Lake Tanganyika is a large East African rift valley system holding about 1/6 of the world's liquid freshwater with about 2000 species of organisms (fauna and flora), of which about 700 are endemic. The lake faces a number of threats including excess sedimentation, overfishing, pollution, habitat destruction, along with climate change. Efforts to better understand these involved an assessment of the magnitude of the threats, through the Lake Tanganyika Biodiversity project (LTBP) in which a number of outputs such as Draft conventions, special study reports and the Strategic Action Programme were achieved. The preparation of detailed projects to address the threats through the Lake Tanganyika Management Planning Projects (LTMPP) was another strategy, as well as projects prepared for management of catchment and pollution control, along with fishing management. It can be concluded that Lake Tanganyika faces essentially man-induced threats compounded by climate change, probably resulting in declining productivity of the lake and declining water levels. It is concluded that in order to maintain sustainability of the lake, both regional and global joint efforts are required.
Lake Tanganyika is a large East African Rift Valley lake situated between 3° 20′ and 8° 48′ S, and 29° 05′ and 31°15′ E with an elevation of 773 m above sea level. It is the second oldest lake in the world with the age of 9–12 Ma (Cohen et al., 1993). The lake has a mean width of 50 km, mean length of 650 km, mean depth of 570 m and maximum depth of 1470 m (Coulter, 1991) and a lake shoreline perimeter of 1838 km, of which 43% is rocky substrate, 21% is mixed rock and sand substrate, 31% is sand substrate and less than 10% is marshy substrate (Cohen et al., 1993). Lake Tanganyika has a surface area of 33,000 km2 and the total volume of water is 19000 km3; thus it contains almost 17% of the world's free freshwater (Edmond et al., 1993; Coulter, 1994).
Major inflows into the lake are Rusizi and Malagarasi rivers entering through the northern side from Lake Kivu and the eastern side respectively, and small rivers and streams due to steep mountains that keep the drainage areas small. The Lukuga River is the only major outflow, which empties into the Congo River drainage.
The catchment area of the lake covers 231,000 km2. The population of the catchment area of Lake Tanganyika is estimated to be around 10 million and is growing rapidly at the rate of about 3.5%. The majority of these people rely on small-scale agriculture for their food and income. Pollution in the catchment from industrial activities is localised, with the specific exception of Bujumbura, still at a low level owing to the largely undeveloped nature of the basin.
According to Coulter, (1994) as cited by Palacios-Fest et al., (2005), Lake Tanganyika is a reservoir of extraordinary lacustrine biodiversity, with over 2000 aquatic organisms, at least 700 of which are endemic to the lake. Most of this diversity in terms of species richness and endemism resides in littoral and sub-littoral habitats; whereas the pelagic zone is relatively species poor. Coulter (1991) makes the following delineation: littoral zone-from shore to 10 m depths; sub-littoral zone-from 10 m to 40 m depth; benthic zone-from 40 m to the end of the oxygenated zone. The endemic species are dominated by cichlid and non-cichlid species, molluscs and crustaceans and are renowned for their morphological and behavioural diversity (Coulter, 1991; Kawanabe et al., 1997; Rossiter and Kawanabe, 2000). The lake is valuable not only for the presence of these unique species, but also as a microcosm in which to study the processes of evolution that have led to this diversity.
Further, the lake has historically supported one of the world's most productive pelagic fisheries (Nixon, 1988), making it an important source of both nutrition and revenue to the riparian countries of Burundi, Democratic Republic of Congo (DRC), Tanzania and Zambia. There are about one million people around the lake whose livelihoods wholly depend on fish resources. Fish is also transported to distant urban centres where it is part of the preferred diet.
The lake provides a key transport and communications link, supporting the economic and social development of lakeshore communities and it is a permanent source of water for industrial and agricultural development as well as for domestic use.
The lake is currently facing a number of threats that are unfortunately man-induced. These threats have started impacting the health of the lake resulting in declining productivity. Sustainable management interventions need to be urgently addressed for safeguarding the lake ecosystem.
Threats to Lake Tanganyika
The lake is severely threatened by both watershed degradation (Alin et al., 2002) and global climate changes (O'Reilly et al., 2003; Verburg et al., 2003). These can be categorized as man induced and natural threats.
Man induced threats
The main threats to the biological richness of the lake and the sustainable use of the lake resources result from the intensification of human activities. The accelerating rate of environmental change caused by human activities is now much faster than the fauna's adaptive capabilities and the absorptive capacity of the environment.
