An ace in Lake Kariba weed fight?

An invasive water weed poses a major threat to aquatic life and economy around one of Africa’s most significant lakes — Lake Kariba. But a tiny, ravenous weevil could help turn the tide against the invasive weeds. In this report, our Agric, Environment & Innovations Editor Sifelani Tsiko (ST) speaks to Zambezi River Authority chief executive Engineer Munyaradzi Munodawafa (MM) on the interventions his organisation is making to manage the water weed at Lake Kariba with 181 cubic kilometres of water straddling Zimbabwe and Zambia on the Zambezi River.

ST: Lake Kariba is the most important freshwater lake for Zimbabwe and Zambia, holding up the bulk of water for the generation of power, fisheries, tourism, wildlife and human consumption, but the water hyacinth is an invasive weed posing a grave threat to Lake Kariba. Can you tell us the extent of coverage of this dangerous aquatic weed on the lake?

MM: Lake Kariba is about 280 kilometres long and up to 40 kilometres in width. It covers an area of 5 580 square kilometres and its water storage capacity is 181 billion cubic meters. Only a fraction of this immense surface area records the presence of water hyacinth. The presence of water hyacinth on Lake Kariba varies, and this also depends on the time of the year. 

A larger part of the lake water body does not generally show any pronounced presence of these floating weeds, except for a few isolated cases. These isolated cases are more visible during the onset of the rainfall season when the weeds float from the upstream section of the Zambezi River into Lake Kariba as well in instances when some of the weeds located in hotspot areas along the lake shoreline may shear off due to wind action coupled with rising water levels and float across the water body. 

The weeds tend to be more pronounced in some lake shore areas where there is reduced movement of the lake waters, commonly referred to as the weed hotspots. In the hotspot areas, the weeds form larger weed mats covering a larger surface area of the lake shore waters. 

The weed hotspot areas tend to register a higher weed population with an increase in water levels at the onset of the rainfall season. The hotspot areas are more prone to human activities, for example, agricultural activities that use fertilisers, that may introduce nutrients that tend to sustain the weeds.

ST: What are the main dangers of this aquatic weed to Lake Kariba? Tell us about the likely impact on people who depend on the lake for survival.

MM: If the weeds were to proliferate to an extent where they cover the whole of Lake Kariba, they would negatively impact the continued availability of adequate dissolved oxygen in the lake waters which is key to supporting aquatic life. 

The proliferation of the weeds could lead to a situation where there is a choking of aquatic life due to reduction in the levels of dissolved oxygen. This would impact the large kapenta and tilapia fishing and aquaculture community due to reduction in fish populations due to the associated environmental change in the fish habitat. 

The proliferation of the weeds could also hinder lake navigation for the local communities as well as the tourism sectors where there would a reduction in boating, fishing and other water based tourism activities — for example, the Tiger fish has been officially recognised by the International Game Fishing Association as a game fish and a world renowned tournament is held annually on Lake Kariba. 

Regards other potential impacts on people, this would include the risks such as those posed to power generation at the power stations. In this case, the proliferation of the water hyacinth weeds in a water body like Lake Kariba is a cause for concern because, if left unchecked, the weeds could then float enmasse to the dam wall and power station area, and subsequently find their way into the power station water intakes and clog the system, thereby presenting power generation operational challenges.

ST: What do you think are the major drivers of the spread of the invasive weed on Lake Kariba? Which are the most affected parts of the lake?

MM: Water hyacinth growth is supported mostly by the presence of nutrients, principally phosphates and nitrates. The nutrients maybe indirectly introduced into water bodies through human activity such as agriculture where there is application of fertilisers and the associated run-off finds its way into the lake environment. 

For Kariba, it has been noted that though the plants do not cover the wider surface area of the lake, they are more visible in a few shallow bays where there is slow moving water where such nutrients could be present. The hot humid weather around the lake also tends to favour the growth of the plant in these areas. The more visible presence of the weed around Lake Kariba is in Mlibizi Area, Binga Area, Ume River area and part of the Eastern Basin of Lake Kariba near University Lake Kariba Research Station, Crocodile Farm and along the Gatche Gatche River area.

ST: The Zambezi River Authority has over the past years taken measures to control the invasive water hyacinth weed which include chemical control. Is the authority still using chemical control of the weed? If not, why has the authority stopped using chemical control of the weeds?

MM: Chemical control is currently not in use. Its use is discouraged due to the risk of some effects it could pose to the environment. Chemical control was last used in the late 90s when the water hyacinth coverage at that time was more pronounced, but even at that time, the use of chemicals was in a restricted and controlled manner and only in areas where the presence of the weeds was more pronounced. 

