Cosmas Butunyi
16 September 2008
Nairobi — When the mass of vegetation that overran the surface of Lake Victoria disappeared, there was a sigh of relief.
Fishermen applauded because they could access the depths of the lake in search of fish and transporters, whose activities had been hampered by the weeds, got a new lease of life.
However, experts say that life could have become easier for the users, but it is not time to celebrate. The problem just got more complex and in the long run, it could mark the beginning of the end of the world's second largest freshwater lake.
According to the scientists, water hyacinth and hippo grass covering more than 3,000 hectares are decomposing at the bottom of the lake where it sunk.
So great is the concern that the new turn of events has generated that the Lake Victoria Basin Commission has ordered a study to investigate the effects of the sinking vegetation.
"The study is meant to trace the succession of the weeds throughout the lake," says Dr John Gichuki, a researcher at the Kenya Marine and Fisheries Research Institute, who led the team.
The team of biologists, ecologists, plant scientists, public health specialists, crop experts and socio-economists, traversed the water mass to study the weeds and also talked to fishermen on the beaches.
Geographic information systems were also used to extrapolate the developments, comparing the latest maps with the situation in 2005.
Dr Gichuki says preliminary findings of the study indicate that the water hyacinth is the genesis of the problem.
On the water hyacinth, a new weed, the hippo grass, flourished. It found not only a source of nutrients, but also anchorage.
"The huge grasses grew and shaded the hyacinth which eventually died off and left a mat that acted as a substrate," explains Dr Gichuki.
The grasses' lush growth was aided by the water hyacinth weevils that were introduced by the Kenya Agricultural Research Institute (Kari) in 1997 to control the weed.
After exhausting the nutrients, the grasses that are usually land plants, had no surface to hold onto. They normally inhabit the fringes of water bodies.
"Before the emergence of the water hyacinth, the hippo grass never used to float in the open waters," says Dr Jembe Tsuma, also a researcher at the fisheries institute. Gradually, the water hyacinth mats that provide a substrate for the grasses to hold onto are depleted, and the heavy vegetation comes tumbling down to the bottom of the lake.
Submerged under water, the plants eventually die off and begin rotting with profound effects on the water quality.
Bacterial decay of the organic matter releases ions into the water, leading to increased conductivity -- the ability to transmit heat. "Conductivity has risen by about 20 per cent since the 1990s," says Dr Gichuki.
The breakdown of the plants uses up oxygen from the water column and in turn releases noxious gases. This has impacted on the quantity of fish and diversity of catches.
Fish species that require environments with high levels of dissolved oxygen flee from the stressful waters. This includes the commercially valuable Nile Perch, which rakes in Sh6 billion annually from exports.
"The change in the characteristics of the bottom of the lake from rocky and sandy to one that is largely muddy however leads to an increase in species such as the mudfish and catfish," Dr Tsuma says.
Latest studies, he adds, indicate that the Lake Victoria fishery is transforming from one that is dominated by Nile Perch to one based on endemic species.
There has been an upsurge in species such as catfish and mudfish that are locally known as mumi and kamongo respectively, which only five years ago were a rare catch in the Nyanza Gulf.
"The other species that are non-tolerant to the conditions move towards the mouth of the gulf that has been less affected by the weeds such as Mbita and Lwanda Gembe," Dr Tsuma explains.
The process of decomposition of the weeds also increases turbidity in the lake water. This affects the growth of phytoplankton -- microscopic plants that fish feed on.
Consequently, he argues, though the decomposing weeds release nutrients, mainly phosphates and nitrates into the water column, productivity of the water body remains abysmally low.
He says that growth of useful phytoplankton is greatly reduced and some that is produced is toxic and makes the water hostile for fish. "The micro-organisms also clog the gills of fish and cause irritation," he adds.
In addition to the effects on aquatic life, the new development is likely to have huge socio-economic implications on the fishing communities.
Dr Richard Abila, the deputy director at the fisheries institute says the health of the fishermen and communities living around the lake is now, more than never before, at a higher risk of disease. The decomposing weeds have also compromised the quality of the lake water for drinking, watering livestock and domestic use.
"The weeds have increased vectors for bilharzia and malaria around the lake region," he adds.
Prof Alfred Ofulla, a researcher at the Maseno University's School of Public Health, says the larval stages of malaria-transmitting mosquitoes reside in the weed. He adds that investigations are on to correlate the hippo grass and the occurrence of malaria and bilharzia.
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