Lake Tanganyika.
Lake Tanganyika is approximately 12 million years old. Its area is 31,900 km2, shoreline is 21,000 km, average depth is 570 m (maximum 1,470 m). The average surface level of Tanganyika’s water mirror is 774 m above sea level. Salt composition of the water (mg/l): Na – 57, K – 35, Ca – 9.8, Mg – 43.3, Cl – 26.5, SO4 – 8, Na2CO3 – 125. Hardness 10-15°, pH 8.6-9.2, temperature 25-27°C.
The lakes owe their origin to tectonic forces, as a result of which a once large East African area was fractured into numerous deep 
cracks, subsequently filled with water. Extinct or still active volcanoes are located along the edges of the lakes, forming peculiar accumulations. Among the largest lakes on Earth, Victoria, Tanganyika, and Malawi occupy 7th, 8th, and 9th places respectively. These are isolated lakes with no connection to major river systems. Lake Tanganyika is the longest freshwater lake in the world. Translated from the local dialect, the name of the lake means “sail”.
Extended from north to south, it has a length of about 650 km with a relatively small width of 40-80 km. In terms of area, the lake ranks second among the East African lakes, yielding (by exactly half) only to Lake Victoria. However, the most remarkable feature of Tanganyika is not its horizontal dimensions, but its incredible depth – 1,470 m. Of all the lakes on the globe, it is second in depth only to Lake Baikal (1,520 m).
Of all East African lakes, Tanganyika has the longest “stint” of continuous existence. Having formed once, it has never dried up; its ancient aquatic fauna, formed back in the Tertiary period, did not disappear, but “survived” to this day and during this time additionally enriched itself with new species that arose in the course of biological evolution. Many ancient species, widespread in the past, now survive only in Tanganyika, becoming its endemics. Another reason for the high endemism of the lake fauna lies in the long-term isolation of the lake from other East African lakes.
According to scientists, throughout most of its history, Tanganyika was an endorheic lake, and its ichthyofauna developed in separation from the fauna of neighboring hydrographic basins. The lake itself consists of three main basins: northern (“Kigoma”), central (“Kalemie”), and southern (“Zongwe”), each of which arose independently of the other; the central basin likely appeared first (9-12 million years ago), the northern basin formed approximately 7-8 million years ago, and the southern (the youngest) – 2-4 million years ago. The conventional boundaries of these basins are outlined by mountain ranges on the coast of Lake Tanganyika.
The shores and waters of the lake belong to four African states – Congo (Zaire), Burundi, Tanzania, and Zambia. The northern part of the lake is located where the border between Congo and Burundi runs. From here to the equator in a northern direction is only a few hundred kilometers. The coast of the lake is topographically diverse, and along the entire coastline, different types alternate: rocky regions neighbor sandy beaches, and in places with particularly steep cliffs – beaches covered with pebbles (gravel) or small stones. River mouths stand out with striking vegetation along the edges of the lake. Most rivers and streams are temporary in nature and fill with water only during the rainy season.
Such large rivers as the Malagarasi, Kalambo, and Ifume (Tanzania), Ruzizi (Burundi), Lukuga and Lualunga (Congo), Lufubu (Zambia) have had (and still have) a significant impact on the distribution of rock-dwelling cichlids, which formed at the border between two neighboring, but anatomically different populations. The Ruzizi River carries highly mineralized water from Lake Kivu into Tanganyika. Moreover, the Ruzizi River contributes not only to the chemical composition of the water in the lake, but also to its current water level. It is noteworthy that after the formation of the Ruzizi River (about 10,000 years ago) between Lakes Kivu and Tanganyika, the water level of the lake rose by 75 meters. The only outflowing river, the Lukuga, begins in the middle part of the western coast and flows westward, connecting with the Zaire River, which flows into the Atlantic.
Coastal landscapes are usually huge cliffs, and only on the eastern side are the shores gently sloping. On the western coast, the steep side walls of the East African Rift system, forming the coastline, reach 2,000 meters in height. The coastline is dotted with bays and gulfs. The largest of them is Burton Gulf. The river mouth areas of the lake are heavily overgrown with *Vallisneria*, *Ceratophyllum*, *Myriophyllum*, *Elodea*, reeds, and bulrushes. The water is opaque. These areas are characterized by *Astatoreochromis*, *Limnotilapia*, and *Oreochromis*.
In a simplified view, Lake Tanganyika cannot “boast” of a variety of biotopes. Firstly, these are rocky biotopes, which in their vertical and, especially, horizontal extent often occupy large sections of the coast. These can be beaches composed of small gravel deposits, piles of stones the size of a fist or a football, or huge boulders taller than a person. Often all these stones and pebbles can be found together on the same stretch of shore. Such areas underwater are especially rich in diverse fauna and flora, since brightly lit rocky stones are the best place for the settlement of various algae and other microorganisms. And if a coastal stone has a complex external structure with numerous depressions, holes, and cracks, it means you will find a wide variety of fish around it.
