Scientific name: Adansonia digitata Shona: Muwuyu, Muuyu Ndebele: Umkhomo The baobab has been woven into the history,…
Invasive Aquatic Species
Zimbabwe is experiencing a disturbing rise in the extent of several invasive aquatic species, taking over the surfaces of rivers and dams throughout the country. The following looks at the main species involved, and the various negative effects of these invasive plants. Finally, you will find a report by Lelee Maasdorp on the effects of these invasive species on the Bream Ponds at Rifa.
| Species | Common name | Origin | Area in Zimbabwe where highly invasive |
| Azolla filiculoides | Water lettuce | Tropical south America (Argentina, Brazil, Uruguay, Peru) | Northern, Central & eastern parts of Zimbabwe |
| Eichhornia crassipes | Water hyacinth | Tropical/subtropical South America, in particular Brazil & Ecuador | Rivers & lakes in all provinces except semi-arid Matabeleland South |
| Pistia stratiotes | Water cabbage/lettuce, Nile cabbage | Uncertain, probably pan-tropical but often held to originate in the Nile, near Lake Victoria | In stagnant water bodies throughout the country |
| Salvinia auriculata | Water fern, eared water moss | The Americas – Mexico to Argentina and Chile | Infested the Zambezi River |
| Salvinia molesta | Kariba weed, Giant salvinia | Brazil | Mostly found in Kariba but also scattered in some inland water impoundments |
The diagram below sums up the desirable characteristics of indigenous aquatic plant species alongside the undesirable characteristics of invasive aquatic species.
Negative Impacts of Invasive Aquatic Species
- Degrades water quality
- High biomass production and subsequent decomposition – reduces oxygen levels in the water adversely affecting fish and other aquatic organisms
- Reduce species diversity – native aquatic plants and animals are outcompeted and displaced
- Suppress desirable native plants, localized extinction of native species.
- Alter animal community interactions – previous food chains broken and various species disappear
- Dense plant beds provide a place for small forage fish to hide from
- predatory fish – leads to over-population of small, stunted forage fishes and few game fishes
- Increases extinction rate of rare, threatened and endangered species
- Increases detritus buildup at the bottom of the waterbody
- Changes sediment chemistry
Negative Impacts of invasive aquatic species on humans and the economy
- Impairs drinking water (taste and odour)
- Interferes with recreational boating impairment
- Interferes with safe swimming (increased drowning risk)
- Interferes with fishing, recreational and commercial
- Interferes with irrigation, clogging systems
- Reducing water availability for agriculture and increasing maintenance costs
- Endangers human health – provides a habitat for insect-borne disease vectors
- Disrupts hydropower generation by blocking water intakes and reducing the efficiency of turbines
- Increases flood frequency, duration and intensity
Leslee Maasdorp, Dec. 1994, Rifa/Hunters Association publication
In a small case study Leslee Maasdorp describes developments at Bream Pools, part of the ancient Zambezi River System near Rifa Conservation Camp over a period of 6 years. The Pools were naturally lightly covered with indigenous water plants such as water lilies, water gentian and fringing sedges, bulrushes and water ferns. In 1987, though water lilies still covered part of the water surface in the rainy season, Kariba weed (Salvinia molesta) was well established. By 1994 this weed, together with other alien waterweeds, formed a thick mat with Cyperus spp creating a double layer over parts. The appearance was of a solid weed surface with occasional clearings showing in the water made by hippo and elephant.
| 1988 | 1994 |
| Cover – 40% of surface | Cover – 96% |
| Cyperus spp. (indigenous sedge usually fringing water bodies) Marsilia spp. (indigenous fern) Nymphaea caerulea (indigenous waterlily) Pistia statiotes (alien Nile cabbage or water lettuce, originates from S. America) Salvinia molesta (alien Kariba weed) | Cyperus spp. (indigenous sedge – now growing on the sudd) Marsilia spp. (indigenous fern) Pistia statiotes (alien Nile cabbage) – almost gone Azolla nilotica (alien fern, turns reddish, covers water surface rapidly) Salvinia molesta (alien Kariba weed, from south Brazil but now called “Kariba weed” because it has spread so fast on Kariba) |
The thick floating mat of plants greatly affected the oxygen content and amount of light in the water which in its turn affected microscopic and macroscopic plant and animal life in the water. Leslee Maasdorp focused on the birdlife on or around the Pools as her measure of the effects.
Birds resident at or visiting for feeding at Bream Pools
| 1988 | 1994 |
| Great white egret Little egret Rufous-bellied heron Open-billed stork Fish eagle Black crake African jacana White-browed coucal Pied kingfisher Malachite kingfisher | Rufous-bellied heron Black crake African jacana White-browed coucal |
Most of the birds that were visiting the Pools were hunting fish or amphibians that could no longer live in the pools by 1994. Birds like the Jacana, and the Black crake could still be present because they are adapted to be able to run across the top of the weed mat hunting insects like dragonflies.
……………………………………………………………………..
As far back as 1994 Leslee Maasdorp and her colleagues were seeing the disastrous effects of uncontrolled alien aquatic vegetation choking our waterways and water bodies. We all need to contribute in whatever way we can to efforts to bring these invasive species under control.
…………………………………………………………………………………………………………………………………
POSTSCRIPT
In 2024 Dr. Rob Rees reported to me (verbally) that the Rifa Bream Pools had disappeared entirely as a result of the halting of annual flooding of the Zambezi in the wake of the building of Kariba Dam.
In March 2026, however, after a good rainy season, the Bream Pools are alive and well again – though still covered in many places with invasive aquatic plants.
More to Explore
