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Brief summary: Limnology and paleolimnology of African
lakes
Part I: Limnology of African lakes
Location:
Between latitude 37°N and 35°S
Climate:
The main determinant of tropical weather variability is the annual north-south migration of
the ITCZ and associated monsoonal wind systems which advect moisture from the tropical
oceans to the hot adjacent continents. There are two rain seasons in tropical Africa (MarchMay = long rains; October-December = short rains). Most of the precipitation during these
periods is coming from the Indian Ocean and thus subjected to dynamic interaction between
ocean, atmosphere and land in the Indian monsoon region.
The Intertropical Convergence Zone (ITCZ) is an area of low pressure that forms where
the Northeast Trade Winds meet the Southeast Trade Winds near the earth's equator. As
these winds converge, moist air is forced upward. This causes water vapor to condense, or
be "squeezed" out, as the air cools and rises, resulting in a band of heavy precipitation
around the globe. This band moves seasonally, always being drawn toward the area of most
intense solar heating, or warmest surface temperatures. It moves toward the Southern
Hemisphere from September through February and reverses direction in preparation for
Northern Hemisphere Summer that occurs in the middle of the calendar year. The result of
ITCZ is not only seasonal wind patterns but also seasonal rainfall.
Landscape – rift formation
- Few folded mountain belts (Ruwenzori in East Africa, Draken’s mountains in South Africa,
Atlas in North Africa, Tibesti mountains in central Sahara,…)
- The Great Rift is an extension of mid-oceanic ridges. It extends southward from the Dead
Sea through the Red Sea, coming ashore at Afar and passing through Ethiopia to split into
two branches, the Western Rift from Lake Albert and Ruwenzori through Lake Tanganyika
and beyong to Lake Malawi and Chilwa (through DR Congo, Uganda, Rwanda, Burundi,
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Tanzania, Zambia and Malawi) – and the Eastern Rift from Lake Turkana to Basotu (through
Kenya and Tanzania).
- The Rift system of the Cameroon highlands (has also been important for the formation of
lakes)
Origin of lakes:
- Tectonic lakes: The largest and deepest lakes were formed directly by tectonic activity
(displacements of the earth’s crust). Such crustal activity has gone on intermittently since
Precambrian time, but most of the conspicuous features of the present Rift have been
formed by tectonic activity that began in the Miocene and continues to present day. Lakes
Albert, Turkana (Rudolf), Tanganyika, Mweru and Malawi occupy basins formed by
downthrust fault blocks (grabens, which are often long, narrow and deep; mainly between
8°N and 15°S). Lake Victoria, the largest lake of Africa and the world’s second largest
freshwater lake is occupying a shallow depression between the shoulders of the West and
East African Rift; it was formed by uplifting of the entire basin (=upwarping; such lakes
are large and fairly shallow). There is little geological evidence for deciding which is the
oldest of the tectonic lakes: without radiometric dating and paleontologic study of complete
sections through their sediments, the ages of the lake basins and especially during which
they have held water continuously, cannot be determined with assurance (Lake Tanganyika
holds >1500 m of sediments, and Albert at least 1200 m). Many people believe that
Tanganyika and Malawi are actually the oldest.
- Lakes associated with volcanic activity: Volcanism associated with rifting has been a
fertile source of African lakes. (1) Some lake districts, such as that of Kigezi in southwest
Uganda, were formed mostly (but not exclusively) by lava damming of river valleys. Few of
these lakes have been dated; Lake Kivu is the largest lava-dammed lake. (2) Other large
and deep lakes occupy the craters of calderas (= collapsed or exploded volcanoes;
basins formed by the subsidence of the roof of a partially emptied magmatic chamber, like
Lake Empakai in Tanzania. (3) In several places, volcanic explosions, with or without the
ejection of substantial amounts of volcanic ash, have created a large number of small (<2
km), round and often deep (>100 m) maar lakes (=explosion craters; some appear to be
about 5000 years old). Maars result from lava coming into contact with groundwater or from
degassing of magma. Maar regions include western Uganda (exceptionally diverse!!),
Bishoftu crater lakes in Ethiopia, and several lakes in Tanzania and central Kenya.
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- Lakes formed by glaciers and ice, such as on the higher (extinct) volcanoes (Mt Kenya)
and also in the Ruwenzori mountains. Most of them are amphitheater-shaped (quite small
and shallow) depressions caused by glaciers scouring glaciated valleys (=cirque lakes;
paternoster lakes). Fiord lakes are absent, and moraine-dammed lakes are scarce (those
that appear to be moraine-dammed are landslide-dammed or glacier-dammed instead). Most
are formed during Pleistocene glaciation and more recently (unlikely that they are much older
than 15000 years). During the Pleistocene, the tropical snowline did not as a rule extent
below 3000 m, so most lakes in tropical mountains are to be found from 3000 m upwards.
- Lakes formed by wind = deflation lakes, sometimes with accompanying erosion by
wallowing mammals. Examples include saline playas in arid areas
- Lakes in interdune hollows, the most spectacular being the lakes in northern Chad
(Ounianga serir/kebir)
- Lakes associated with shorelines, rivers… African rivers have the usual complement of
oxbow ponds and cutoff channels
- Man-made lakes (artificial lakes): series of very large man-made lakes is created since
the late 1950s (Lake Volta is the world largest!)
