Water extraction in Siwa, Egypt © Kevin Smith
Percentage of freshwater species affected by the major threats in Africa
The leading threats to African freshwater species are habitat loss and degradation due to agriculture, and deforestation. How this and other threats affect freshwater ecosystems is outlined below:
Deforestation, particularly of upper catchment areas, has led to increased runoff carrying greater sediment loads into rivers and lake systems. Sedimentation of rivers and lake habitats leads to the deterioration of many habitats important to numerous species. Sedimentation is also a problem in many areas where crops are cultivated right to the rivers edge leading to the increased sediment loads. Sedimentation is primary threat to Fish and Molluscs. As more terrestrial species, habitat loss due to agriculture is primary threat to Odonata and Plants. In total, 10% of Africa’s freshwater taxa are threatened by habitat loss due to agriculture, and 12% due to deforestation, with more than 15% threatened by sedimentation (predominantly as a result of these two activities).
The main sources of water pollution are sedimentation, agricultural runoff, domestic and industrial effluents. Eutrophication is one of the most prevalent global problems of our era. It is a process by which lakes, rivers, and coastal waters become increasingly rich in plant biomass as a result of the enhanced input of plant nutrients mainly nitrogen (N) and phosphorus. Eutrophication of water bodies leads to increased species mortality, changes in species assemblages and loss of aquatic flora and fauna diversity. In Africa 28% of lakes and reservoirs are impaired by eutrophication. Examples include Lake Victoria, which is shared between Uganda, Tanzania, and Kenya, Lake Chivero in Zimbabwe, Lake Albert on the boundary between Uganda and Congo, the Zeekoevlei in South Africa, and the rift lakes in Ethiopia.
Pesticides used in agriculture and against human pest vectors (e.g. tsetse fly) are an emerging problem for species survival and water quality in wetland areas. Pollutants from mining activities (such as lead, cadmium, iron, and copper) and organic wastes from leaking sewage systems can accumulate in rivers and other freshwater bodies and affect water quality and species survival. Bioaccumulation of metals in fish and crabs may become health problem in the future as population, uncontrolled effluent discharges and intensive agriculture increases. Some freshwater systems are able to retain nutrients (N and P), which can act as buffers to eutrophication, and constitute an important role in maintaining lake and river ecosystems. However, encroachment on freshwater habitats and increased wastewater production in urban areas, have increased nutrient loading beyond their buffering capacity, thus impacting on freshwater ecosystems.
Throughout history, human activities have severely affected the condition of freshwater ecosystems. Surface water and groundwater are being degraded in almost all regions of the world by intensive agriculture and rapid urbanization. Africa has seen the largest regional population rise for the period 1990-2000. Over the next 25 years, population projections indicate an expected increase of a further 65 percent. Africa also has the lowest total water supply coverage of any region, with only 62 percent of the population having access to improved water supply since 1990. Water resources are unevenly distributed across the African continent, with about 50 per cent of the total surface water on the continent contained within a single basin — the Congo River basin — and 75 percent of total water resources concentrated in eight major river basins, the Congo, Niger, Ogooue (Gabon), Zambezi, Nile, Sanga, Chari-Logone and Volta. This has led to an increase in water transfers from water-rich catchments to those where water is limited with major implications for channel integrity and ecological functioning, and leading to a frequent loss of biodiversity. Water transfers produce: flow reduction in the donor rivers and increased flow in the recipient rivers; changes in the physical and chemical status of river’s water; introduction of fine sediments from one river to another leading to a loss of benthic habitats such as gravel spawning beds for fishes; and spread of alien fish species, floating aquatic plants, and animal diseases and their vectors.
To meet the needs of the increasing population, sustain long-term human welfare, and conserve functioning ecosystems that provide us with the wide variety of goods and services we depend on, consumption of renewable natural resources must stay within the limits of biological capacity over the long term. By sector, the highest water use is for agriculture for irrigation followed by domestic use and industrial use. Four per cent of Fish, 9% of Dragonflies and Damselflies, and 6% of Molluscs assessed are directly affected by groundwater extraction but it is believed that the indirect impacts fro freshwater ecosystems are much more dramatic.
