Boundaries

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.

Topography

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.

Freshwater habitats

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).

Description of endemic fishes

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)

Other noteworthy fishes

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).

Justification for delineation

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).

Level of taxonomic exploration

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.