Non-nutrient chemistry
Main chemical sources come from:
(1) inflow and seepage, depending quantitatively and qualitatively upon the general
hydrology and geochemical character of the catchment plus cultivation, fringing samps,
human settlement etc. The waters in Subsaharan Africa are dominated to an unusual extent
by ions derived from the incongruent solution of sodium silicate rocks, rather than ions
derived from congruent solution1 of carbonates as in North America, Asia and especially
Europe. There is also a strong tendency, accordingly, for most African waters, to be solutions
of sodium bicarbonate (Na+, HCO3- and CO2-), rather than calcium bicarbonate. Many of
them have accompanying pH of 10 or more. The typical chemistry of African lakes is in fact a
combination of chemical peculiarities of the volcanic rocks that surround many of them, plus
(!) evaporative concentration with selective removal of less soluble salts.
incongruent solution: more soluble parts of the solid go into solution first, producing a
solution and leaving solid residue with different composition
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(2) atmospheric precipitation both wet and dry. E.g., near the sea coast, and in some
cases inland, there is a considerable contribution of cyclic sea salt.
(3) from gaseous exchange with the atmosphere. Gase exchange is determined by the
mixing regime. Other factors playing a role here are e.g. layers (free-floating mats) of
macrophytes widespread in both shallow and deeper waters include the species Pistia
stratiotes, Eichhornia crassipes, and Salvinia molesta. Such mats can lead to reduced
concentrations in the waters below (i.e., they can create particular barriers to vertical
exchange)
The chemical composition of African waters is largely controlled by rock weathering,
evaporative concentration and precipitation of calciumcarbonate.
Anions: Lakes are largely bicarbonate-carbonate (cf. alkalinity) dominated. Various studies
show that carbonate-bicarbonate (and hence alkalinity), chloride and sulphate all increase
regularly with increasing conductivity in African waters. Chloride-dominated waters are most
strongly represented in coastal or near coastal situation with present or past ingress of seawater (lagoons in North and West Africa; coastal lakes, but also inland, like Lake Katwe, and
Sahara salt lakes, like Dawada). Concentrations of fluoride can be very high! Perhaps our
evolution in an environment where the natural waters are so fluoride-rich generated the
anomalously high human requirement for this ion. In the wider world, we must supplement
the fluoride content of our drinking water to develop sound caries-resistant teeth.
Cations: In the commonest type, bicarbonate-carbonate, increase in salinity (or conductivity)
is associated with a steady rise in the concentrations of Na+ rather less regular rise of K+,
and final loss of precipitation of Ca2+ and Mg2+ (as carbonates). In fact, calcium and
magnesium also increase linearly with conductivity, but only up to about 200 µS/cm and 1000
µS/cm,
respectively.
Precipication
of
calcium
carbonates is easily documented;
precipication of magnesium is less common. Precipication of sodium carbonates also exist
e.g. Lake Magadi in Kenya supports an industry exploiting the trona of this lake. Thus the
ratio of monovalent to divalent cations increases and in saline waters Na+ is dominant. West
Rift lakes of the volcanic sources (Virunga volcanic field) are unusually rich in Mg2+ and K+.
Concentration of salt is often generated by enhanced evaporation under dry conditions. The
lake waters encompass virtually all of the very wide range of salinity. pH generally rises with
increasing alkalinity, locally depressed by accumulation of CO2 and raised by active removal
in photosynthesis.
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≠ Temperate lakes: The chemical composition is very different from that of temperate lakes
where calcium and bicarbonate tend to dominate.
Soda lakes
Shallow soda lakes cover large areas of the Rift Valley in Ethiopia, Kenya and Tanzania,
and occur in southern Africa. Small but numerous saline lakes are scattered through the
western Rift Valley, especially in Uganda, and stretch northeast from Lake Chad.
Soda lakes can be devided into two morphometric groups: broad, shallow pans and
small, deep depressions. The broad, shallow pans usually stratify and mix each day, with
calm stratified mornings followed by wind-mixing afternoons. Many of the small, deep lakes
are chemically stratified and meromictic, but mix daily to the chemocline. Suspended mineral
particles or dense phytoplankton are typical of these soda lakes, and vertical decrease of
light is often very high. The Secchidisk commonly disappears within 30 cm (1% light at 6 cm
depth, in Ngorongoro crater, Tanzania)
Soda lakes are systems of extremes. Intervals of near-dryness alternate with periods of
flooding and high water while salinity ranges through orders of magnitude in these lakes (e.g.
Nakuru, Elementeita,….).
The biota of soda lakes is depauperate. Nearly unispecific algal blooms can persist for
years, and photosynthetic rates approach the highest measurements for natural
communities. The soda lakes of tropical Africa are recognized as among the world’s most
productive ecosystems. Persistent populations of the cyanophycean Spirulina platensis are
commonly dominant supporting huge flocks of lesser flamingos (> 2 million); these are also
used for human food. In these systems, we find typical species of algae, hemiptera,
chironomid Diptera, anostraca etc.
Solid salt deposits are a conspicuous feature of the margins of many shallow soda lakes.
Some periodic removal by wind may occur. The relatively insoluble carbonates are
particularly common, notable trona (Na2CO3.NaHCO3.2H2O), and less frequent a tufa of
calcium and magnesium carbonates. Sodiumchloride occurs occasionally, e.g. in Lake Katwe
(western Uganda).
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Nakuru, Elementeita and Magadi, three broad shallow soda lakes lying in the Gregory Rift
Valley of central Kenya, may be used to illustrate the biogeochemical dynamics of African
alkaline saline lakes. The three lakes lie in semi-arid basins of internal drainage, and their
levels and salinities fluctuate drastically in response to changes in rainfall and evaporation
(dilution and evaporative concentrations)
Generation of sediments
Main sources are: (1) Allochtonous transfer of particles; (2) Chemical precipication; (3)
Biological deposition.
