Download Limnology and Paleolimnology of African Lakes

Limnology and Paleolimnology
of African Lakes
Dr. Hilde Eggermont
Limnology research group, Ghent University
http://www.ecology.ugent.be/limno/HE.php
Part I:
Limnology of African Lakes
Part I: Limnology of African lakes
Introduction (location – climate - landscape)
Origin of African lakes
Non-nutrient chemistry
Soda lakes
Generation of sediments
Hydrology, stratification and mixing
Nutrient fluxes
Tanganyika: general limnology and some biological aspects (endemism)
Human impact
Location: 37°N and 35°S
Mean determinant of tropical weather variability: annual north-south migration of the
Intertropical Convergence Zone (ITCZ) and associated monsoonal wind systems
ITCZ
ITCZ
850 mbar
(~1500 m)
CAB
CAB
To Southern hemisphere from sept – febr
=> Seasonal rainfall and wind patterns
To Northern hemisphere from march-aug
Vegetation zones
(White 1983):
Landscape:
Distribution of waterbodies in Africa
Origin of lakes:
A. Tectonic lakes:
Largest and deepest lakes were formed directly by tectonic activity (i.e. displacements
the earth’s crust), which has gone on continuously since Precambrium time
(most conspicuous features: Miocene to present)
2 types:
- Basins formed by downthrust fault blocks
(grabens);
Long, deep and narrow lakes
e.g., Lakes Tanganyika, Albert, Turkana, Malawi and
Mweru
- Basins formed by uplifting of the entire basin
(upwarping);
Large and fairly shallow lakes
e.g., Lake Victoria (largest lake of Africa; world’s
2nd largest freshwater lake)
Oldest type? > we need radiometric dating and
paleontologic study of complete sections through
their sediments.
Origin of lakes:
B. Volcanic lakes – 3 types:
1. Lava-dammed lakes:
formed by lava damming of river valleys
lava-dammed
lakes
Lava-dammed lake:
Lake Kivu (Rwanda)
Origin of lakes:
B. Volcanic lakes – 3 types:
2. Caldera-lakes: lakes formed by subsidence of the roof of
a (partially) emptied magmatic chamber (caldera=
collapsed or exploded volcano); fairly large and deep
caldera
lakes
Caldera lake:
Empakai crater (Tanzania)
Malha crater (northern Sudan)
Origin of lakes:
B. Volcanic lakes – 3 types:
3. Maar lakes: lakes occupying explosion craters (maars),
formed when lava comes into contact with groundwater or
degassing of magma. Often small (<2km²), round and
deep (>100 m)
maar
lakes
Maar lake districts in western Uganda
N
0
500
1000 m
Bishoftu lake district in Ethiopia
Cameroon volcanic line
Origin of lakes:
C. Lakes formed by glaciers and ice – 2 types:
1. paternoster-lakes:
series of lakes dammed by moraines or landslides, on the route of glacial
meltwater streams
2. cirque lakes:
lakes lying in amphitheater-shaped (quite small and shallow) depressions
caused by glaciers scouring glaciated valleys
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
Ruwenzori Mountains
(Uganda)
Lower Kitandara Lake (Ruwenzori Mountains, Uganda)
Middle and Lower Kachope (Ruwenzori Mountains, Uganda)
Mt. Kenya
(Kenya)
Hausburg Tarn
Mt. Kenya (Kenya)
Origin of lakes:
D. Lakes formed by wind (deflation lakes):
e.g. saline playas in arid areas
Lake Bunyampaka (Uganda)
Lake Nhsenyi (Uganda)
Origin of lakes:
E. Lakes in interdune hollows
Ounianga lakes (northeast Chad)
Ounianga lakes (northeast Chad)
Ounianga lakes (northeast Chad)
Origin of lakes:
