Aquatic in literal meaning refers to water. As an ecosystem, widely taken, it includes freshwater (rivers, reservoirs and lakes), marine (oceans and seas) and estuarine (coastal, bays, tidal) ecosystems. The Ethiopian aquatic ecosystem has high diversity areas such as major rivers and lakes that are of great national and international importance. The country is well known for its richness in water potential. There are about 30 major lakes that are located in different ecological zones. These lakes are situated at altitudes ranging from about 150 m below sea level high up to 4000 m. The surface area of the lakes vary considerably from less than 1 km² to over 3600 km² and mean depths range from few meters to over 260 meters. However, the major lakes that are of economic importance are concentrated in the Rift Valley.
River and Stream Systems
Ethiopia is endowed with over 7000 km of rivers, whose drainage basins cover almost the whole country including the semi-arid lowlands. There are 12 major river basins with total annual surface runoff of 110 billion m³. The direction of river flows is dictated by the topography of the country. Accordingly, the main drainage basins are flowing away from the rift system either towards the Nile System in the West or to the Indian Ocean in the Southeast. For instance, Tekeze and Angereb rivers, the headwaters of Atbara, drain the plateaus north of Lake Tana; the Central plateau is drained by Blue Nile rising from the same Lake Tana (Diddessa and Dabbus, rising in the high rainfall western regions being its main tributaries).
West of the Rift valley, the Omo River (part of the Omo-Gibe Drainage System) flows south to Lake Turkana. The Wabishebele and Genale Dawa river watersheds drain the plateaus of the southeast to Indian Ocean. The rivers that flow in the rift system form closed basins, the Awash River being the largest which originates in the plateau of West of the Rift valley moving south of Addis Ababa northward into the closed Lake Abbe in the Afar Depression (Tudorancea et al., 1999).
Lake Tana is found at an altitude of 1830 m and 110 37’N, 370 25’E. It is the largest lake in Ethiopia with total area of 3,600,000 hectares and with the maximum width and length of 67 and 75 km, respectively. The maximum depth is 14 m. and the average 8.91 m. It is found in Amhara Regional State, and it includes 37 island many of them hosting ancient churches and monasteries. Extensive beds of papyrus (used for locally made boat or ‘tankwa’ is found on water. Echinochola spp. Filla, Grasses and Polygonum with other several aquatic pants including the water Lilly are present.
The Ethiopian Rift Lakes
The Ethiopian Rift Valley Lakes are categorized into two groups: Those fed by perennial rivers which include lakes within the Rift Valley and those fed by the Awash River. Those located in the Afar depression such as Lakes Asal and Beseka (Lake Matahara). Three closed basins presently represent these main Rift Lakes: – Zuwai-Langano-Abijata-Shala lakes located in the northern part of the valley, also called as “Galla” lakes (found in Oromia). Lake Awasa-with smaller area and completely enclosed in by faulting; similar to Abijata and Shala by being without visible outlet; Tikur Wuha river is its main tributary; (found in SNNPRS). Abbaya-Chamo- lies in the Southern part of the Rift Valley.
The Crater lakes, particularly the Bishoftu, lie at an altitude of about 1900masl. They are found near the small town Bishoftu, at a distance of about 50 km Southeast of Addis. These lakes are five in number, namely Lake Arenguade, Bisoftu, Chitu, Babugaya, Hora and Cheleleka. The crater lakes are generally circular in shape with surface areas ranging from 0.5 and 1 Km2. They vary in maximum depth from 26 m (Lake Kilote) to 8.7m (Lake Bishoftu) (Tudorancea et al., 1999).
Similarity is observed between the Ethiopian Crater and Rift lakes and other East African waters regarding chemical composition (Talling and Talling, 1965). They are alkaline; sodium, carbonate and bicarbonate ions being in high concentration; the pH value ranges from 9 to more than 10.
High Altitude Lakes
Because of their Size and locations, the Ethiopian high mountain lakes are not much studied. They are also not much known to the public like the Rift Lakes and Crater Lakes of Ethiopia. And yet, scientists used to visit them since the 1930’s and discovered that the lakes support a wealth of biological resources.
