Coast KZN

30 May 2017

Update: Eyes turn to Craigieburn waste water treatment plant as lettuce infestation spreads

Juan Venter (South Coast Herald) Picture: Various departments working in unison to combat the water lettuce infestation which has consumed much of the Mahlongwana River discuss the way forward at a site meeting last week.


The water lettuce infestation is quickly becoming an environmental disaster

UPDATE, May 30:
Another month has passed in what has, according to locals, become one of the worst water lettuce infestations the South Coast has ever seen.

On Wednesday, May 24 a site meeting was held at the Mahlongwana River, which was attended by members of the national Department of Environmental Affairs, various metro departments such as Natural Resources, Parks and Health as well as the Conservancy itself.

The Conservancy took to social media to update concerned parties on the matter and explained that the weevils – which were first introduced to the river some months ago – are doing an excellent job.
Concerns initially were plentiful, especially as weevils – Neohydronomous affinis – have never been introduced into an infected river this close to the ocean. “Much of the water lettuce was in distress which could be seen by the yellowing of the leaves and holes in the plant.”

In order to make a major impact on the plants, it is suspected that the weevils could take up to two full seasons. “On the other hand, large and sudden die-offs have occurred elsewhere so there is no standard method of knowing what to expect.”

What was further shared during the meeting was the concerning fact that the water lettuce had spread upriver towards places from which it had previously been eradicated.

This, according to the Conservancy, suggests that the weed has seeded. It was advised that the waste water treatment plant upstream had to be strictly monitored so as to ensure that the effluent released was of an acceptable quality and meets both Metro and Water Affairs’ standards.

“If this was not done and high levels of phosphates, rather than nitrates, were being released, it would then prevent the bio-control – the weevils – from eliminating the weed.”

Concerns were raised towards the big ‘what if’ moment when the weed would begin to sink to the bed of the river.

“There would be so much biomass in the river that this would bring additional problems and could not be permitted to happen.”
Locals who were formerly keen on having the river breached would be displeased to read that this could not, however, be done. “Environmentally speaking, if the river was breached the water would flow out but the major portion of the weed would not, as the outflow current would not be on the surface and the weed would be left within the system.”

There were also legal concerns raised during the site meeting. “Neither the Metro nor the DEA would support the breaching of the river and both felt it unlikely that the national DEA would approve the breaching.”

As a result, an operation is expected to commence this week in which between 40 and 50 percent of the weed will be mechanically removed ’as a matter of urgency’, the Conservancy said.

It was advised that the relevant parties will commence with the hand removal of the weed at the beach area as this was the most accessible area. The weed, once removed, would be placed in the inter-tidal zone and so flow out to sea.

Crookes Bothers were kind enough to agree that workers access the river area through its grounds on the southern bank.

However, workers would need to be careful when accessing this area as there are various protected plant species on the banks. The banks are also very steep which makes it very difficult for machinery (which would require further permission to be granted) to access the river.

Locals will, however, be pleased to read that steps are being taken to eliminate the mosquito problem which has occurred as a result of the water lettuce infestation in Widenham.

Fears of bilharzia were also ruled out some weeks ago when tests confirmed that the disease was not present in the river.