559: Lake Malawi

Lake Malawi (also known as Lake Niassa or Nyasa) and its influents, Lake Malombe and the Shire River in between the two lakes, form this globally distinctive ecoregion (Tweddle et al. 1979; Ribbink 2001). The ecoregion hosts highly endemic species flocks of fishes, which make up one of the richest lake fish faunas in the world. Lake Malawi/Niassa/Nyasa (hereafter referred to as Lake Malawi ) is the southernmost lake of the Rift Valley and is bordered by the countries of Malawi , Mozambique , and Tanzania .

Lake Malawi is the ninth largest lake in the world, the fourth deepest, and has a surface area of about 28 to 31,000 km² (Bootsma & Hecky 1993; Ribbink 2001). More than 200 rivers flow into Lake Malawi . Most are short and many flow only in the rainy season. Major rivers include the Lufira, Songwe, Rukuru, Dwangwa, Bua, and Linthipe on the western coast, and the Ruhuhu and Rio Lunho on the eastern coast. The Shire River , which links Lakes Malawi and Malombe, shows seasonal and long-term variability in flow. Flows in this river diminished steadily from 1896 until they ceased in 1915. In 1935 lake levels rose sufficiently for flows to resume and they have continued until the present day, varying in magnitude with rainfall (Beadle 1981; Lopes 2001). Lake Malombe is almost entirely dependent upon the flow of the Shire River, and the lake dried out when flows ceased between 1915 and 1935 (Beadle 1981).

Lake Malawi is the ninth largest lake in the world, the fourth deepest, and has a surface area of about 28 to 31,000 km² (Bootsma & Hecky 1993; Ribbink 2001). More than 200 rivers flow into Lake Malawi . Most are short and many flow only in the rainy season. Major rivers include the Lufira, Songwe, Rukuru, Dwangwa, Bua, and Linthipe on the western coast, and the Ruhuhu and Rio Lunho on the eastern coast. The Shire River , which links Lakes Malawi and Malombe, shows seasonal and long-term variability in flow. Flows in this river diminished steadily from 1896 until they ceased in 1915. In 1935 lake levels rose sufficiently for flows to resume and they have continued until the present day, varying in magnitude with rainfall (Beadle 1981; Lopes 2001). Lake Malombe is almost entirely dependent upon the flow of the Shire River, and the lake dried out when flows ceased between 1915 and 1935 (Beadle 1981).

The topography in and around Lake Malawi is notable. The Livingstone Mountains in the north-east drop precipitously to cliffs at the lakeshore and continue their descent underwater (Beadle 1981; Hughes & Hughes 1992). The Lake Malawi shoreline tends to be steep and rocky in most places in the north, while the topography is less steep in the southern part of the ecoregion, where sandy bays and coastal plains occur more frequently (Hughes & Hughes 1992). The lake itself contains numerous islands, islets, rocky outcrops, and reefs, which provide a rich and diverse underwater habitat for fish.

Three seasons are recognized. During the cool and dry period from May through August, temperatures near the Lake Malawi shore may drop to 15^oC, with a daily average of 20^oC to 22^oC. During the hot and dry phase between September and November, average air temperature is about 28^oC but may reach 40^oC. Average daily temperatures during the wet season from late November through April are 25^oC (Lopes 2001). Rainfall varies from about 600 to 2,200 mm per year depending primarily on location, with areas of higher altitude generally having higher rainfall (Hughes & Hughes 1992). Lake levels rise during the wet season, and annual fluctuations range between 0.4m and 1.8m (Lopes 2001). Flooding of the coastal plains occurs with increased rainfall in the wet season (Beadle 1981).