It is a crater lake located at about 2030 m. It is found in the Amhara Regional State, at the vicinity of the town of Dessie. It was observed that the quality of the water has been changing (Turdonacea et al., 1998). This lake is unusually clean with transparency of 9 m and low productivity. It has a similar chemistry with other Ethiopian lakes especially with Lake Pawlos and Bishoftu crater lakes (Tudorancea et al., 1999).
Lake Alemaya is another highland lake found in Oromiya Regional State. In the Bale Mountains, East of the Ethiopian Rift Valley, Also found in Oromiyaa, is a group of other high altitude lakes of Ethiopia. Loffler (1978) visited about fifteen lakes and ponds, including Lake Garba Guracha, located in the Bale Mountains at altitudes above 4000masl. According to Loffler (1978) Except Lake Garba Guracha that is a moronic lake located in the valley of a river, all of the lakes in this area are of glacial nature or were influenced by glacial activities. These lakes are very shallow. These lakes have similar chemical composition with other rift lakes. Some of the Bale mountain lakes are Lake Garba Guraca, Hora Orgona, Tarn, Tarn 2 etc.
Few reservoirs are found in Ethiopia. Koka, at about 1589m, formed on the Awash River Southeast of Addis in the Rift Valley. Another important reservoir is the Lege Dadhi reservoir found near Addis and it is the main water supply for the city.
According to Wood (1965), Bishoftu crater lakes are grouped into three based on the algal dominance. Microcystis aeruginosa; Lake Arenguade by Spirulina platensis (Arthrospira fusiforrmis); Lake Kilotes by Chroococcus-like algae.
In lake Hayiq, a number of species are listed and some of them are Microcystis, Phormidium, Sutirella, Pediastrum, Amphora, Synedra, etc. Species of characteristic to saline water such as Nitzschia and Gyrosigma are also recorded. According to recent findings the Diatom Navicula sp. and Cynophytes such as Microcystis delicatissima and M. elachista are dominant ones (Kebede et al., 1992).
The most common species inhabiting the Rift Valley Lakes (Zuwai, Abbayya, Chamo and Awasa) are Scirpus spp., Cyperrus spp., Typha angustifolia, Paspalidium geminatum, Nymphaea coerulea and Potamogeton spp. Aquatic macrophyts are not found in the saline lakes of Abiyata and Shala, as well as in the crater lakes (Tudorancea et al., 1999). Eichornia crassipes (Water hyacinth), the notorious invasive weed of aquatic ecosystem occurs in Koka Dam along Awash River and in Gambella along Baro and Gilo Rivers (Getachew Aweke, 1997).
Green and Seyoum Mengistu (1991) carried out sampling on rotifer communities of thirty lakes. The altitudes of the lakes ranged from 1100 m- 2000 m that can be arranged on salinity gradient. The salinity values range from 0.12% (Finchaa) to 56.3% (Lake Matahaara). Tudorancea et al., 1999, have reported a decrease in species richness from an average of 8 species per lake for the lakes with salinity under 2% to 3 species for those above 2%.
The rotifer communities such as B. plicatilis dominated Lakes like Lake Shala, which has salinity above 2%. The limiting factor for rotifer communities is suggested to be the tolerance of species to salinity. In Lake Awasa Brachionus and Keratela species were dominant (Seyoum Mengistu et al., 1991). Along the littoral inhabitants Colurella sp. and Trichocrca sp. were recorded. According to Tudorancea et al. (1999) and Elias Dabebo (1988) rotifers were recorded in the guts of fish species of Oreochromis niloticus and Clarias mossambicus.