The increase in fire outbreaks, demand for fuel wood and building materials, along with the demand for agricultural land for food production (anthropogenic activities) and other economic activities have led to deforestation (exposing the land) particularly in steep slopes, leading to rapid weathering, accelerating the erosion rate and reducing soil fertility in recent years. The progressive deforestation of watersheds surrounding the lake increases the runoff of rain during the rain season (Dettman et al., 2005). The deforested watershed discharges more sediment into the Lake Tanganyika basin, thus impairing water quality (Nkotagu, 2005). Lake margin habitats adjacent to areas of extensive deforestation are subject to rapid pulses of sediment erosion, with consequences that may include reduction in habitat heterogeneity from siltation, increased near shore inputs of soil nutrients, reduction in light penetration and reduction in ability of visual predators and fish using color cues for breeding to survive (e.g. Seehausen et al., 1997; Donohue and Irvine, 2004). Subtler top down and bottom up effects are also likely consequences of siltation in this lake, for example reductions in biomass specific net productivity as herbivore grazing efficiency declines (O'Reilly, 2001) and changing patterns of herbivory and parasitism (Irvine et al., 2000; McIntyre et al., 2005). Eroded sediments enter the lake resulting in habitat destruction (Palacios-Fest et al., 2005) and increased sediment accumulation rates along the shorelines of deforested watersheds and finally disturbing the primary production on which many organisms depend. Records show that remarkable sediment accumulations rates have increased 10 fold during the late 20th century, (McKee et al., 2005) consistent with earlier work at the same area (Wells et al., 1999).
Poorly planned urbanisation near the lakeshore is a phenomenon that creates a different set of threats. Domestic sewage, household and industrial wastes find their way into the watercourses and ultimately into the lake. These unwanted pollutants are slowly distributed throughout the lake by wind-driven currents and seiches.
The agricultural run-off from the rivers Malagarasi and Rusizi leads to increased pollution in the lake as well. This is due to the fact that the agricultural expansion has been accompanied by an increase in the use of agrochemicals, such as artificial fertilizers, pesticides and herbicides.
Industrial pollution is mainly centered at Bujumbura in Burundi. Oil spills at the Kigoma and Mpulungu ports are also observed to be on the rise. The pollutants do destroy the lake habitat for many biological communities in a variety of ways.
The increasing demand for fish for local consumption and for sale to distant markets increases fishing pressure and leads to high fishing intensity. Fishing with destructive methods also threatens the biodiversity in the lake. The data show that the estimated annual harvest in recent years is between 165,000 and 200,000 metric tons (54–66 kg ha− 1) with an equivalent value of tens of millions of US dollars (Mölsä et al., 1999). Long-term overfishing may result in the reduction of the fishery potential and the unique high biodiversity in general.
Various studies have attributed the large (30–50%) decline in the clupeid catch (Stolothrissa tanganicae andLimnothrissa miodon) since the late 1970s partially to environmental factors rather than overfishing because the lake had sustained high yields under similar fishing pressure for the previous 15–20 years (Shirakihara et al., 1992; Mannini, 1998; Mölsä et al., 1999; Van Zwieten et al., 2002). This decline was coincident to the disappearance of previously strong seasonal patterns in the catches (Shirakihara et al., 1992; Van Zwieten et al., 2002), suggesting a decoupling from ecosystem processes due to the weakening of hydrodynamic patterns. The contribution (by weight) of the sedentary Lates species to the fish catch declined from 20–60% in 1955 to 2% after 1977 (Van Zwieten et al., 2002), resulting from prolonged high fishing pressure (Coulter, 1991). Sarvala et al. (2006a, 2006b) nevertheless specifically ascribe declining fish trends to increased fishing pressure and practices and argue that there are no data to substantiate claims that these are due to global warming.
Natural vs. anthropogenic threat
Global climate change
Recent studies have shown that climate change contributes to a significant alteration of environment of Lake Tanganyika. Climate change causes variations in sediment accumulation rates, charcoal accumulation, lake level fluctuation and water chemistry (Cohen et al., 2005). According to Verburg et al. (2003) temperatures in the north basin have increased between 1913 and 2000 by 0.2°C near the lakebed and by 0.9°C at 100 m below the lake surface. About 50% of the heat gained by the lake is at the upper 330 m thus increasing vertical temperature gradient. Air temperature at the north end increased by 0.81°C over the last 27 years. This is much above the global average of +0.42°C.
Another study by O'Reilly et al. (2003) found that the average annual air temperatures shows a raise of 0.5–0.7°C, and deep-water temperature increased from 23.10°C in 1938 to 23.41°C in 2003. This increase of 0.31°C in deep-water is comparable to that found in other African Great Lakes: Lake Victoria has warmed 0.3°C between the 1960s and 1991 (Hecky et al., 1994), deep waters of Lake Malawi have warmed 0.29°C since 1953 (Beauchamp, 1953; Patterson and Kachinjika, 1995), and Lake Albert has warmed 0.5°C since 1963 (Lehman et al., 1998). Wind velocities in the Lake Tanganyika watershed have declined by 30% since the late 1970s. O'Reilly et al. (2003) recorded a monthly constant wind velocity during the cool windy season in the north as 2.2 ± 0.4 ms− 1 until 1985 along with a significant decrease to 1.6 ± 0.3 ms− 1 and 5.8 ± 1.6 ms− 1 to 4.0 ± 1.3 ms− 1 after 1977 in the south.
The stability of the water column during the non-windy season defined by Idso, (1973) as the energy required to mix the water column to uniform density, increased by 97% from 84.4 kJm− 2 in 1913 to 166.3 kJm− 2 in 2003 (O'Reilly, 2003), due to increased temperatures and decreasing wind speed resulting in reduced mixing depth. The depth of the oxygenated zone also showed a significant shallowing trend of 1.3 ± 0.2 m yr− 1 since 1939 to a current depth of around 80m (Beauchamp, 1939; Degens et al., 1971; Craig, 1974; Edmond et al., 1993), thus supporting the reduced mixing depth.
