Lagarosiphon major may not be a name you know, but once you have seen a photo you’ll probably recognise it as the oxygen weed that you had in your fish pond or aquarium when you were a kid.
Originally introduced to New Zealand through the aquarium trade, lagarosiphon became naturalised in 1950 and quickly spread between waterways mostly thanks to human assistance. Its sale and distribution was finally banned in 1982, however by then considerable damage had been done.
Lagarosiophon spreads vegetatively; even a piece as small as your little toe could potentially take root and create a new colony. Fortunately, we have only female plants in New Zealand, therefore it is unable to spread through seed dispersal, however fragmentation as a dispersal mechanism is quite sufficient thank you very much! It can grow up to a metre in a month and by having brittle stems and a solid root system it certainly maximises its spread potential.
All of this is bad news for waterways where lagarosiphon is absent, as it quickly outcompetes native plants and can grow to the surface from its maximum depth of 6.5m. Once at the surface it can be a hazard for swimmers and boat users, interfere with fishing success, cause visual amenity issues, and ‘rafts’ of weed can clog screens on hydroelectric power stations.
In 1972 lagarosiphon was discovered in Lake Wanaka, reputedly introduced by someone liberating their goldfish. The weed established extensively within the lake, and in 1992 when Lake Dunstan was filled, weed fragments from Lake Wanaka made their way down the Clutha River and rapidly colonised the new lakes’ littoral zone. Interestingly, lagarosiphon was identified as a potential risk to the hydro-generation, amenity and recreational values of the lake, the Ministry of Works and Development (MWD) sought advice from the then-Department of Scientific and Industrial Research (DSIR) on potential environmental issues with the filling lake.
Advice was provided by the now Chief Science Advisor at NIWA, Clive Howard-Williams, who pointed out the need for ‘sculpting’ (with bulldozers) the lake shore profile before the dam filled. It was advised to cut the southern lake edge straight down like a swimming pool to a depth of at least 4 metres to minimise the areas of aquatic weed establishment and particularly surface growths. Unfortunately, somewhere along the line the message didn’t make it through to the engineers and the naturally gentle bathymetry of Lake Dunstan’s foreshore was soon extensively colonised.
Comprehensive management programmes for both Lake Wanaka and Dunstan are in place: led by LINZ, managed by Boffa Miskell and supported by various agencies and authorities. Control methods include applying herbicide via helicopter and boat, weed cutting and hand removal using commercial divers.
(Photo of Lagarosiphon major: RohanWells/NIWA)
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For further information please contact Marcus Girvan
2 August 2018