Infrastructure development is the primary source of threats for crabs, second biggest threat to aquatic plants, and third biggest threat to fish. Ten per cent of Africa’s freshwater species are threatened by infrastructure development, of which the most common forms are human settlement and dams.
Four per cent of Africa’s freshwater taxa are threatened directly by dams. There are more than 1,300 large and medium sized dams, of which approx 40% are in South Africa. Although dams create new wetland habitats, the ecosystem impacts of dams (particulalry large ones) are more negative than positive, and they have in many cases led to significant and irreversible loss of species. As well as changing ecosystem for all species present, they also can prevent species' migration and dispersal, particularly of fishes.
"Efforts to date to counter the ecosystem impacts of large dams have met with limited success due to the lack of attention to anticipating and avoiding such impacts, the poor quality and uncertainty of predictions, the difficulty of coping with all impacts, and the only partial implementation and success of mitigation measures” (World Commission on Dams 2000)
With the growing demands for hydropower, the number of dams is likely to increase across Africa.
The introduction of alien species can have a profound and devastating impact upon an ecosystem. Lake Victoria remains one of the best examples of the effect that invasive species can have on an ecosystem from anywhere in the world. Before the 1970s Lake Victoria contained hundreds of species (350 - 500+) species of fish in the cichlid family of which 90% were endemic, comprising 80% of the fish biomass and much of the fish catch. This represented one of the diverse and unique assemblages of fish in the world. Since the introduction of the Nile Perch and Nile Tilapia, half the species are either extinct or only occur in very small populations. Nile perch now comprise 60% of the fish biomass. Their introduction has probably led to the extinction of 200 species of fish. This project has assessed 191 species found in Lake Victoria which are African endemics, of which 85 (45%) are threatened, including 49 (26%) which are now Possibly Extinct. There is evidence that the decline in cichlid numbers may have decreased or even reversed partly due to fishing pressures and better land management in recent years. This emphasises the importance of assessment work, in that to know to put such management in place it is firstly necessary to be able to identify areas which are of concern.
The picture of damage by invasive species extends through many river systems across the continent. For example in southern Africa the introduction of many North American species (smallmouth, largemouth and spotted bass, and the bluegill sunfish) together with banded and Nile Tilapia from further north in Africa pose particular threats to other fish species. Less obvious threats from invasive species come from the impact of alien tree species which shade out the habitats of sun loving dragonflies and damselflies. South Africa's "Working for Water Programme" whose aim is to rehabilitate water supplies for humans is now removing these tree species providing a good example of how multiple benefits can be derived in terms of jobs for people from amongst the most marginalised section of society. This delivers not only a win in terms of more water for people but also for the dragon and damselflies.
According to Brainerd (1995), the majority of the fishery resources in Africa are overexploited, fully exploited or about to reach their maximum level of exploitation. In total, are 791 freshwater species are known to be threatened by harvesting (including fishing and gathering), 775 of which are freshwater fish. Local over fishing for food is affecting to 530 of the freshwater fish assessed and 149 (6%) of the threatened taxa. As a consequence, this is considered one of the main causes of threat for freshwater fish in the region together with water pollution. In many freshwater bodies, increased harvesting has led to changes in fish community structures and distributions, with an overall reduction in recruitment. Over fishing also causes a decline in average fish size and often lowers trophic levels of fish communities following the disappearance of larger species. It can also reduce genetic diversity, especially when the stock size is greatly reduced from natural levels.
Wetlands provide an important source of income for African local communities and their livelihoods. It is estimated that in Africa inland fisheries land nearly 2.5 million tonnes per year, which accounts for nearly 25% of the world’s inland waters capture fisheries, providing essential nutrition for the poorest of communities and employment and income for many more. Tilapias (Oreochromis) are the most targeted group of freshwater fish in the continent. For example Oreochromis karongae is one of the most valuable food fishes in Malawi, but populations collapsed in the 1990s due to overfishing, and it is now assessed as Endangered.