In Lake Awasa eight cyclopoid copepods and five cladocerans constituted the crustacean zooplankton assemblage while the numerically dominant species are: Mesocyclops aequatorialis similis, Thermocyclops cosimilis, Diaphanosoma excisum and Alona diaphana. Cycloids M. aequaterialis and Eucyclops spp., Calanoids Lovenula africana and Cladocerans Daphnia barbata, D. longispina and Ceriodaphina cornta inhabit Abijata and Langano lakes (Kassahun Wedajo and Amha Belay, 1984). In highland lakes the cladoceran Daphina obtusa and the copepods Arctodiaptomus and Megacyclops viridus were recorded (Dussart, 1974; Baxter and Golobitsh, 1970)
Harisson and Hynes (1988), in their study on Ethiopian stream and river bentic communities, have identified Dugesia (Planariidae), Baetis harrisoni, Pseudocloeon spp., Centroptilum sudaafricanus, Afronorus peringueyi, Neoperla spp., Hydropsyche spp., Simulium spp., nymphs of Aeshna and chironomida larva in the stony runs and torrents. In the slow flowing streams on the Rift Valley were identified B. harrisoni, Cloen cf. Crassi, Pseudagrison, Enochrus sp., and chironomid larvae such as Pracladius brevipetiolatus, Cironomus formisipennis, Einfeldia disparilis and Dicrotendipens punctatus.
The Ethiopian mountain streams are the areas of occurrence for the cool-adapted snail Ancylus fluviatilis. A. Trichoptera genus Mesophylax (Aethiopicus malicku) is found in the bale Mountain area. Among the bentic species chironomids are the most diverse group, 19 species, next being the nematods, 16 species and ostacods are 13 species. Some of these species are recorded in other Afro-tropical and temperate waters: the nematodes Ironus tenuicudatus, Trypila glomerans and Rhabditoides stigmatus and the Ostracod Darwinula Stevensoni. Six nematode species including a new genus (Lanzavecchia)-(Zullini, 1998) and an endemic (sub) specific cluster of the ostracod Limnocythere (Martens and Tudorancea 1991) are new to science. The chironomid species Cladotanytarsus pseudomancus and Microchironomus deribae identified in Ethiopian Lakes and widely distributed in many African waters.
Bulinus natalensis, Biomphlaria sudanica, Ceratophalus bicarinatus, Lymnaea natalensis and Melanoides tuberculata are the only gastropod species of Mollusks recorded inhabiting littoral vegetation areas.
In Ethiopia there exist more than 101 species of Fishes of which at least 4 are endemic. Fish resources of the country are currently utilized mainly for food and sport fishing as well. Some of these are Oreochromis niloticus, Bagrus docmac, Mormyrus mormyrus, etc.
10.4 Uses and Values
Water is an important natural resource and a limiting factor for the economy of a country. This ecosystem has paramount importance as the major source for domestic and industrial water supply of the nation. In addition, the Ethiopian water bodies are a “natural laboratory” for education and scientific research. Generally the Rift Valley Lakes of Ethiopia serve as important sources of food for a good number of local inhabitants. Some of the commercial fishing areas, for example, lake Ziway, Langano Abaya, are also known to support a high number of waterfowls such as Great White Pelicans, Flamingoes that have both national and international importance (EWNHS, 1996). Most importantly most of the ecosystem has got a dual value as attractive to tourists, with some amount of income to the country.
The economic importance of aquatic ecosystem is significant in the areas of hydroelectric power generation, irrigation and transportation. The possibility of developing fishery industry in the country and small-scale aquaculture are also another prospective economic value of the ecosystem.
Generally the ecological value of the ecosystem is of paramount importance. It is the home for aquatic biota. It is an important place where vital role in nutrient cycling and energy flow to sustain aquatic organisms is played. Still more, the ecosystem is a site of endemism. Benthic nematode endemism is quite high and some rotifers and copepods described were also new to science
10.5 Threats and Rates of Change
Ethiopian streams and rivers are much influenced by various development activities. Land and water development, pollution, introduction of exotic species, over exploitation of fish stocks, etc are some of these activities. The effects of these activities have resulted to the demise of some aquatic biota that cannot tolerate conditions created by human activity, an increase in others cultured by human beings or favored by human-induced alterations in this aquatic environment (McClanahan et al., 1996).
