Why are water weeds so much of a problem? They have many social, environmental and economic effects that I will discuss later, however the reason they are so problematic is that they are so hard to deal with. Once an aquatic weed becomes established, it is very difficult, expensive and sometimes impossible to deal with. With terrestrial weeds you can deal with them. It may be hard, and cost a lot of time and money, but if you stick to the job you have a chance of prevailing eventually. Even such a simple thing as getting access to the weed is a problem in an aquatic situation. In the bush, even if you have to use a bulldozer you can get to the site. You can’t use a bulldozer in a lake!

Cabomba Harvester Operator Ross Paulger.Social impacts include increased mosquito breeding, navigation hindered, risk of drowning by entanglement, water based activities such as boating, fishing, kayaking and swimming can be prohibited.

Environmental impacts include displacement of flora & fauna, reduction of bird diversity, light penetration greatly reduced; reduction in fish stocks, lower stream flow leading to sedimentation, water quality and oxygen levels can be reduced particularly during decomposition and the water ecosystem can be altered.

Economic issues relating to aquatic weeds include clogged irrigation intakes, high transpiration reducing volumes of water available, tourism can be adversely affected, livestock can be lost mistaking the weed mass for pasture, crops such as rice can be quarantined, and the cost of control can be high.

Most aquatic plants have been introduced into Australia as ornamentals, quite a few via the aquarium industry. These plants then find their way into our water bodies and streams, either deliberately or accidentally. Once established they multiply rapidly, generally as they have no predators which might exist in their country of origin. They can be spread by a number of methods, usually called vectors. These include boats, recreational craft, trailers, eel traps, fishing nets, earthmoving equipment, and sometimes deliberate introductions. You will notice that these are all human based activities. Yes, birds do occasionally spread aquatic weeds. They may transport very small aquatic plants like duckweeds, but generally not the larger aquatic macrophytes. I say this with some confidence as I have seen many dams full of aquatic weeds, next to dams with none…and birds happily flying from one to the other.

Cabomba in water

In Australia the Federal Government announced 20 Weeds of National Significance [WoNS] in 1999. These plants were chosen for their invasiveness, and potential to spread with associated impacts. There are three aquatic species on that list. These are Fanwort [Cabomba caroliniana], Salvinia [Salvinia molesta] and Alligator weed [Alternanthera philoxeroides]. In the Noosa Biosphere the most damaging are Cabomba and Salvinia. Cabomba is an attached, submerged plant that grows to a depth of up to 5 metres. Lake Macdonald has 36% of all Australia’s cabomba. This weed costs approximately $140,000 per year just to manage in Lake Macdonald. This entails mechanical harvesting and readers may have seen the harvester operating on the lake.  If operating in an area not recently harvested, the operator can remove up to 10 tonnes of cabomba per day! This Lake is currently managed by SEQ Water. It is believed that Cabomba was introduced into Lake Macdonald from an aquarium in 1993. Currently control is limited to harvesting and work is being carried out by CSIRO on a biological control program.

Cabomba close up

Salvinia is a free floating aquatic fern that can quickly double its area. In our area, whilst it is not in Lake Macdonald, it is in a number of creeks and farm dams. In 2004 in the Hawkesbury River in NSW, Salvinia infested 88 kilometres of the river, closing one part of the river for a period of four months. The weed had a surface area of 347 hectares. 1.8 million dollars was spent to control this weed. We have a very good biological control agent for this weed [Cyrtobagous salviniae], which greatly assists in its control

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Salvinia in farm dam

Alligator weed is not yet identified in the Noosa Biosphere. It is common around the Hunter and Sydney regions, and also in Brisbane and Pine Rivers. An infestation was recently discovered in Miriamvale. This weed is a major problem in rice crops, irrigation channels and also for the turf industry. It can grow both in water and on land. It is extremely difficult to control with herbicides being of limited affect, and the biological control agent [Agasicles Hygrophila] is limited to aquatic situations.

Alligator weed

Given the difficulties of treating aquatic weeds, early detection is paramount. If you can get an infestation [of any aquatic weed] early enough, you may have a chance of stopping it. So you can help by keeping you eye out for something different in our water bodies. Noosa & District Landcare Group have been running Aquatic plant identification training sessions all over Australia to help weeds officers to be able to recognise weeds early. There are many useful websites on aquatic weeds.  www.weeds.gov.au  and the Qld Government site http://www.dpi.qld.gov.au/cps/rde/dpi/hs.xsl/4790_8331_ENA_HTML.htm