ALTHOUGH it generally lies quietly on the surface of major lakes, dams and rivers, the water hyacinth, is still proving difficult to contain as it often becomes aggressive and active, multiplying at a faster rate during hot and rainy seasons.
BY TONDERAYI MATONHO
According to environmental conservationists, the water hyacinth is among proliferating invasive alien species constituting the third largest cause of biodiversity loss on the planet. They cover the entire gamut of living organisms, ranging from plants, both aquatic and non-aquatic, to rodents, insects, fish, trees and micro-organisms.
The International Union for the Conservation of Nature (IUCN) has so far identified 217 culprits, with the 100 most dangerous ones featuring on its black list. The water hyacinth is recognised as one of the top 10 worst weeds in the world.
A visit to Lake Chivero and other major lakes or dams around the country shows that most of these fresh-water bodies have been greatly affected. Parks authorities are finding it difficult to tame it and water experts can only explain its origins, but not offer solutions on how to effectively contain it.
Parks and wildlife authorities at one time had to resort to hand-spraying using the knapsack method with a chemical (glyphosate) which, according to experiment results, could kill the water hyacinth in five days.
Around 1986, when the weed was first detected at Lake Chivero, weed-barriers were constructed in the upper-reaches of the lake to prevent the weed from entering the lake. However, with the passage of time, both methods came to nought as the weed continued to spread.
“The water hyacinth becomes inactive during the winter season, but when it starts getting hot and rainy, it multiplies very fast,” said Harare-based agronomist, Kingstone Chaitezvi.
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“Between 1971 and 1972, the weed invaded Lake Chivero (then known as Lake Mcllwaine) and a controversial herbicide had to be used to control it.”
He said the herbicide, 24-D, was said to be harmful to both humans and aquatic life but the city council used it regardless. The weed was put under control only to come up again around the end of 1985.
United Nations Environment Programme (UNEP) notes that the spread of invasive alien species is neither easy to manage nor reverse, threatening not only biodiversity, but also economic development and human wellbeing.
The water hyacinth, according to water experts, is believed to have originated from South America.
According to top water expert and University of Zimbabwe professor Chris Magadza, the water hyacinth invaded Africa through human activity.
“Water hyacinth is found across the tropical and subtropical regions. Native to the Amazon Basin in South America, its entry into Africa, Asia, Australia, and North America was facilitated by human activities,” he told local media recently.
He highlighted that water hyacinth has emerged as a major weed in more than 50 countries in the tropical and subtropical regions of the world with profuse and permanent impacts.
With climate change also allowing the spread of water hyacinth to higher latitudes, Magadza said intensified monitoring, mitigation and management measures were needed to minimise the problem.
In Africa, where water hyacinth is listed by law as a noxious weed, it is the most widespread and damaging aquatic plant species.
UNEP notes that the economic impact of the weed in seven African countries has been estimated at between $20 million and $50 million every year. Across Africa costs may be as much as $100 million annually.
In a review of water hyacinth infestation in eastern, southern and central Africa, experts reports note that the weed was first recorded in Zimbabwe in 1937.
It colonised important water bodies, such as the Nkomati River in Mozambique in 1946, the Zambezi River and some important rivers in Ethiopia in 1956.
Rivers in Rwanda and Burundi were colonised in the late 1950s while Sigi and Pangani rivers in Tanzania were infested in 1955 and 1959.
The plant colonised Kafue River in Zambia in the 1960s, the Shire River in Malawi in 1968 and Lake Naivasha in Kenya in 1986.
Lake Victoria in Uganda is the second largest freshwater lake in the world and currently supports approximately 30 million people.
According to the World Agro-Forestry Centre, infestation of water hyacinth in the lake has been a serious nuisance, generating public outcry. At its peak, it was estimated that the weed was growing at the rate of three hectares per day on the lake.
Today, biological alien invasions are a major driver of biodiversity loss worldwide. Water hyacinth is challenging the ecological stability of freshwater bodies, out-competing all other species growing in the vicinity, posing a threat to aquatic biodiversity.
Experts note that besides suppressing the growth of native plants and negatively affecting microbes, water hyacinth prevents the growth and abundance of phytoplankton under large mats, ultimately affecting fisheries.
Floating mats of water hyacinth support organisms that are detrimental to human health. The ability of its mass of fibrous, free-floating roots and semi-submerged leaves and stems to decrease water currents increases breeding habitat for malaria-causing anopheles mosquito as evidenced in Lake Victoria.
Snails serving as vector for the parasite of Schistosomiasis (Bilharzia) reside in the tangled weed mat.
Water hyacinth has also been implicated in harbouring the causative agent for cholera. For example, from 1994 to 2008, Nyanza province in Kenya, which borders Lake Victoria, accounted for a larger proportion of cholera cases than expected given its population size (38,7% of cholera cases versus 15,3% of national population).
At the local level, increased incidences of crocodile attacks have been attributed to the heavy infestation of the weed which provides cover to the reptiles and poisonous snakes.
However, experts point out that water hyacinth control is absolutely essential. Control methods that are often used include mechanical, chemical and biological control.
But existing methods have often been insufficient to contain the aggressive propagation of the weed and viability of its seeds despite substantial monetary investments over the years, mainly due to lack of continued policy and management support by governments.
The weed infestation on Lake Chivero which supplies water to Harare, was controlled and declined from 42% in 1976 to 22% in 2000.
Another re-invasion emerged in 2005 and included massive amounts of another invasive plant, spaghetti weed.
However, in an effort to turn the challenge on its head, researchers note that some communities have come up with a solution — using the alien species as a source of income; for example, the water hyacinth can be turned into compost, biogas, fish-meal and even paper.
Researchers note that market gardeners on the banks of the River Niger have used water hyacinth to make compost. Today more than 20 market-gardening co-operatives are producing compost from it.
In Thailand, South East Asia, plaited hyacinth stems are used to make furniture and basket-work for sale in Europe.
In Burkina Faso, the water lettuce (pistia stratiotes) is being used as a water purifier.
Experts say exchanging experiences on prevention and control strategies is crucial, be it between one country and another, or between continents.
The IUCN has adopted the principle that “every alien species needs to be managed as if it is potentially invasive”.
However, this is a huge challenge at a time when global trade and mass tourism are creating perfect conditions for a wide variety of species to be on the move.