With depth, the amount of growth on the stones decreases and, depending on the position of the shore and proximity to rivers, the volume of sediment increases. Accordingly, the picture of the animal world also changes. In the rock zone, the shores drop steeply to a depth of 5 to 15 m, the bottom is covered with huge rock fragments forming large caves. There are no shallow waters here, nor are there higher aquatic plants, but a powerful carpet of algae can be found even at great depth. Representatives of the genera *Chalinochromis*, *Cyprichromis*, *Julidochromis*, *Ophtalmochromis*, *Perissodus*, *Petrochromis*, *Telmatochromis*, and *Tropheus* live here.
The second important biotope is underwater sandy plains. Here species diversity is noticeably poorer, although the density of living organisms can be quite high. Visibility depth here is also “modest”, so this biotope looks paler. Near large sandy bays, there are extensive coastal areas also covered with sand. A detailed study of this biotope is yet to come. On a silty sandy bottom, for example, in river alluvium areas, the number of species is even sparser, approaching zero. The sandy beach zone is divided into areas covered with clean white-red sand and swampy river mouth areas. The former are characterized by an almost complete absence of shelters (except for reed and bulrush thickets in shallow water) and crystal-clear water. The fish inhabiting this biotope are mostly schooling and flee when in danger.
Among cichlids, there are those that like to dig in the substrate. They live mainly near the rocky bottom in Lake Tanganyika. Most often these fish lie near their shelters, resting on their pectoral and ventral fins. Like most fish on the planet, the inhabitants of Tanganyika camouflage themselves to match their environment. Therefore, species living in the sandy zone are colored in light tones, while cichlids inhabiting the rocky landscape are darker and more contrasting; as a consequence, the inhabitants of the transitional zone between the two characteristic biotopes possess a mix of light and dark tones. Of course, there are exceptions to all these “rules”. They are represented by the genera *Xenotilapia*, *Callochromis*, *Cyathopharynx*, and *Lamprologus*.
Another habitat is the transitional zone between rock and sand. Due to the presence of all resources from both biotopes, the highest animal density is observed here. Cichlids of the genera *Eretmodus*, *Julidochromis*, *Spathodus*, *Tanganjicodus*, *Telmatochromis*, and *Tropheus* inhabit this zone. And only those species that cannot exist in an environment with an increased sediment inflow feel uncomfortable here and are therefore found in very small numbers. However, it is within the boundaries of this biotope that representatives of the new genera *Cunningtonia* and *Cyathopharynx* can be seen. In some places, plants are found, in the thickets of which *Limnotilapia* and *Ctenochromis* live. Usually, these fish settle only in this zone. The river mouth areas of the lake are heavily overgrown with *Vallisneria*, *Ceratophyllum*, *Myriophyllum*, *Elodea*, reeds, and bulrushes. The water is opaque.
Cichlids living in the open water zone are of almost no interest to aquarists due to their size and specific habits. The best known is *Boulengerochromis microlepis*, reaching a length of 80 cm. *Bathybates*, *Cyprichromis*, *Hemibates*, *Limnochromis*, *Perissodus*, and *Trematocara* also live here. Most of these fish are schooling and have an elongated, cylindrical body.
An average of about 1,200 mm of precipitation falls on the surface of Tanganyika per year. Tanganyika’s tributaries bring in 23.8 cubic km of water annually. Consequently, the total annual water input to the lake is 64.6 cubic km, of which 63% comes from precipitation and 37% from catchment runoff. Precipitation during the rainy season, which falls in October-December and late February-April in the north and November-March in the south, noticeably increases the water level in the lake and washes a large amount of sediment into the lake. In the northern part, due to the proximity to the equator, bad weather lasts longer. In the south, the rains are more intense. However, the amount of precipitation in both the south and the north is approximately the same.
Continuous, prolonged rains affect the reproduction of plankton and the transparency of local waters, which are crystal clear at normal times. The water in Tanganyika is generally very clear. Visibility is very good. The water transparency in the lake reaches more than 20 meters and only in sandy or muddy regions can it drop to 10 meters due to suspended sediment particles. More than 94% of all water expenditure accounts for water loss through evaporation from the lake surface (61.2 cubic km), while runoff through the Lukuga accounts for less than 6% (3.4 cubic km).
According to foreign researchers, temperatures in the upper layer of the lake (epilimnion) fluctuate throughout the year from 23.8 degrees to 26.6 degrees in the northern and from 23.6 degrees to 26.5 degrees in the southern basin of Tanganyika. Seasonal fluctuations in the temperature of the surface water layers are small: in winter, in August, it drops to 22–25°C, while in mid-summer, in January, it rises to 28–32°C. These changes are smoothed out due to the great depth of the lakes, and thus the temperature of the inner layers of water remains practically unchanged. The thermocline layer, usually well expressed, varies in depth in the northern part of the lake from 40 meters in the hottest months to 100 meters in the colder time of year; in the southern part, fluctuations in its position are more significant. At great depths, a temperature of about 23 degrees is maintained all year round.
Cichlids of Lake Tanganyika. Main guidelines for fish habitats.
Lake Malawi. Mbuna and Utaka cichlid groups. 