The tropical setting confers thermal stability to Lake Malawi , which is characterized by permanent stratification. This is maintained by temperature and density differences between the upper epilimnion between 0-125 m, the metalimnion between 125 m and 230 m, and the anoxic hypolimnion below 230 m (Gonfiantini et al. 1979). Full atmospheric exchange is restricted to the surface mixed layer that is well oxygenated but can be as little as 40 m in depth. Limited vertical mixing does occur between strata, and oscillations of the thermocline due to internal waves and wind-induced upwelling may cause advections of the metalimnion in the southeastern arm of the lake. The lengthwise orientation of the lake coincides with the southeast winds (mwera) that are channeled along the length of the lake and push surface waters north, to be replaced with deeper, cooler, nutrient-rich waters in the south. This upwelling maintains a thermal longitudinal gradient, with cooler surface waters in the southern part of the lake throughout the year. This upwelling system is the basis of the productive fisheries in the southern arms of the lake (Eccles 1974). Currents affect ecological processes such as nutrient cycling, plume dispersal, and fish productivity (Fryer & Iles 1972). Some indications suggest that there is a clockwise circular surface current within Lake Malawi, controlled by winds and radiant energy exchange as well as by inputs to the lake from rivers (Ribbink 2001). The flushing time of the lake is about 750 years (Bootsma & Hecky 1993).

Algae are the primary producers and the main source of organic carbon input into the lake, with phytoplankton in the pelagic zone and demersal algae on the lake floor. Nutrient cycles are driven by algae, which are dependent on the inflow of organic and inorganic nutrients from the catchments and by atmospheric deposition and the cycling of these nutrients in the lake.  Certain forms of nitrogen and phosphorous have been shown to be contributed to the lake mainly through dry deposition and precipation (Bootsma & Hecky 1993). Plant nutrient concentrations are found to increase with depth, with ammonia the dominant form of inorganic nitrogen in the deeper anoxic waters (Bootsma 1993; Patterson & Kachinjika 1995).

The deep (more than 500 m) rift valley lake habitat type is considered globally rare because there are fewer than eight such lakes worldwide (Thieme et al. 2005).

There are about 800 species of fishes, but in addition, there are clearly recognizable geographic sub-populations, some having economic value - such as those sold as ornamental fishes. It has been suggested that as many as 3,000 recognizable fish taxa (species and populations) might be found in the lake. The latter number probably represents the largest number of fish taxa for any lake in the world (Ribbink 2001). There are about 70 non-cichlid species (14 families) in the basin of which a few, such as the lungfish and the trout, are introduced; circa fifty belonging to 11 families are living in the lake itself (Snoeks 1999b).

Worth mentioning are the endemic, mostly deep-water dwelling large catfishes of the genus Bathyclarias, which have been shown to have originated from a widespread, generalist species, Clarias gariepinus, which still occurs in the lake (Agnese & Teugels 2001). Several cyprinids are economically important such as the sardine-like pelagic cyprinid Engraulicypris sardella (usipa), the salmon-like Opsaridium microlepis (mpasa) and the trout-like O. microcephalum (sanjika). It is difficult to summarize the large variety and complex nature of the over 800 species of cichlids. Only just over 300 are scientifically described. The cichlids are mainly represented by lineages of mouth-brooding haplochromines. An important exception is the chambo, endemic Oreochromis spp., which are important food fish. The haplochromines can roughly be divided in the smaller, beautifully coloured, mainly rock-dwelling mbuna and the non-mbuna. Malawi cichlids in general, but especially the mbuna are very popular aquarium fishes. Several groups can be distinguished within the non-mbuna. There are two lineages of merely pelagic, predatory and zooplanktivorous cichlids Rhamphochromis (ncheni) and Diplotaxodon (ndunduma). Utaka are a group of zoo- and phytoplanktivorous schooling cichlids living over reefs and sandy habitats. Most of the remaining, very diverse demersal species are locally named chisawasawa or kambuzi (Turner 1995).

The cichlids share several distinguishing characteristics. Many practice parental care and most species in the lake basin are maternal mouthbrooders, breeding and releasing young in the same habitat. These cichlids also grow relatively slowly and produce few young. Given these characteristics, most of the cichlids of this ecoregion are vulnerable to habitat degradation, sensitive to exploitation, and slow to recover from population declines (Ribbink 2001). All but four of the cichlid species are endemic to the lake (Ribbink 2001).