Hydrology
Ratio of ponded water to river flow through it. At one extreme end you have systems
whose flushing time is short. At the other extreme are closed lakes, or ones so nearly closed
as Tanganyika [at modern rate flow the effluent Lukuga would take 5000 to 6000 years to
discharge the volume of water equal that that which in ponded in the lake]:
Seasonal changes in hydrology are particularly great in floodplain lakes of
seasonally variable rivers; some completely evaporate to dryness. With it are associated
species that have adapted their life histories in spectacular fashions (e.g., airbreathing
lungfish, diapausing eggs, burrowing to a depth of 3 m into the dry sandy beds, …)
Closed lakes (=no surface outflow) are commonly less seasonal partly because they
are sustained by a groundwater reservoir that fluctuates less than rivers. Changes occur
more slowly as a results of runs of relatively wet or dry years. For many lakes, including
Victoria and Tanganyika, the main water losses are direct evaporation from the water
surface. Hence, the lake balance is determined by evaporation-precipitation lake balance.
[e.g., wet 1960s caused Tanganyika’s and Victoria’s lake water to rise several meters]
Stratification and Mixing
General notes:
Stability (S) per unit area of a lake is the quantity of work or mechanical energy required to
mix the entire volume of water to uniform temperature without addition or substraction of
heat. The stability of a lake is strongly influenced by its size and morphometry.
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Density differences are known to regulate stratification in lakes. The density difference
between water at a given temperature and 1°C lower (= density difference per degree
lowering) increase markedly at temperatures above and below 4°C. The amount of energy to
mix water of different densities increases proportionally to the density differences. E.g., the
amount of energy to mix layered water masses between 29° and 30°C is 40x, and between
24° and 25°C is 30x that required for the same masses between 4° and 5°C.
In temperate lakes, the main determinant of the circulation are seasonal fluctuations of
radiation and atmospheric temperature. The annual cycle of stratification in temperate
lakes is dominated by the great difference in radiant heat income between summer and
winter.
Density stratification has its constraints on vertical distribution and aspects of photobiology
that include O2/CO2 exchange, pH etc.
Tropics, general features:
We stated that the density difference per degree lowering is increasing markedly above
4°C so that the amount of energy to mix layered water masses between 29° and 30°C is 40x
that required for the same masses between 4° and 5°C. This means in tropical lakes a
relatively small difference in temperature between surface and deep waters may provide
considerable thermal stability.
What about seasonal fluctuations of radiation and atmospheric temperature in the
tropics? Within the extensive range of latitudes (37°N-35°S) occupied by the African landmass, lakes are found under varied annual climatic cycles. Seasonal variation in radiation
and hence temperature changes systematically with latitude, both as regard the amplitude
(minimal near the equator) and phase. Within the tropical belt, latitudes <23°, the amplitude
of seasonal variation of solar radiation and temperature are minimal. As northern starting
point: a maroccan mountain lake at 2050 m elevation: an annual cycle similar to the northern
temperate zone with low (= 5°C) winter temperatures and summer stratification, established
by warming after March and ended by cooling and mixing in November-December. But when
proceeding southwards, there are some important differences with the temperate zone: here,
the major determinant of the circulation, and hence production, is wind-driven evaporative
cooling (enhanced by low atmospheric humidity) rather than seasonal fluctuations of
radiation and atmospheric temperature, although the influx of cool rains and reduced
insolation during wet seasons can also be significant over a large part of Africa. Wind regime
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and other components of the energy budget (like atmospheric turbidity/clouds) mostly
display seasonal variation bearing no relationship to solar energy income. The best site
example is Lake Pawlo among the Bishoftu crater lakes in Ethiopia: here a seasonal phase
of cooling, typically in October-December, takes place not in the season of low solar
radiation, but under combination of relatively high solar radation and low humidity. For other
lakes, e.g. Victoria (0-2°), mixing is happening during the main incidence of the SE Trade
winds in June-July. Other examples include mixing in Lake Opi (Nigerian savannah) during
the annual impact of the dry harmattan wind. For this combination estimates of evaporative
heat loss were high. In conclusion, at higher latitudes the ultimate and general controlling
mechanism is undoubtedly the variable input of solar radiation, but near the equator
seasonal differentiation is muted and hence other factors like wind regime, atmospheric
humidity and turbidity, are more important. Overall, the tropics tend to be less windy than the
temperate zones, but in many areas of subsaharan Africa are so windy that the work of the
wind in disrupting stratification is quite high. Note that in tropical lakes, the anoxic lower layer
is not always the equivalent of the hypolimnion.
Tropics, various types of lakes and mixing regime (interplay between morphometry
and factors of the energy budget):
(1) Shallow lakes at low to mid-elevation (< 5m depth; <3000 m altitude): warm
polymictic (frequent periods of circulation at temperatures well above 4°C). There are
various case studies on such lakes, e.g. Lake George (area 250 km², mean depth 2.4 m).
These lakes can undergo three types of cyclic environmental change: (a) a diel (24h)
cycle of heat storage, stratification and mixing. The diurnal thermocline that quickly
develops in the late morning and afternoon (cf. density stratification at >12-14°C!) affects
phytoplanktonic sinking and accumulation or depletion of dissolved gases and mineral
nutrients, much as do the seasonal thermoclines of the temperate zones. The daily
stratification is ended by increased vertical mixing due mainly to a combination of reduced
radiation input, increased wind strenght late in the day, and transfer of sensible heat to a
cooler nocturnal atmosphere. Depending on the region in Africa, winds may or may not have
a pronounced seasonality (e.g. July impact of the S.E. trades influential is part of East and
central Africa). During the night, there is enhanced cooling and finally mixing. Exceptions
include lakes with very good wind shelter!; (b) an annual cycle. This can have radically
different forms depending on whether it is or not accompanied by entensive volume change
during a flood-drying cycle (well-developed in many African floodplains). In a shallow basin of
gently shelving morphometry, large changes of water volume due to hydrological factors lead
to corresponding large horizontal excursions of water level as well as changes in mean
depth. E.g. in the river-fed Lake Nakuru, there are radial extensions and contractions; Lake
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Chilwa is more complex with asymmetric river inflow and shallows plus swamp which lead to
freshwater rings; Lake Chad is even more complex as it consists of 2 basins, the deeper at
greater distance from the main inflow. This fact and the development at low level of aquatic
vegetation as obstacle to flow between basins, led to isolation of the deeper basin during low
levels of 1973 and final drying out afterwards; (c) cycles of longer period, resulting from the
succession of several dry or wet year (climate variation at time-scale of decades or longer,
see Part II: Paleolimnology).