F. Lakes associated with shorelines, rivers ...
oxbows, cutoff channels
G. Man-made lakes (artificial lakes)
Lake Volta (Ghana) – Akosombo dam, 50s
Non-nutrient chemistry
Main chemical sources
A. Inflow and seepage:
Depending qualitatively and quantitatively on the general hydrology and
geochemical character of the catchment (incl. cultivation, fringing swamps, human
settlement, …)
African waters are dominated to an unusual extent by ions derived from the incongruent
solution of sodium (Na+) silicate rocks (i.e. the more soluble parts go into solution first,
producing a solution and leaving solid residue with different composition) => strong
tendency to be solutions of sodium bicarbonate (Na+, HCO3- and CO32-), whereas
temperate lakes are often dominated by calcium bicarbonate
The chemical composition of African waters is largely controlled by rock weathering,
evaporative concentration and precipitation of calciumcarbonate
Non-nutrient chemistry
Main chemical sources
B. Atmospheric precipitation:
Wet and dry, e.g.: considerable contribution of cyclic sea salt near the coast
C. Gaseous exchange with the atmosphere
Determined by mixing regime (determined by depth and elevation)
Other factors, e.g. free-floating mats of macrophytes (Pistia stratiotes, Eichhornia
crassipes, Salvina molesta,…) as such mats can lead to reduced concentrations in the
waters below (i.e. they can create particular barriers to vertical exchange)
Salvinia molesta
Waterhyacinth
Non-nutrient chemistry
Main anions:
- Largely bicarbonate-carbonate
- Anions all increase regularly with increasing conductivity in African waters
- Chloride dominated waters
are strongly represented in
Coastal/near coastal areas with
past or present ingress on sea
Water
conductivity
-Floride concentrations can be
fairly high
Non-nutrient chemistry
Main cations:
- Sodium (Na+)
conductivity
- In the commonest type, bicarbonate-carbonate, increase in salinity is associated with a
steady rise in Na+, a rather less regular rise of K+, and final loss of precipitation of Ca2+
and Mg2+ (as carbonates)
Lake Katwe (Uganda)
Trona (sodium carbonates)
Lake Magadi (Kenya)
- African lake waters encompass virtually all of the very wide range of salinity
- pH generally rises with increasing alkalinity. It can (locally) be depressed by accumulation of CO2
and raised by active removal in photosynthesis
Soda lakes
Cover large areas of the Rift valley
and also occur in southern Africa,
northeast of Chad, etc.
Lake Elementeita (Kenya)
Two morphometric groups
1. Broad, shallow pans:
usually stratify and mix each day
(calm stratified mornings
followed by wind-mixing afternoons)
Lake Nakuru (Kenya)
Lake Kitagata (Uganda)
Two morphometric groups
1. Broad, shallow pans:
usually stratify and mix each day
(calm stratified mornings
followed by wind-mixing afternoons)
2. Small, deep depressions:
chemically stratified and
meromictic, but daily mix to the
Chemocline
Typified by suspended mineral
materials or dense phytoplankton;
vertical decrease of light is often
very high
Soda lake biota is species poor, though amongst the
world’s most productive ecosystems
Lake Katwe (Uganda)
Trona (sodium carbonates)
Lake Magadi (Kenya)
Solid salt deposits along
the margins of many
shallow soda lakes
Generation of sediments
(1) allochtonous particles; (2) chemical precipitation; (3)
biological deposition
Hydrology
Flushing time (Volume/Outflow): Ratio of ponded water to river flow through it
open lakes (short flushing time) ----------------------------- (nearly) closed lakes
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 result 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.
=> Lake balance, determined by evaporation-precipitation balance
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
is spectacular fashions (e.g., airbreathing lungfish, diapausing eggs,...
Lake Nakuru (Kenya)
Lake Chad (Niger, Nigeria, Chad, Cameroon)
1963
1987
1973
1997
Stratification & Mixing
Stability (S) per unit area of lake is the quantity of work or mechanic energy required to
mix the entire volume of water to uniform temperature without addition or substraction of
heat. The stability is strongly influenced by temperature of the lake water, size and
morphometry.
Density difference: the density difference between water at a given temperature and 1°C
lower (=density difference per degree lowering) increases markedly at T above and below 4°C.