Currently, the authority uses an integrated tool kit as control measures for the short and long term. Firstly, through the long-term biological control measure that encompasses the use of biological control agents such as weevils. Physical/mechanical means are also used to remove weeds, especially those that float in the lake area adjacent to the dam wall.

ST: The ZRA has also used equipment for the mechanical control of the Kariba weeds. Are you still using mechanical methods? Has this been successful? If not, why have you stopped using this method?

MM: Yes, as and when necessary. Mechanical methods have been successful when removing weeds when they float in the area adjacent to the dam wall so as to prevent the weeds from getting too near the power station water intakes and subsequently entering the power station and clogging the power generation system.

ST: In Zimbabwe and other parts of Africa, water weeds have been controlled through the use of mechanical and manual removal, herbicide application and biological control. In the 2020 ZRA report, the water authority says it is now using the weevils of the Nichortina species as biological agents to constrain the growth of the invasive plant species. Why has the ZRA chosen to use biological control of weeds? To what extent has this been successful? Is this now the only methods being used at the Lake?

MM: I have covered some aspects of this question. With respect to the use of biological control through introduction of weevils of the Neochetina species, this has been the preferred weed control approach for the last 15 years as it is a more natural approach compared to the use of chemicals over a long period of time that could lead to possible increase in the risks of negatively impacting the lake environment and the associated biodiversity. Considering that pesticide control is not involved, the process of controlling the proliferation of the weeds using the weevils is cheaper and more sustainable. 

The use of weevils is now globally known to suppress their growth and reproduction thereby reducing the biomass and area coverage of the weeds. This is key in limiting the possibility of proliferation of the weeds over the wider area of a water body like Lake Kariba. 

The adult and larval weevils eat water hyacinth and, by tunnelling into the plant, allow bacteria and fungi to invade the plant. Water also enters these tunnels, sinking the mats. Once submerged, water and wave action continue to degrade the plant matter. 

Additionally, the fluctuation in the lake water level at Kariba has also helped to control the proliferation of water hyacinth coverage, especially when the lake level reduces during the dry months of the year, leaving weeds stranded to dry up on the lake shores.

ST: Water weeds don’t recognise national borders or boundaries. What do you think needs to be done to enlist the support of local communities, nations and the international community in the fight against the weed menace on the lake?

MM: Accordingly, ZRA, being a bilateral entity, has also taken advantage of this approach and is part of such local, regional and international efforts. 

At a local level, ZRA continues to liaise with the local communities as well as other government agencies such as the Environmental Management Agency of Zimbabwe (EMA) and the Zambia Environmental Management Agency (ZEMA), as well catchment management institutions such as the Zimbabwe National Water Authority (ZINWA) of Zimbabwe and Zambia’s Water Resources Management Agency (WARMA). 

Full article on: www.herald.co.zw

At an international level, ZRA has in the past received support in efforts to address various challenges around Lake Kariba, including technical capacitation from other International Organisations like the French Fund for Environment (FFEM/FGEF), the Swedish Government through Swedish International Development Agency (Sida) and other Cooperating Partners. 

We work with all key stakeholders — local communities, government agencies, donors and NGOs to tackle problems such as water weeds. 

More recently, the Authority is in the process of taking advantage of the Zambezi Water Course Commission’s (ZAMCOM) roll out of a regional strategic plan where one of the pillars of the plan is environmental considerations, including issues of environmental management.

ST: Funding still remains a huge handicap in all efforts to contain the water hyacinth menace on the lake. How much has been mobilised to support weed control programmes on the lake? What is the funding gap for the programme? Which international partners are supporting the ZRA in this programme?

MM: ZRA has an annual internal budget and associated action plan that continues to support the environmental monitoring programmes on Lake Kariba that incorporates the monitoring of the weeds. The authority is also involved in activities that could also bring additional funding so that the monitoring is scaled up in terms of frequency.

ST: Are there any researchers from Zambia and Zimbabwe focusing on weed control on the lake? If so, what are some of the research outcomes aiming to help control the weed menace?

MM: There are several researchers from the two nations’ agencies, universities and research institutions, for example, the University of Zimbabwe and University of Zambia to mention a few. These can be directly engaged to obtain more details on the subject matter in question.

ST: Looking ahead, how do you see the fight against the weed menace going in terms of using new technological innovations?

MM: The involvement of tools such as the Global Positioning System (GPS), Satellite Remote Sensing (applying satellite imagery) and Geographic Information Systems (GIS) as well as drones and other technologies continue to allow institutions to enhance their monitoring mechanisms around weed control. 

The spread of the weeds in water bodies is curtailed where organisations continue to take advantage of such tools in making informed decisions under environmental management and monitoring programmes they implement that cover various aspects, including weed control to promote the continued health and well-being of water bodies.