(2) Lakes of intermediate depth, at low to mid-elevation (<100 m, <3800 m altitude):
oligomictic, one to several mixing periods at irregular intervals throughout the year by
occasional cooling of the surface.). For example, in Lake Victoria and Kariba, mixing is an
annual event, sometimes accompanied by spread of anoxic water and massive fish kills.
Lake Victoria has an enourmous surface area but is relatively shallow (max depth of 79 m;
average depth of 40 m). From September to January there is a trend leading ultimately to
stratification with a thermocline at 40 to 60 m between January and May (no more than 2°C
difference between surface and bottom!). Breakdown of stratification happens during May to
July: this is a period of the heavy southeast trade winds, resulting in increased evaporation
and there may also be greater loss of heat by radiation at night. The onset of the calmer
weather in August inaugurates another period of progressively increasing stratificiation.
(3) The deeper lakes, at low to mid-elevation (>100 m, < 3800 m altitude): meromictic,
staying stratified yearround (strata: monimolimnion, chemocline, mixolimnion). Especially
tectonic and volcanic ones that are very deep relative to their linear extent ànd often windsheltered by crater rims or surrounding highlands, stratification may persist for many years.
Above the level of permanent stratification, or chemocline, there may be a seasonal
thermocline subject to annual or more frequent breakdown by evaporative cooling of
surface waters during the windy season. Examples include Lakes Tanganyika and Malawi.
Both lie in deep rifts orientated north-south, up which the powerful southeast trade winds are
funnelled during the period from April to September. The seasonal alteration between this
period of strong unidirectional winds with low rainfall, and the remainder of the year with
variable and generally less violent winds and more rain, is the dominant influence on the
hydrological regime. The power, direction, duration of the wind; the orientation of the lake
and the surrounding topography all favour this condition.
Partial mixing results in recycling of nutrients crucial for maintaining lake
productivity (cf. endless summer of high surface T yearround accelerates all biological
processes). Internal loading processes dominate nutrient cycling in deep tropical lakes. This
includes erosion of the upper part of the anoxic layer by turbulence at the interface and
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through movements in the anoxic layer itself associated with internal waves. Chemical and
biological consequences of mixing: distribution of oxygen, transport of nutrients from depth to
illuminated surface where phytoplankton can grow.
Note that in some lakes, thermal stratification is frequently increased by a salinity
difference between deep and surface waters, e.g. seasonally freshwater cap in closed lakes
above residuum of more saline water built up by evaporation.
Some pecularities: Some of these meromictic lakes display a strong temperature
inversion (e.g., maar lake Mahega in western Uganda, has a Tmax of 40°C with cooler water
above and below; a bloom of bacteria and blue-green algae at 1m depth absorbing solar
energy to produce a warm layer). In other lakes like Lake Kivu, which is strongly stratified
(~2000 years) by dissolved salts that are believed to come from sublacustrine saline springs,
you have an accumulation gases (methane and carbon dioxide) to concentrations that
exceed saturation at surface pressure! You can pump the gases up… In such heavily
stratified lake upwelling of deep water rich in toxic gases (or very low in oxygen!) may cause
widespread fish kills at the time (season) of circulation. Although no kill sufficiently extensive
to destroy the fish fauna has been observed in modern times, probably such catastrophes
are the reason why the fish fauan of this lake is much pooreer than that of its neighbors.
Other examples include customary annual overturn in August 1976 in Lake Bosumtwi
(Ghana) resulting in heavy mortality of cichlid fishes, and the 1984-1986 gas disasters of
Lake Nyos and Monoun in Cameroon (killing >1700 people and life stock)
(4) Tropical high mountain lakes (>3800-4000 m) = cold polymictic lakes, with frequent
circulations during both day and nightstand of T below 12°C (normally slightly below or near
4°C). Glacial lakes present an interesting combination of equatorial insolation and very low
temperatures. They lack any stable stratification (e.g., lakes at the foot of glaciers >5000 m
on Mount Kenya are permanently below 4°C from top to bottom) because of relatively slight
differences in the density of water within that thermal range. Two main items differentiate
them from cold monomictic lakes (= lakes with frequent circulations in summer at low
temperatures, typically even below 4°C): cold polymictic lakes enjoy throughout the year
more or less constant light conditions at least with regards to the daily length of insolation
(and more or less permanent fog keeps insolation on an equal level) and they mostly lack
permanent ice covers (sometimes they do throughout the year or consecutive years by
fluctuations of the snowline = amictic). Great number of cold polymictic lakes are found in the
Andes (here we find several thousands, including the world’s biggest tropicial mountain Lake
Titicaca and lakes located above 6000 m altitude), sub-tropical Himalaya (>6000 m), and
~100 lakes are known to occur in the East African mountains and Ethiopian mountains. Such
lakes display even distribution of oxygen because of frequent mixing; those above the
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vegetation belt often have no noteworthy amount of organic material and the chemical
properties of lake water distinctly reflect the petrography of the catchment area. On Mt
Kenya, they are embedded in maily pure alkaline rocks extremely poor in Mg; conductivity is
almost pure a function of sodium concentration and neither Ca or Mg increases with growing
conductivities.