Hence, the amount of energy to mix water of different densities increases proportionally to the
density differences
>> In warm waters, a relatively small difference in temperature between surface and deep
waters may already provide considerable thermal stability [e.g. The amount of energy to mix layered
water masses between 29°C and 30°C is 40x greater than that required for the same masses between 4°C and
5°C.]
Temperate lakes:
Mixing is determined by seasonal fluctations of radiation and atmospheric
temperature (difference in radiation between summer and winter)
Seasonal variation in radiation and
hence temperature changes
systematically with latitude, both as
regard the amplitude (minimal near
the equator) and phase
Radiation captured at different latitudes
Tropical lakes:
Within the tropical belt
(latitudes <23°C) the
amplitude of seasonal
variation of solar radiation
and T are minimal
- wind-induced evaporative
cooling is the main
determinant of circulation
- other factors: atmospheric
humidity, atmospheric
turbidity, salinity, elevation,
morphometry
E.g. Lake Pawlo (Ethiopia): seasonal
phase of cooling around Oct-Dec. This
takes place not in the season of low solar
radiation, but under a combination of
relatively high solar radiation and low
humidity
equator
Tropical lakes:
(1) Shallow tropical lakes, at low to mid-elevation
<5 m depth; < 3000-3800 m altitude
Warm polymictic, with frequent periods of circulation at temperatures well above 4°C
Lake George (Uganda)
area: 250 m², mean depth 2.4 m
Diel cycle of heat storage, (midday) stratification and (nightime)
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.
! In shallow saline lakes, diel mixing is incomplete due to high salinities (high density differences)
e.g. Pretoria salt pan (South Africa)
Tropical lakes:
(2) Tropical lakes of intermediate depth, at low to mid-elevation
<100 m depth; < 3000-3800 m altitude
Oligomictic, one to several periods of mixing at irregular intervals throughout the year by
occasional cooling of the surface
Lake Victoria (Uganda):
area: 68,800 km²; max. depth: 79m; average depth: 40 m. In this lake, mixing is an annual
event, sometimes accompanied by spread of anoxic water and massive fish kills
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 stimulates another period of
progressively increasing stratificiation.
Tropical lakes:
(3) Deep tropical lakes, at low to mid-elevation
>100 m depth; < 3000-3800 m altitude
Meromictic, staying stratified yearround (strata: monimolimnion, chemocline, mixolimnion)
Especially tectonic and volcanic lakes that are very deep relative to their linear extent and often windsheltered by a craterrim or surrounding highlands
Above the level of permanent stratification (i.e. in the mixolimnion), there may be a seasonal
thermocline subjected to annual or more frequent breakdown by evaporative cooling of surface
waters during the windy season. E.g., Lake 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.
Lakes Tanganyika and Malawi:
Partial mixing results in recycling of
nutrients (>erosion of the upper part of
the anoxic layer by turbulence at the
interface and through movements in the
anoxic layer itself associated with
internal waves). This is crucial for
maintaining lake productivity (cf.