Direct human effects such as damming and diversion of rivers, channeling and building water distribution facilities; and indirect influences such as removal of vegetation cover of drainage basins for agricultural, urban, industrial, mining etc. expansion have played a great role in changing the habitat. Conversion of lotic to lentic systems above dams is an effect of damming. Along with these changes in hydrological systems, temperature conditions, nutrient levels and sediment deposits at dams are negative effects observed in aquatic ecosystem. Dams act as barriers to migrating fish species, disrupt the hydrological or chemical cues needed to induce migratory and spawning behavior of fish. River and steam diversion reduces habitat diversity and, consequently, aquatic biodiversity, whereas canals provide pathways for the dispersion of exotic species and vectors for pathogens (McClanahan et al., 1996).
The removal of vegetation cover increases erosion and sediment in-outs into water bodies. Farming of the catchments contributes to enhanced nutrients and particulate runoff where grasslands are overgrazed or fields are fertilized. It is known that some of the soluble fertilizers pass into the water body and leads to eutrophication (Tudorancea et al., 1999). As a result of these effects sediments will cover bentic communities, scrap the bodies of aquatic animals, inhibit oxygen penetration and photosynthesis.
The introduction of fertilizers into water bodies through runoff will result to increased nutrient load of the ecosystem- eutrophication, accumulation of toxic chemicals in the bodies of fish, etc. Eutrophication results to changes in algal composition, especially cyanobacteria, and biomass. This in turn results to fish kill caused by decreased biochemical oxygen demand and other interrelated consequences (Amha Belay and Wood, 1982; Kassahun Wodajo, 1982; Tudorancea et al., 1999).
The introductions of organic sewage and domestic wastes into water bodies have become threats to aquatic ecosystem. It causes large increase in microbial respiration, a decrease in the level of oxygen, causes appearance of “sewage fungus” and disappearance of most macroinvertebrates (McClanahan et al., 1966).
The possible cause for the elimination of freshwater crabs and reduction of number of Veliiae and Gerridae from northern areas of the Rift Valley might be the washing of clothes and introduction of detergents in to steams and rivers (Tudorancea et al., 1999). Disappearance of Trichoptera and many Ephemeropteans from the zone of Abbo- Kebena River is due to pollution from domestic wastes.
One of the causes for the deterioration of aquatic ecosystem is manifested in a decrease in level of the lakes’ water. This could result due to natural phenomena of shortage of rainfall and human phenomena of a number of irrigation schemes, especially of rivers flowing into lakes. Rivers Meki and Katar, Bulbula and Gogessa which flow into lakes Ziwai and Abijata respectively, being used for irrigation decreased the lakes’ water level and consequently resulted to drastic effects on the fish and other aquatic communities. Fish species like Oreochromis niloticus that spawns in shallow parts of the lake are adversely affected by the change in water level (Tudorancea et al., 1999).
The introduction of exotic fish species had a considerable effect on the freshwater fish species. Since 1930’s a number of exotic species were introduced into Ethiopian fresh waters for purposes such as sport fishing, as food, effective utilization of the ecosystems, improvement of local stock and control of disease vectors and aquatic weeds. Among them is Gambusia and Exos from Italy to Lake Tana, Cyprinus carpio and the North American Salmo gairdneri into a man made Lake Abba Samuel, S. trutta from Kenya by UNESCO into Bale lakes, rivers Sibila, Chacha, Beresa and Muger (Tudorancea et al., 1999).
The threat on the biodiversity of Lake Abijata due to the Soda Ash extraction project, which has diminished the lakes water level at a faster rate is an issue of due attention. A remarkable decline in water level, the increase in salinity and alkalinity of lake, the effect of water diversion on the fish population associated with the project is still on move.
10.6 Conservation status
The conservation aspect of the aquatic ecosystem of Ethiopia is almost very limited. From the points of view of environmental protection and development, no significant work has been done. Water policies alone may not be effective to protect or conserve the aquatic ecosystems, and they have to be integrated with other policies like land use policy and forestry policy.