Potential Impacts of Cabomba
Aquatic plants are an important part of many freshwater bodies. They add oxygen to the water, provide shelter and a spawning medium for invertebrates and fish, and consolidate river banks and beds. However, excessive amounts of aquatic vegetation can cause many problems. They can: increase resistance to flow, resulting in stagnating water; increase siltation, which can impact on benthic communities; degrade water quality; and increase flooding and river encroachment onto surrounding lands. They can also block pumps, impede navigation and recreational activities such as fishing, and can actually be a hazard for swimmers and divers due to entanglement. Excessive amounts of one type of aquatic plant may also lead to the displacement of other plant types, resulting in a reduction in plant diversity. This may affect other wildlife that depend on the displaced plant communities.
Cabomba is an aggressive invader of freshwater ecosystems. It is recognised as a Weed of National Significance (WONS). In the NT, cabomba is declared under the NT Weeds Management Act as a Class A (to be eradicated) and Class C (not to be introduced) weed. Infestations are typically dense, monospecific, and can cover substantial areas.
In 2004 Cabomba established in several reaches of Darwin River and in associated billabongs. In one 2 km section, cabomba had established along both sides of the river in an almost unbroken stand, it often extended up to 15 m from the banks to the middle of the river. In another section, cabomba covered approximately 75% of the water body, taking up nearly the entire water column from the riverbed to the water surface.
If left untreated, this relatively small, but significant population of Cabomba could have severely impacted upon the NT's economy and ecological and cultural values. The Darwin River infestations, though now under control (current estimates place the populations at <1% of those observed in 2004), must be continuously monitored. Should cabomba populations restablish undetected, in these or other areas, there is still the potential to:
- Reduce biodiversity and ecosystem functioning.
- Reduce water quality.
- Reduce water storage capacity of dams.
- Block water distribution infrastructure.
- Severely impede recreational activities including swimming, fishing and boating.
- Create suitable habitat for mosquito breeding.
The impacts on water quality and dam capacity are particularly relevant at Darwin River. Just 6 km upstream from the original infestation is Darwin River Dam, which supplies 90% of Darwin’s potable water and covers an area of 5000 hectares when full. There is no herbicide registered for cabomba control in potable water.
