, beetles, biting midges (including Bezzia), chironomids, mayflies (including baetids, Thraulophlebia inconspicua and Tasmanocoenis) and dragonflies. The presence of crayfish holes in the banks indicated that yabbies and introduced marron also inhabited the lower reach of this river. The baetid and leptophlebiid mayflies were the only sensitive macroinvertebrates collected since they are typically found among clear, flowing water habitats. The other species recorded were common, generalist species that are often found among more polluted habitats in the State. The lack of worms, mites, shrimp, waterbugs, damselflies, caddisflies, and limited number of aquatic flies collected was unexpected, given the extent of vegetative cover remaining in the catchment. Predation by crayfish and the limited habitat provided by deep pool habitats may limit the ability for this reach of the river to support a more diverse range of species.
The water was fresh (salinity ranged from 464-508 mg/L), well oxygenated (87-120% saturation), clear and coloured, and with low to moderate concentrations of phosphorus (0.01-0.03 mg/L) and high nitrogen concentrations (0.58-1.06 mg/L).
The sediments were dominated by detritus, sand and silt with smaller amounts of boulder and clay also present. Samples taken from below the surface were well-aerated grey sands and silts in autumn but when flows reduced during spring, the sediments had blackened, turned anaerobic, released sulphide, and generally lacked oxygen. Over 10 cm of silt was noted in the bottom of the river on occasion but this fine sediment was not sourced locally, since there was no evidence of any significant areas of bank erosion at the site. In addition, the only sign of any animal droppings in the vicinity of the river were from kangaroos.
There was a small growth of phytoplankton (chlorophyll a ranged from 0.9-1.6 μg/L) but no evidence of any filamentous algal growths recorded during 2013. Over 35% of the channel was covered by rushes (Juncus) and Water Ribbons (Triglochin) in places. The riparian zone was dominated by native shrubs (including casuarinas and hakeas) over rushes, sedges and bracken. The surrounding vegetation near the river comprised dense native forest dominated by casuarinas and gums.
Special environmental features
De Mole River provides a permanently wet, freshwater habitat that supports a few significant mayfly species on the island.
Pressures and management responses
|Moderate nutrient and sediment inputs from diffuse sources in the catchment (leading to extensive growth of algae and aquatic weeds as well as increased turbidity and smothering of habitat).||The Kangaroo Island NRM Board has funded the fencing of significant areas of riparian vegetation in the catchment and continues to work with landowners to increase the fencing of watercourses.|