Saving SA’s water bodies from aquatic plant invaders with a sustainable twist

Water lettuce is an alien invasive species from South America. It can affect local water biodiversity, human wellbeing, business, tourism and the broader environment. (Photo: Julia Evans)

By Kristin Engel

28 Aug 2024

From the Vaal River to Hartbeespoort Dam, the effects of invasive aquatic plants have been detrimental to the health of communities and the environment. In a Daily Maverick webinar, experts discussed solutions to this growing problem.

Aquatic plant invaders, also known as aquatic alien invasive species, are covering many of South Africa’s water bodies, forming dense mats of vegetation that block sunlight and prevent indigenous plants from growing. They’re a threat to our daily lives, increasing flood risks and spreading diseases like malaria.

In a Daily Maverick webinar on Wednesday, the deputy director of the Centre for Biological Control at Rhodes University, Julie Coetzee, and Ferrial Adam, the manager of WaterCAN, an initiative of Outa, joined Our Burning Planet journalist Julia Evans to discuss aquatic plant invaders and solutions to this growing problem.

https://youtu.be/b10XDylfHLw

From the Vaal River to the infestation in Hartbeespoort Dam, aquatic plant invaders have been detrimental to the health of SA communities and the environment.

Adam explained that the high level of nutrients in SA water bodies speeds up the reproduction of these rapidly growing plants, “So they basically just explode.

“In simple terms, we have a lot of sewage and agricultural fertilisers and pesticides that are going into our rivers… That nutrient then creates a cycle of eutrophication, which is an excess of nitrates and phosphates, and that is like a growing food for these plants.

“We need early detection. We need to be able to not wait for things to explode before there’s any action. We don’t have to wait for the whole of Hartbeespoort Dam to be covered before we realise there’s a problem. That’s something we need in terms of environmental issues across the board. There has to be early detection and monitoring of our water systems,” she said.

Methods of control

When reporting on the impact of water hyacinth and water lettuce on the Vaal River community this year, Evans found that the first thing community members did was physical removal — pulling the plants out.

sa water bodies aquatic plant invaders The Taaibosspruit (a tributary of the Vaal River) is inundated with alien invasive species, with water lettuce covering the stream for kilometres. (Photo: Julia Evans)

Read more: Rapid water lettuce spread threatens Vaal River — weevils could be the solution

Coetzee said this was not a long-term solution, because the plants required frequent removal.

“The plants just outgrow any removal, particularly in the warmer summer months. On smaller systems, sure — there’s a system in Boksburg on Witfield Lake where they did manage to remove the plants. But it’s usually temporary because these plants have huge seed banks. The seeds remain dormant while there’s a full cover of the plant over the surface,” said Coetzee.

Once that plant cover is removed and the water warms up, then germination occurs and new plants cover the surface.

Another measure to combat aquatic plant invaders is chemical control. Coetzee said that in South Africa, glyphosate was used, but this was controversial.

“It is very effective. It’s expensive. But again, it’s short-term. You’ll have to keep applying herbicides if that’s the route that you’re going to go, but the plants will germinate from seeds,” she said.

Read more: Winning the war against the aggressive water hyacinth – A Cape Town community’s tireless effort

When considering this option, Coetzee said the impact of glyphosate on aquatic life had to be taken into account.

The measure that Coetzee and her team at Rhodes University use is biological control — releasing insects that “only feed on these plants. But … we’ve changed our approach to an inundative approach, where … we have to constantly apply these insects to get a faster knockdown,” said Coetzee.

Unlike physical or chemical removal, Coetzee said this method was sustainable and affordable, with no negative impact on the environment — biological controls reduce toxicity in water.

She said the final control measure was an integrated approach, using a combination of the three methods, which, she said, usually produced the best outcomes.

Where do the alien invaders come from?

alien plant invaders water lettuce Water lettuce is an alien invasive species from South America. It can affect local water biodiversity, human wellbeing, business, tourism and the broader environment. (Photo: Julia Evans)

Water hyacinth originates in the Amazon Basin in South America, as does water lettuce.

Because these aquatic plant invaders are not indigenous to SA, they have an impact on the environment itself.

They reduce water flow in dams and rivers, increase the accumulation of sediment and elevate the nutrient concentration, which reduces oxygen and affects all life within that environment, resulting in biodiversity loss.

“Then there’s an ecosystem function that it affects directly. It affects the water flow … and if you don’t have proper flow of the rivers with heavy rains, then you’re going to have floods, etc. That then becomes a byproduct of having these species in your water,” said Coetzee. DM

https://www.youtube.com/watch?v=REeWvTRUpMk

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Methods of control

Where do the alien invaders come from?