Endemism in the ecoregion is remarkably high. Among fish, 99% of the more than 800 species of cichlids and over 70% of the 17 clariids are endemic (Snoeks 1999a; Ribbink 2001)

Several fish such as the nchila (Labeo mesops), sanjika and mpasa are potamodromous, migrating annually up inflowing rivers to spawn. Large numbers of individuals congregate at river mouths and within the rivers prior to these spawning migrations, making them easy targets for fisheries. These potamodromous species are of special concern: most are endemic, they are prized as food fish, and they are subject to the twin threats of heavy exploitation and degraded spawning habitats in the rivers (Tweddle 1996).

Richness of taxa is exceedingly high in the ecoregion, with about 200 mammal, 650 bird, over 30 freshwater mollusc, and over 5,500 plant species (Brown 1994; Ribbink 2001). This richness is also reflected in the invertebrates and algae (Fryer 1959; Abdallah 2000). The lacustrine invertebrates appear to have high levels of endemism, although exact numbers are unknown for these largely unstudied groups (Fryer 1959; Abdallah 2000).

Lake Malawi is outstanding because of its extraordinary species radiations, including 50 endemic cichlid genera and 1 endemic cyprinid genus (Engraulicypris). The evolutionary processes that have produced such incredibly high numbers of endemic cichlids have fascinated biologists for some time, leading to the use of metaphors such as ‘explosive speciation’ to describe the rapid evolution of taxa (Fryer & Iles 1972; Liem 1980). The most widely accepted scenario of evolution of this highly diverse cichlid fauna is that riverine species with broad habitat ranges colonized the young lake and slowly became specialized to their new habitats within Lake Malawi . Changing lake levels during this time would have forced shallow-water stenotypes to live in habitats for which they were not adapted, placing them under severe selection pressure and causing incipient species flocks to become adapted to rocky, sandy, shallow, deep, vegetated, or mixed habitats (Fryer 1977; Owen et al. 1990). A prominent role for sexual selection has been suggested, amongst other things based on the observation that many closely related species differ mainly in male color pattern (see Seehausen 2000 for a larger discussion)(Seehaus.

This ecoregion is based on boundaries of Lake Malawi and its basin. The fauna of Lake Malawi is separated from that of the Lower Zambezi by the Murchinson Falls in the lower Shire River with the lowermost element of the cataracts, the Kapachira Falls, providing an absolute physical barrier to upstream movement of fish species (Tweddle et al. 1979). Lake Malombe is included within this ecoregion because the majority of its fishes and invertebrates are common to Lake Malawi and Lake Malombe, and as far as the fishes are concerned, most species are endemic to the two lakes (Turner 1996).

Lake Malawi developed as a consequence of tectonic activities in the Miocene, during the formation of the Great Rift Valley (Livingstone & Melack 1984). The rift formed when the land between two parallel faults subsided and the valley filled with water from the rivers that flowed into it. Although the rifting that formed the basin has a history of more than 20 million years, it is generally believed that water has been continuously present in the basin for about two million years (Fryer & Iles 1972; Crossley 1979; Johnson & Ng'ang'a 1990). The fishes that originally colonized the lake did so from the rivers that flowed into the Rift Valley (Lowe-McConnell 1987). Colonisation was followed by explosive speciation resulting from narrow specialization as the cichlid fishes adapted to particular habitats. This evolutionary spectacle is an outstanding phenomenon, reflecting what is arguably the most rapid evolution and speciation of vertebrates anywhere in the world (Fryer & Iles 1972; Greenwood 1974; Ribbink 1994; Ribbink 2001).

Fair. Overall, data on the systematics and ecology of the lake are poor and extensive work is needed to improve the knowledge of the system. Sampling in the ecoregion has been concentrated only in certain areas, such that few lake-wide datasets are available for informed decisions. However, the recently completed SADC/GEF Lake Malawi/Nyasa Biodiversity Conservation Project included a large fish systematics component (Snoeks 1999a), and a more recent EU Programme that included studies on the trophic ecology of the demersal fish community (Irvine pers. comm.). Data from these studies have improved the knowledge base. Additionally, data on commercially utilised fish and fisheries along the Malawi shores, particularly in the southeast arm of the lake, are good.