Harare — THE latest episode of massive fish deaths in Lake Chivero due to deoxygenation compounded by excessive pollution indicates the gravity of the ecological disaster on Harare's principal water supply dam.
There have been several incidents of fish deaths in Lake Chivero and its feeder tributaries -- Nyatsime, Marimba and Manyame in the past and no consistent and meaningful, practical measures were taken to improve the ecological conditions that are responsible for the fish deaths.
Zimbabwean scientists have since 1952; when Lake Chivero was created, made several studies on the lake's ecology. They also made important recommendations to Government and the City of Harare on how to improve the quality of water, reduce pollution levels and enhance the ecological conditions on Lake Chivero, the source of drinking water for more than 3 million people now.
The last major work on the ecology of Lake Chivero was in 1996/7 by Prof. Ngonidzashe Moyo, formerly with the University of Zimbabwe's Department of Biological Sciences and other scientists drawn from the field of chemistry, freshwater biology, engineers, urban planners and environmental studies.
In the compilation titled: "Lake Chivero: A Polluted Lake" Zimbabwean scientists gave invaluable information on the status of pollution in Lake Chivero and its catchment area, the causes and impact of pollution and possible solutions to enhance the ecological conditions of the lake were suggested.
This project came after more than a million fish died over a four-week period in Lake Chivero in 1996 as a result of deoxygenation worsened by ammonia toxins.
In the wake of the disaster, Government banned commercial fishing and recreational activity for a period of 10 weeks, stepped up pressure on Harare City Council to upgrade sewage treatment plants, tightened pollution laws and embarked on a campaign to raise awareness on the need to reduce pollution in rivers. However, over the years, a combination of factors have led to an increase in the discharge of sub-standard sewage effluent into rivers that feed Lake Chivero.
Industrial effluent discharges are rising leading to an increased level of various heavy metals in the lake. Rapid urbanisation both within the Greater Harare area, dormitory towns and outlying communal areas has worsened the pollution of the lake.
"The pollution levels in Lake Chivero are increasing and it's uncharacteristic for fish to die now. It's unusual," said Prof. Moyo, a freshwater biologist now working at the University of Limpopo in South Africa.
He said fish deaths normally occurred when it was cold or when there is an overturn around April/May.
Fish deaths, he said, could be triggered by seasonal turnover/inversion that mixes the lake's water layers. When the surface temperature drops sharply, the upper layer, that is less toxic becomes colder and denser than the bottom layer, which then rises to replace the denser layer. More often than not, the bottom layer will have more toxins that kill fish upon inversion.
Prof. Moyo suggests that episodes of oxygen deficiency have become increasingly severe due to excessive pollution of the lake.
"The fact that we are having these fish deaths is indicative of the excessive pollution levels. There may be other factors that are causing the fish deaths and there is need to do more investigations. The issue is much more complicated even though we may point to high Ammonia levels," he said.
Ammonia is toxic to fish gills and Prof. Moyo said the preliminary investigations he conducted in the past demonstrate that: "If the mouth (of the fish) is wide open and has a bulging head, this indicates that there are high ammonia levels and little oxygen."
The discharge of sludge, the product of water clarification with aluminium sulphate at the Morton Jaffray water works, which contains high concentrations of aluminium has led to the accumulation of deep layers of anoxic sludge in the Manyame River bed which are often deposited in the lake when rains come.
Prof. Moyo concurs with the findings made by Prof. Christopher Magadza, a fish biologist, in 1996 that the flushing of this sludge deposit into Lake Chivero may have been responsible for the fish deaths reported recently. Biologists say changes in the water chemistry of the lake have led to the deterioration of water quality.
The main indicators include:
lHigh concentrations of heavy metals and other pollutants.
lThe massive water hyacinth infestation that removed nutrients from the water and locked it up as plant biomass.
lHigh levels of compounds like ammonia and others.
lEutrophication of the lake -- nutrient pollution which may promote excessive plant growth and decay, weedy species reducing the quality of water.
lThe density of the blue-green algae which produce toxins that have caused the seasonal outbreaks of gastronomic diseases in the city.
Experts have, in the past, also identified the major problems that have contributed to the ecological crisis in Lake Chivero, which they say include:
lThe City of Harare lies within its own catchment area and most of the pollution problems being experienced now are a result of this poor original planning. This means that all the city's waste, which passes through the heavily industrialised and densely populated areas, flows into the lake. This has compromised the quality of the city's water and contributed to the accumulation ammonia compounds that are causing fish deaths every year.
lLack of resources to upgrade sewage treatment works in Harare, Chitungwiza and Ruwa, which have become overloaded owing to rising urban population.
lLack of funding for water quality monitoring activities and for research purposes into the lake's ecology.
lLegislation that does not force the polluters to bear the cost of their activities, penalties are too low to act as deterrents.
lOvercrowding and the housing crisis have led to the development of informal settlements along the city's main rivers that feed Lake Chivero.
lBureaucracy -- there are no clear guidelines on how various stakeholders can participate in the management of the lake's ecology.
lIn the Lake Chivero catchment stream, self-purification has been lost through environmental degradation.
lPoor management of wetlands of the Manyame and Nyatsime rivers. These wetlands and sponges regulate the hydrology of the streams and control the quality of water flowing into the lake.
"In ecological terms, we have identified this lake as a hot spot of pollution. Lake Chivero and the Kafue River in Zambia are the most polluted rivers in southern Africa.
Sources in the water-engineering sector say it is now becoming increasingly difficult for the city's water to reach World Health Organisation standards. This is a sensitive and controversial issue for politicians who often keep the lid tight.
Prof. Moyo said the ecological disaster in Lake Chivero had led to increased water treatment costs, rising costs of maintaining water treatment works, water shortages, lack of funds to procure chemicals for water treatment and loss of biodiversity.
The flight of skilled engineers, freshwater biologists, fish pathologists, ecologists and other environmental experts and urban planners to other countries has worsened Zimbabwe's capacity to respond to ecological disasters. Experts say solutions to the ecological crisis in Lake Chivero include:
lPrioritising the construction of Kunzvi Dam -- this will reduce water treatment costs as the water will be free from industrial pollution and sewage effluent.
lUpgrading sewage treatment works in Harare, Chitungwiza and Ruwa.
lPutting special considerations in research and management of wetlands and streambed structure to improve water quality management.
lEnhancing the capacity of the Environmental Management Agency to monitor and investigate industries and other polluting sectors.
lIncreasing funding for monitoring activities, equipment and the training of manpower involved in managing the environment.
lInvolving the private sector in environmental programmes.
lStrengthening environmental awareness programmes.
Conducting environmental impact assessment, developing and applying water quality systems, the polluter-pays principle, self-regulation, providing economic incentives, public participation and tightening regulation and enforcement can enhance water quality management and pollution control.
"Water is a basic right, we are all entitled to clean water. Its sad if no special attention is given to Lake Chivero because this is what millions of people survive on," said Prof. Moyo.
"Politicians react when the crisis is dramatic and affects many people. And when its not dramatic there is little attention on the unfolding ecological disaster."