These data suggest that the increased thermal stability, coupled with a decrease in wind velocity, has reduced the mixing depth in the lake. This reduced mixing impairs the natural nutrient hydrodynamics processes resulting in diminished deep-water nutrient inputs to the surface waters, subsequently causing a decline in primary production rates. This inferred decrease in primary productivity would explain the recent decline by roughly 30% in the pelagic fish catches (O'Reilly et al., 2003) and an expansion of the anoxic water mass (Verburg et al., 2003). This view is opposed by Sarvala et al. (2006a, 2006b), who argue that long-term fisheries data indicate that declining catch rates are related to increased fishing efforts and that the observed changes in dissolved silica and transparency are insufficient evidence for decreasing primary productivity. Verburg et al. (2007) agree that, although in their opinion sufficient evidence has been provided for reduced productivity due to warming, available fisheries data do not support O'Reilly's theory. There is a need for more continuous limnological and fisheries monitoring in Lake Tanganyika.
To some, the available data provide evidence that climate change has contributed to diminished productivity in Lake Tanganyika over the past 80 years. However, Hulme et al., (2001) predict air temperature increases of 1.3–1.7°C for the Great Lakes region of East Africa within the next 80 years. This may further increase thermal stability provided that wind velocities remain low and reduce productivity in these essential natural resources for regional economies. Hence increased poverty will result for the lake dependent population.
Sustainable management strategies
The management strategies for the conservation and sustainable management of the Lake Tanganyika started well in 1992 after the Rio de Janeiro meeting. Then a proposal was prepared to address the conservation measures of the lake. In 1995 a project named the Lake Tanganyika Biodiversity Project (LTBP) commenced under the UNDP/GEF support and ended in the year 2000. The Lake Tanganyika Biodiversity Project (LTBP) had three major outputs including: Five special studies report to address the threats facing Lake Tanganyika such as excessive sedimentation, pollution, overfishing and habitat destruction. Consequently, the special studies concerned: sediments, pollution, fishing practices, biodiversity, socio-economics and awareness education campaigns (http://www.ltbp.org/FTP/SS.PDF:2000). These studies were conducted in each riparian state of the lake and aimed at provision of baseline data on which interventions measures could be based.
Second, the Strategic Action Programme (SAP) was formulated. In the SAP the threats were analyzed and prioritized through a Transboundary Diagnostic Analysis (TDA) covering all the riparian states in each country accordingly.
Third, a convention signed in 2003 by all riparian states initially ratified by Burundi, Tanzania and Zambia and recently by DR Congo was put in place. In this convention, legal experts from the riparian states jointly formulated 44 articles including five appendices for the conservation of biodiversity along with water quality for future sustainable management of Lake Tanganyika.
The Lake Tanganyika Management Planning Project (LTMPP) succeeded the LTBP in the year 2002 under UNDP/GEF support. The project focused on preparing fundable proposals for future interventions against the threats. Proposals addressing sedimentation and pollution for Tanzania were made; while DR Congo and Zambia only addressed the former and Burundi the latter. Intervention for the overfishing threat was already separately undertaken through the African Development Bank initiative and covers all riparian countries. Similar efforts by Food and Agricultural Organizations (FAO) and Finnish International Development Agency (FINNIDA) through the Lake Tanganyika Research Project (LTR) were undertaken to assess the fisheries potential of the lake. The project concluded that the decline in fish production in the lake is due to fishing pressure and to other directly anthropogenic activities. It was noted that the ongoing climate change contributes to the decline in fish abundance due to the intensification of the dry season, which leads to increased runoff and sedimentation in the lake, decreasing overall productivity.
Conclusions and recommendations
The paper enumerates the threats facing Lake Tanganyika today and lists the efforts made by the riparian states to cope with these. However, these efforts need to be sustainable for the future health of the lake. They need to be conducted jointly by all riparian states in an integrated fashion by addressing all the threats using innovative techniques.
Since some of the threats facing the lake are evidently due to factors beyond the borders of national and riparian states, for example the predicted air temperature increase of 1.3–1.7°C within the next 80 years for the Great Lakes in the region (Hulme et al., 2001), we recommend that global collective efforts be implemented to halt such an accelerated temperature increase. Finally, regional monitoring should be undertaken on the lake in order to assess the effectiveness of the intervention measures and to provide answers to the controversial opinions concerning the lake's productivity and the fishing pressure.
I thank the United Nations Office of Project Services HQ New York (UNOPS) and the United Nations Development Programme (UNDP) office in Dar es Salaam for enabling me to participate in both phases of Lake Tanganyika related projects in various capacities. The paper has greatly benefited from the anonymous reviewers whose suggestions and constructive criticisms are acknowledged. The United States National Science Foundation (NSF) is also thanked for allowing me to participate in the Nyanza Project, where I learned of the importance of interdisciplinary projects.