Concerning mixing regime in tropical high mountain lakes, even a lake like Titicaca,
16°S, 3800 m and max. depth 280 m has frequent circulation! If it would occur at lower
altitude, such lake would be permanently stratified.
Nutrient fluxes (these are in fact not so well-studied in Africa)
Main fluxes are from:
(1) River inputs: salt transportation by tropical rivers
(2) Rain inputs: high inputs of total nitrogen from rain (fine terrigenous dust and
photochemical reactions) may be common in Africa
(3) Swamp inputs: many lakes are fringed by extensive and highly productive swamps
(Cyperus papyrus, sedges, grasses…; living and dead stems, roots, decomposing peat).
Such swamps can lead to nitrogen fixation and phosphorus absorption, though nutrient flux
from such swamps is likely to be very large but not so well understood. Many of these
(paryrus) swamps are more similar to a large septictank than to a filter: large amounts of
debris are produced and settle to form a sludge, which provides nitrogen and enriched
detritus for the often productive biota in the neighbouring waters.
(4) Zooplankton: regeneration of nutrient; zooplankton excretion
(5) Phytoplankton: mineralisation of dissolved organics excreted by phytoplankton or
eroded off the sheaths of colonial blue-green algae; direct release of ammonia by
phytoplankton.
(6) other (and higher) levels of the food chain, zoobenthos, fishes, piscivorous fishes,
crocodiles, birds - like flamingoes (feed on Spirulina) in the shallow soda lakes (e.g. Nakuru),
can consume as much as 93% of the primary production
(7) Hippo excretion (very common in the Kazinga channel, Uganda)
(8) Export of fish
(9) Sediments: release or uptake
In temperate lakes, nutrient cycles are dominated by the physical processes governed by the
thermal regime and light limitation. More specifically, outburst of productivity is triggered
by rising temperature and illumination in spring, when surface waters are enriched by
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nutrients during winter mixing. In the tropics, on the other hand, there is ‘endless summer’
(i.e., termperature and illumination are adequate yearround), and therefore biological control
dominates the cycles of essential elements in lakes all year, rather than a few monts of the
summer. Hence, production will be maximum for the available resources, and is stimulated
by the onset of seasonal heavy winds which bring to the surface nutrients, which had been
depleted above and had accumulated below during stratification.
Smaller lakes seem to be more more P-limited and larger lakes seems to be more N-limited;
annual productivity seems to be unaffected by like size. Large deep lakes are as productive
as small shallow lakes. So, African lakes appear to violate the ‘first law of limnology’ (Fee
1979) stating that shallow lakes are more productive than deep lakes. However, meromictic
lakes are evidently less productive than they would otherwise be if they were periodically
completely overturned. The stagnant bottom layer is continuously accumulating nutrients
derived from organic decompostion, of which only a portion is brought to the surface and reenters the production cycle.
Of the major nutrient, Si is typically present – by world standards – in considerable
concentration but liable to depletion by diatom growth; forms of N (nitrate) are typically
present in low concentration (except for eutrophic systems) with some influence from
denitrification; and P is often in considerable concentration, especially in many but not all
soda lakes.
Turbidity
Can be considerable in shallow lakes (<5 m depth) where it is associated with particulate
content, heavy development of phytoplankton, or from suspension of sediments and plant
detritus (by wind, or influshing during the rainy season). Turbidity, or its reverse
transparancy, can be measured by the depth of visibility of a Secchi disc.
Lake Tanganyika: general limnology and some biological aspects
General features:
- Mean depth: 570 m; max depth 1470 m, deepest lake in East Africa (3rd on global scale);
largest by volume in East Africa
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- 75% of its volume is anoxic; the upper 200 m of oxygenated waters support a spectacular
lacustrine biota (no measurable oxygen below 100 m in the north and 200 m in the south)
- seasonal thermocline between 25 and 75m depth; and a permanent chemocline at
greater depth
- deep waters rich in phosphorus, silica and menthane (but not as much as in Kivu)
- inflows relatively small, the Ruzzizi river being the largest (carrying so much dissolved
material that its water dominates the chemistry of Tanganyika), coming from Lake Kivu. Also
important is the Malagarasi from the east. Of the water loss, 90% is by evaporation from the
free surface, only 10% goes out (water flowing westwards to the ocean through the Lukuga
and Zaïre)
- Nearly no nutrient input by rivers. Euphotic zone is nourished by regeneration of nutrient
internally (recycling in the surface waters) and from the depths (through the thermocline and
chemocline; and localized seasonal upwelling of deep water near the south end, especially
from July to September and from November to December, when strong southerly winds
blows up the lake. This upwelling, also featured in the similarly shaped and oriented Malawi,
is associated with a phytoplankton pulse).
- >1500 m of sediments, with sedimentation differing widely in different parts of the lake (in
rate and nature!)
- very transparant, with a Secchi disc visibility of as much as 20 m; low rate of nutrient
loading from effluents = misconception that Tanganyika is an oligotrophic unproductive
lake (thousands of tons of fish each year)
- lake level changes during its history have been considerable. During pleistocene, when
African lake levels were generally low, the level dropped several hundred of metres.
Endemism
In Tanganyika, you find lots of endemic species and genera, especially in the fish fauna
(173 out of 193 species are endemic; 40 genera are endemic vs Kivu only 17-18 species
present of which 7 or 8 are endemic); there are also many endemic ostracods, copepods and
molluscs, and even 2 endemic snakes. Although ancient lakes in other parts of the world
contain species that are endemic to them, and Lake Baikal on top, no other lake district
presents so many examples of radiative speciation from a limited number of parent stocks.
