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Kenya: Lake Victoria faces new threat as grasses overrun the waters
Experts warn the encroachment of the lake by grasses is an indication that something is amiss in the Lake Victoria ecosystem. The water mass has, for the last seven years, been choking under massive water hyacinth growth.

Dr Richard Abila, the deputy director of the Kenya Marine and Fisheries Institute, says the presence of vegetation in the lake is a symptom of a problem.

“The growth of the plants shows that there are excess nutrients getting into the lake,” concurs Mr Okoth Mireri, an environmentalist at the Friends of Lake Victoria lobby group.

Plant succession

The invasion by the plants is a phase in plant succession in the lake that could culminate in the reduction of its depth.

“Plant succession is the process by which stronger plants take advantage of the presence of another plant to invade a habitat,” explains Dr Abila.

About the lake getting shallower, he argues that it is not a direct result of the grasses.

“Generally, the presence of vegetation in a water body reduces its depth due to the increased deposition of silt,” he says.

According to Mr Mireri, the purple-flowered water hyacinth, with its long fibrous roots, is partly to blame for the new development.

“The hippo grasses that have invaded Lake Victoria are usually present in rivers, and small pieces find themselves in the lake,” he explains.

The grasses do not appear normally in an open lake ecosystem, and are instead dominant in river basins draining into the lake, such as River Nyando.

Bits of the grasses find their way into the lake. The floating grass cuttings get trapped in the mat formed by the water hyacinth.

In the nutrient-filled water of the lake, the stationary pieces get a medium for growth, leading to sprouting of the grasses.

The rapidly growing plants, he says, will overtake the water hyacinth in the lake, which will consequently shrink leaving the grasses to colonise the water mass.

Two species

Currently, Mr Mireri adds, there are about two species of the hippo grass, but after it subdues the hyacinth, other grasses will come up. There is the hairy vossia species, and a soft deep green cladium species.

“Typha grass and papyrus could soon sprout in the lake leading to floating islands,” says the environmentalist.

The grasses shelter the shores of the lake against wave action, thus making it easy for the floating islands to form.

This has the effect of increasing mud at the bottom of the lake; hence the lake will gradually get shallower.

However, Dr Tom Okurut, the executive secretary of the Lake Victoria Basin Commission, says hippo grasses cannot survive in deep parts of the lake.

“The grass is usually common in the shallow areas of the lake, next to the shore,” he explains.

Dr Okurut adds that little is known about the succession system of the aquatic weeds in Lake Victoria and studies are on going.

For instance, he says, it is not known whether the wind can influence distribution of the grasses as is usually the case with water hyacinth.

Dr Abila clarifies that, like water hyacinth, the grasses can be blown to other areas in the lake and back.

“Consequently, the area under the grasses and other vegetation will expand with time,” he predicts.

A project to rid the lake of the water hyacinth had been slated to begin in January but has been delayed. He, however, says that it will take off soon.

Continuous removal

After the project is implemented, there are plans for continuous removal of the weeds after the initial massive removal.

“This strategy has been applied successfully in Uganda and Tanzania,” says Dr Okurut.

Healthwise, the stagnation of the lake water as a result of the presence of the aquatic weeds could provide a fertile breeding ground for mosquitoes which cause malaria.

The aquatic weeds also stand in the way of fishing expeditions, water transport and other economic activities.

However, the plants are not all about causing harm; they have a wide range of beneficial uses besides increasing biodiversity.

“In the long run, the grasses will lead to increased yields of fish species such as lung fish, mud fish and tilapia (ngege),” he says.

Mr Mireri adds that the grasses also aid in natural conservation. The bird population around the lake will increase due to additional habitats provided by the grass bushes.

The aquatic weeds are beneficial as they can be harvested and fed to livestock. This would be particularly helpful in zero-grazing.

“On overall view, the grasses are more economical as compared to the water hyacinth,” says Dr Abila.

All this notwithstanding, Mr Mireri seeks to allay fears that the lake could be on its way to extinction.

“Lake Victoria is not about to disappear, at least not by the growth of the plants,” he says.

Lakes, he adds, depend on recharge from the ground water and the amount of rainfall received and would not disappear unless these are compromised.

“The gradual reduction of rivers Nyando and Sondu Miriu could be a stronger cause for alarm over the disappearance of the lake,” adds Mr Mireri.

But scientists are in agreement that the weeds should be got rid of as soon as possible.

“Mechanical removal would be a good option since the pieces of the plant could be carried away to feed livestock,” says Dr Abila.

This method, he says, failed for water hyacinth because the pieces of the plants were dumped on the lakeshore.

While plans are in progress for the increase in rearing of weevils, Dr Abila says that biological control has not been developed for the grasses.

“A long-term solution for the problem would be reducing with the huge nutrient load in the lake,” he says.

Organised sewage system

Most of the informal settlements in Kisumu town such as Obunga and Nyalenda do not have a sewage system, and the untreated waste from the pit latrines flow underground and get to the lake.

The first step towards solving the nutrient problem would be getting everyone in the town, including the industries, to channel their waste into an organised sewage system.

“We need to deal with nutrient flow in the lake right from the top regions of the catchment,” Mr Mireri recommends.

Moreover, afforestation in the catchment areas should be intensified to reduce erosion, which washes the top fertile soils into the lake, thus increasing the nutrient load.
 
 
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