endless summer of high surface T
yearround accelerates all biological
processes and hence consumption of
nutrients)
Some special features:
Strong temperature inversion
e.g., Lake Mahega (western Uganda):
at 1 m depth T-max of 40°C with cooler water above and below  reason: a bloom of
bacteria and blue-green algae at 1 m depth absorbing solar energy to produce a warm
layer
Some special features:
Gas accumulation
e.g. Lake Kivu (Rwanda) & Cameroon crater lakes:
Strongly stratified, with methane and carbon dioxide trapped in deep water under high
hydrostatic pressure. These gases have accumulated to concentrations that exceed
saturation at surface pressure
Gas exploitation
Extensive fishkills (upwelling of deep water rich in toxic gases or low in oxygen)
Limnic eruption phenomenon & catastropic gas disasters (eg. 1984-1986 Cameroon
killer lakes)
Lakes Monoun and Nyos (Cameroon): 1984 and 1986 gas disasters –
‘Killer lakes’
Tropical lakes:
(4) Tropical high mountain lakes (>3000-4000 m)
Cold polymictic, with frequent circulations during both day and night at T below 12°C (normally
slightly below or near 4°C)
Air temperatures: daily T oscillations between -5 and +20°C, much greater than seasonal
variations; cf. winter every night, summer every day
Water temperatures: ~0-8°C
Lake Batoda (Rwenzori Mountains at 3990 m)
- Slight density differences of water
within that thermal range
Lack any stable stratification
- Even distribution of oxygen because
of frequent mixing
≠ cold monomictic:
- More or less constant light
conditions (at least with regard to
daily length of isolation)
- Mostly (but not always) lack ice
covers
Lake Titicaca (Andes of Peru at 3880 m; 280 m depth)
Frequent circulation, but if such a lake would occur at lower altitude, it would be
permanently stratified
Nutrient fluxes
Main fluxes are from
(1) River inputs:
Salt transportation by tropical rivers
(2) Rains inputs:
Total nitrogen from rain (fine terrigenous dust and photochemical reactions)
(3) Swamp input:
Many African lakes are fringed by extensive and highly productive swamps. Such swamps
can lead to nitrogen fixation and phosphorus absorption.
Rather ‘septic tanc’ than 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 water
Nutrient fluxes
Main fluxes are from
(4) Zooplankton
(5) Phytoplanton
(6) Higher level in the food chain…
(7) Sediments
In temperate lakes:
Nutrient cycles are dominated by the physical processes governed by thermal regime
and light limitation
Outburst of productivity is triggered by rising in T and illumination during spring, when
surface waters are enriched by nutrients during winter mixing
In tropics:
‘Endless summer’: T and illumination are adequate yearround, and therefore
biological control dominates the cycles of essential elements in lakes all year.
Production will be maximum for the available resources, and is stimulated by the onset of
seasonal heavy winds which bring to the surface water nutrients (which had been
depleted above and had accumulated below during stratification)
Summary: Tropical versus temperate lakes:
Often solutions of sodium bicarbonate
Mixing regime:
- slight T difference: stable stratification (cf. density difference)
- not determined by seasonal fluctuations of radiation and temperature
(but: wind regime, humidity, morphometry, elevation, etc!)
Endless summer (high biological productivity!) and biological control of nutrient cycles
Lake Tanganyika - General features:
Mean depth: 570 m
Max depth: 1470, deepest lake in East Africa (3rd on global scale)
75% of volume is anoxic (chemocline at 100 m in N, 200 m in S)
Seasonal thermocline between 25-75 m
Permanent thermocline at greater depth
Relatively small inflows
90% of water loss through evaporation
Internal nutrient recycling!
>1500 m of sediments
Secchi depth of as much as 20 m
Considerable lake level changes during its history
Endemism
Fish fauna (98% of the cichlid fishes: 57% of the non-cichlid fishes)
Species flocks
Gastropod mollusc
Heavy ornameted shells do not
reflect former connection of Lake
Tanganyika and the sea, but are
adaptations to the high wave
energy environments and
predators
Endemism
Various hypotheses
Speciation requires the geographic separation of 2 populations which then diverge sufficiently to
prevent inbreeding (reproductive isolation)
(1) Lake Tanganyika and Malawi: long shorelines with alternating stretches of sandy beach and
rocky cliff – two types of habitats acting as a series of islands, and intervening stretches of the
other habitat providing effective isolation (during periods of low lake level)  reproductive
isolation
(2) Lakes were separated into distinct basins during the course of their history  these closed
basins could have been the home of isolated, differentiating aquatic populations whenever the
main lake dried up completely
(!a fall of 750 m in Lake Tanganyika would separate the lake into three lakes; a fall of 1250 m
would create four)
Human impact
- Deforestation in the catchment  soil erosion and increased sedimentation
- Overfishing
- Eutrophication (persistent anoxia, fish kills, impact on nutrient recycling,...); acidification;
contamination by toxic substances
- Species introduction (e.g., Nile Perch and Tilapia)
Waterhyacinth
Nile perch