Malawi, Victoria and Tanganyika contain >100 endemic fishes (98% of the cichlid fishes;
57% of the non-cichlid fishes are endemic).
What is the reason for high endemism?:
There are various hypotheses. It is
conventionally believed that speciation requires the geographic separation of 2 populations
which then diverge sufficiently to prevent inbreeding. Lakes Malawi and Tanganyika have
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long shorelines consisting of alternating stretches of sandy beach and rocky cliff. It is
postulated that each of these two types of habitats acts as a series of islands, and that the
intervening stretches of the other habitat provide effective isolation; gene flow between the
islands is reduced to such an extent that evolutionary divergence can proceed to the level of
reproductive isolation. Lake-level changes that are known to have occurred in tropical African
lakes over a time-scale of tens of thousands of years would be sufficient to merge or partition
sandy beaches and rocky headlands, so populations could be split and reunited from time to
time. If reproductive isolation had been established by the time of reunion, and if competitive
interaction between the daughter species did not provide an insuperable advantage to either
of them, one would have two species where only one had been before. Lake Victoria has a
much more elaborate shoreline, less easily categorized into separate habitats, but it swarms
with islands that could provide the required geographic separation, and you also have beach
ponds where isolation can occur.
It has also been suggested that the lakes were separated into distinct basins during
the course of their history. Tanganyika does, in fact, contain several distinct basins, and a fall
in water level of 750m would separate it into three lakes; a fall of 1250m would create four.
Lake Victoria may have consisted at an earlier stage in its history, of a number of separate
small lakes (separate basins created by tectonic activity, but now united with the main lake).
Although water level change would not separate it into separate large lakes like Tanganika,
there are several small closed basins which could have been the home of isolated,
differentiating aquatic populations whenever the main lake dried up completely. All these
hypotheses are reasonable, although it must be recognized that they rest more on
hypothesis than evidence. They provide no explanation for the presence in Barombi Mbo, a
small volcanic crater in Cameroon with a simple bowl-shaped basin and uniform shore of 12
endemic species of fish.
A number of other groups on animals, all of them associated with the bottom rather
than open water for at least some critical part of their lives, have developed species flocks.
The gastropod molluscs of Tanganyika have attracted the most attention, in part because
some of them have thick or elaborate shells reminiscent of marine species. After a long
debate of a hypothetical former connection between Tanganyika and the sea, it is now clear
that these gastropods are of fresh-water origin. The heavy or ornamented shells
characteristic for some of the opistobranch gastropods are adaptions to the high wave
energy of this enormous lake, even if though many of the ornamented gastropods live well
below the surge level. It might also be an adaptive advantage in coping with specialized
mollusc predators such as shell-crushing cichlids.
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Human impact
Concerning human impact, it is noteworthy that tropical lakes may not function in exactly the
same way as temperate lakes. Impacts to which these lakes are exposed include:
-
eutrophication (resulting in persistent anoxia, fish kills, impact on nutrient recycling…)
-
soil erosion and increased sedimentation as a result of enhanced deforestation in the
catchment
-
species introduction, e.g. Nile Perch (’50) and Tilapia (’60) for commercial fish catch
(up to 200 kg!) in Victoria let to enormous decrease in number of endemic species
and population size
-
overfishing
-
…
Selected references on African Limnology:
Batterbee, R.W., Gasse, F., Stickley, C. (Eds.), 2004. Past climate variability through Europe and
Africa. Springer, Dordrecht, The Netherlands, 637 pp.
Beadle, L.C., 1974. The inland waters of Tropical Africa – An introduction to tropical limnology.
Longman, London, 365 pp.
Johnson, T.C., Odada, E.O. (Eds.), 1996. The limnology, climatology and paleoclimatology of the East
African lakes. Gordon and Breach Publishers, Toronto, Canada, 664 pp.
Kilham, P., 1990. Mechanisms controlling the chemical composition of lakes and rivers: data from
Africa. Limnol. Oceangr. 35(1): 80-83.
Livingstone, D.A., Melack, J.M. , 1984. Some lakes of subsaharan Africa. In: F.B. Taub (Ed.) Lakes
and reservoirs, Elsevier Science Publishers, Amsterdam, The Netherlands.
Löffler, H., 1964. The limnology of tropical high-mountain lakes. Verh. Int. Ver. Limnol. 15: 176-793.
Löffler, H., 1968. Tropical high mountain lakes. Their distribution, ecology and zoogeographical
importance. Colloquium Geographicum Band 9.
Talling, J.F., 1969. The incidence of vertical mixing, and some biological and chemical consequences,
in tropical African lakes. Verh. Internat. Verein. Limnol. 17: 998-1012.
Talling, J.F., 1992. Environmental regulation in African shallow lakes and wetlands. Rev. hydrobiol.
Trop. 25(2): 87-144.
Talling, J.F., Lemoalle, J. (Eds.), 1998. Ecological dynamics of tropical inland waters. Cambridge
University Press, UK, 452 pp.
Talling, J.F., Talling, I.B., 1965. The chemical composition of African lake waters. Int. Revue ges.
Hydrobiol. 50(3): 421-463.
Wetzel, R.G. (Ed.), 1983. Limnology. Second edition, Saunders College Publishing, US, 767 pp.
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Part II Paleolimnology
II.1. Paleoclimatology
Paleoclimate research in Africa is an international research priority coordinated by the
programmes Past Global Chages (or PAGES) and International Decade of East African
Lakes (or IDEAL).
?Why doing paleoclimate research
Knowledge of past climate variability (at decade-to-century time scales) is the foundation for
understanding and modelling current and future climatic trends. Long-term perspective of
natural climate variability is needed to be able to discover underlying mechanisms of climate
variation and to evaluate the impact of humans on future climate. People are now building a
global network of accurate and high-resolution climate reconstructions, with special attention
going to the tropics because this region is the heat engine of the global climate system, and it
is also the region from which we lack a lot of paleoclimate data.
?Why focusing on East Africa?
Throughout tropical Africa, and dryland regions of East Africa in particular, knowledge of
drought frequency and intensity is crucially important for the development of good landscape
and water-resource management practices in agriculture, freshwater fisheries and hydroelectric power generation.
?How to obtain long-term perspective of climate variability in East Africa?
In East Africa, instrumental weather and documentary records from before the colonial period
are very rare and too short; therefore these records do not offer a lot of information. There is
also limited potential of natural archives traditionally used to reconstruct climate in northern
temperate regions such as tree rings [very few African trees develop distinct annual growth
rings because of muted seasonality, and those that do rarely grow older than 100 years; also
dead logs and construction timber is rarely preserved long enough to be of any use in
extending the historical record], ice cores [very few snow-capped East African mountains; on
Ruwenzori and Mt Kenya climate signals are corrupted by intermittent meltwater percolation;
the only potential lies in the ice field on Mt Kilimanjaro], speleothems and corals [both limited
in geographical distribution, with speleothems only in extra-tropical north and south Africa;
corals in the Indian ocean coast]. Hence, lake sediments that accumulate on the bottom of
climate-sensitive lakes are the principal source of information on the climate history of
tropical East Africa. ‘Climate-sensitive’ lakes are lakes that
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(1) that clearly respond to short-term climate change (=sensitivity)
(that clearly display lake-level fluctuations through time)
AND
(2) that have a continuous sediment record (= longivety)
(that have survived the most arid periods without dessication or erosion of sediments)
These two criteria should certainly be met. In East Africa, it is certainly a huge challenge to
find lakes showing good balance between longivety and hydrological sensitivity.
In the sediment of these lakes, several paleoclimate indicators can be found, not only
biological proxies (species composition of subfossil algae, invertebrates, pollen) but also
non-biological proxies (sediment composition, geochemical signals). Every proxy can tell
something about past climate, every proxy has its strenghts and weaknesses so to get a
more reliable and integrated picture of climate change, one needs to compare
reconstructions based on several independent proxies (= multi-proxy studies)
The best-resolved multi-proxy reconstruction of rainfall and drought yet available from
equatorial east Africa is based on a sediment record recovered from Lake Naivasha,
Kenya (Verschuren et al. 2000). This record covers last 1800 years (~8 m of sediment core),
it has a decade-scale resolution and excellent age control because all factors confounding
chronology were carefully studied.
Ppt-slide 94: illustrated are lake-level fluctuations over the past 2000 years inferred
from sediment stratigraphy, and salinity fluctuation inferred from species composition of fossil
diatom and fossil midges (chironomids). High salinities correspond with low lake; low
salinities correspond to high lake level, hence there is generally good agreement between
reconstruction based on the different proxies. These data show that the climate of central
Kenya over the past 2000 years has been characterised by a succession of (at least) 7
(decade-scale) dry periods more severe than any drought recorded during the 20th century.
These periods are indicated in yellow [and called Naivasha drought 1 to 7].
Ppt-slide 95: Illustrated is the lake Naivasha lake level reconstruction with focus on
the past 1100 years. During the MWP (Medieval Warm Period; ~1000-1270 AD, when there
was medieval warming in north-temperate regions) climate was significantly drier than today
and that during the LIA (Little Ice Age; ~1270-1850, when Europa enjoyed great cooling)
climate was relatively wet, with a significant highstand between 1670-1770. However, the LIA
was also interrupted by three episodes of severe aridity; especially the dry period at ~200
years was tremendous and a lot of inland waters completely dessicated. From this, it clearly
shows that tropical climates have been anything but stable during the Holocene unlike
previously stated. Further illustrated is the pre-colonial history of East Africa recounted in oral
traditions and historical sources: the dry periods are associated with periods of famine,
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political unrest and large-scale migration, while the wetter periods concide with periodes of
prosperity. This clearly shows the importance of rainfall and drought in agricultural and
pastoral societies, and the strong link between cultural development and climate change.
Altogether, this study certainly underlines the importance of lake-based paleoclimatology.
Of course, one core is not enough to represent climate history throughout equatorial
Africa. Climate records from the large African Rift Lakes Tanganyika and Turkana lack
comparable detail and age control, but are consistent with our inference that tropical East
Africa was drier during 11th and 12th centuries and relatively wet from the late 13th to mid 18th
century. The extreme drought at ~ 200 years ago is also visible from these records, and
similar patterns also occur in several (typically incomplete) records from shallow fluctuating
lakes in the region, e.g. from Lake Chilwa in Malawi and Lake Chad in the Sahel. When
looking at data coming from southeast tropical Africa, dry Medieval period and maximum
aridity at ~200 years ago is also visible (which means these are continental scale events)
while trends during most of the LIA period were opposite (increasing or decreasing moisture
trends). In fact, this apparent alternation between episodes of interregional contrasts in
climate and episodes characterised by continent-wide events is not unexpected as it simply
reflect varying influence of distinct climate-forcing mechanisms working at different timescales.
One core is also not enough to elucidate all external and internal climate-forcing
mechanisms. As for external forcing, it seems that dry Naivasha periodes broadly coincide
with phases of high solar radiation, and intervening wet periods coincide with phases of low
solar radiation. Especially, highest inferred rainfall of the past 1100 years is coeval with the
Maunder minimum in solar radiation (~AD 1645-1715). Hence, solar forcing may have
contributed to decadal-scale rainfall variability in equatorial East Africa. As for internal
forcing, there is indication of influence of Indian monsoon variability but also processes in the
Atlantic ocean seem to be (at least partly) responsible for climate-variability at decade-tocentury time scales.
One core is not enough to evaluate links between climate variability in Northern
hemisphere and tropical regions and to produce realistic forecasts of future climate
change under changed boundary conditions of human-induced climate change.
Temperature rise during the past 2 decades is already outside the range of natural climate
variability. Hence, human impact is certainly an important player to take into account.
It is clear that it is quite a job to answer all these questions. Ongoing research is now
exploring the full potential of African lake sediments as recorders of past climate variability.
Over the past 4-5 years, some 75 permanent standing waters throughout equatorial East
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Africa were studied. Some 60 lakes are located in Uganda and Kenya. Study sites in Uganda
are mostly crater lakes located in four maar-crater districts in Western Uganda. Crater lakes
are preferred sites for climate reconstruction because many of them display reasonably good
balance between longivity and hydrological sensitivity. Study sites in Kenya include all the
main tectonic lakes and associated crater lakes in the dry Eastern Rift valley, freshwater
lakes on its moist shoulders to the east and west, the Nyanza Gulf of Lake Victoria, and
shallow lakes and swamps in Amboseli and Nairobi National Parks. All these Ugandan and
Kenyan lakes constitute a very diverse data set with lakes covering a broad salinity gradient
as they range from very dilute up to lakes 4x as salt as seawater (100-135,400 S S/cm);
they also display diverse combinations of lake area, depth, mixing regime and catchment
disturbance. As such, they represent the full regional diversity of climate-driven
environmental conditions, and can be considered a unique natural laboratory for doing
paleoclimate studies and for calibrating climate indicators.
Field data are of all kind. To evaluate longivity and hydrological sensitivity, lake
morphometry was mapped using echosounding, elevation points were taken along the
craterrim; notes on catchment characteristics were taken often in combination areal
photographs and recent satellite images. In addition, water samples were collected as well
as profiles of oxygen, pH, salinity and T etc. Based on all these data, and on rough
evaluation of local sedimentation conditions and hydrological sensitivity, some 27 sites have
now been selected as potential sites for high-resolution climate reconstruction. From these
lakes, short sediment cores were taken spanning ~100-250 years. More detailed
sedimentological analysis, preliminary sediment dating and rapid biological screening was
necessary to evaluate the real potential of these site for high-resolution climate
reconstruction. Ppt slide 109: illustrated are the results of sediment analyses from 3 short
cores (2 from Uganda – 1 from Kenya); in the sediment stratigraphy, there are clear
signatures of the dry event 200 years ago. However, based on Pb and C14 dating it seems
that none of these lakes display a continuous sediment archive; they likely dessicated during
dry events – hence they do not constitute good sites for long-term climate reconstruction. In
fact, only 10 of the investigated sites display good balance between sensitivity and longivety.
From these sites, long sediment cores were recovered which now form the basis for a
network of climate reconstructions across tropical East Africa over at least the past 2000
year. Lake Wandakara (ppt slide 111) in Western Uganda is such a lake, with clear climate
signals in the sediment record and continuous sedimentation. One of the most promising
sites we discovered for climate reconstruction is Lake Challa (ppt slide 112-113), a deep (97
m) permanently stratified crater lake on the lower East slope of Mount Kilimanjaro
(southeastern Kenya). Seismic-reflection profiling in January 2003 yielded a detailed
bathymetry and revealed very uniform sediment deposition across the central lake bottom.
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Exploratory Pb and C-dating of short cores indicate an average Holocene sedimentation rate
of ~1m per 1000 years, which means that the last 21,000 years are covered by the upper 18
to 20 meters (Late glacial + complete Holocene). The short cores are very finely laminated
over their entire length holding clear signals of climate change
As stated before, the current East African data set is – in many ways - very diverse; hence it
also constitutes the perfect basis for calibrating climate indicators, which means that we
analysed their variation in surface sediments of lakes spanning the full modern gradient of
climatically controlled environmental factors. From all lakes, surficial lake sediments were
collected with recently deposited death assemblages (of diatoms, crustaceans, aquatic
insects, …). Analysis of these subfossil remains provides information on modern species
distribution, information which is extremely valuable when making either qualitative or
quantitative interpretations from long sediment cores.
?Avenues for future research
(1) Multi-proxy study of the long sediment records to obtain regional network of climate
reconstructions. Especially the continuous and finely laminated sediment record of lake
Challa (low East slope of Mt. Kilimanjaro) is a very promising site to reconstruct – with
excellent time resolution and age control – the complete post-Glacial history of climate
variation in equatorial East Africa.
(2) Preliminary analysis of chironomid remains on Mt Kenya and the Ruwenzori Mountains
(Uganda – DR Congo) show clear potential for development of African chironomids as
quantitative indicators for temperature (a paleothermometer) independent of moisturebalance change.
Selected references on African paleolimnology
Verschuren D., Laird K.R. & Cumming B.F, 2000. Rainfall and drought in equatorial East Africa during
the past 1100 years. Nature 403: 410-414.
Verschuren D., 2004. Decadal and century-scale climate variability in tropical Africa during the past
2000 years. In Batterbee R.W. et al. (Eds.). Past climate variability through Europe and Africa.
Kluwer Academic Publishers, Dordrecht, The Netherlands.
II.2. Human impact studies
Paleolimnological techniques can also be used to reconstruct the environmental changes to
which lakes and adjacent landscape are exposed. Case study:
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Verschuren D., Johnson T.C., Kling H.J., Edgington D.N., Leavitt P.R., Brown E.T., Talbot M.R. &
Hecky R., 2001. History and timing of human impact on Lake Victoria, East Africa. Proc. R.
Soc. Lond. B. 269: 289-294.
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