85
the swamp due to lack of wind. To the contrary,
its mixing effect ensures homogeneity out in the
open water. The idea of a swamp may be unattractive to many people. But thorough investigation of the ecology of the lake soon reveals that a
swamp, such as that surrounding Lake Chilwa,
has a vital role to play in the functioning of the
ecosystem as a whole.
The swamp water is shallower, clearer and
less saline and contains less oxygen than that in
the main lake. Less stirring by wind due to the
sheltered condition and less photosynthetic
oxygen liberation due to decreased light intensity
could be the reasons. These could afford reasons
for less oxygen in water. Further, the detritivores
and the decomposers use up whatever little oxygen is left behind. The water is usually devoid of
oxygen in the centre of the swamp.
Nevertheless, the swamp plants represent a
store of energy and also a considerable store of
nutrients. The swamp sediment is usually deoxygenated. Hence, no barrier layer is formed on top
of the sediment and nutrients, such as phosphates
which are released into the water. The plants
take up nutrients through their roots and incorporate them into their tissues. Nitrogen is stored in
the rhizomes while the plant is still alive. But
other chemicals in the rest of the plant remain
there and are released after the plant has died and
rotted away or been converted into faecal pellets.
The open water of the lake usually benefi ts from
this source of food and nutrients only when the
swamp water is mixed with that of the open lake.
This is caused when rivers passing through the
swamp, especially during the rainy season, fl ush
the swamp water into the lake carrying the dissolved nutrients and fi ne particles of plant detritus along with the fl ow of water. Further, wind
also blows lake water deep into the swamp and
thus assists in nutrient transfer to the open lake.
There is a big population of fi sh in Lake
Chilwa which has a signifi cant impact on the
local economy. There is rich abundance of fi sh
around the swamp margins, where there is rich
abundance of planktonic and benthic animals
being sustained by the productivity of the swamp.
20 % of the fi sh catch is generally registered from
within the swamp itself. The dominant commercial
ichthyospecies, notably Clarias mossambicus
and Barbus paludinosus , make up to c 70 % of
the catch. They usually move from the lake into
the rivers and swamps during the rainy season in
order to spawn, after which they return to the lake
as the waters recede.
Fluctuations in water level on a long-term
and wide-seasonal basis seem to be a characteristic feature of many African lakes. However,
this is particularly evident in shallow lakes and
specifi cally those without any outfl ow. It may be
noted here that the water of Lake Chilwa
becomes increasingly saline as its level draws
down. Many plants and animals die in this condition. Some animals produce resting eggs and
take refuge in the swamps and rivers. Animals
which can endure such low water condition can
only survive in such a condition of the lake. The
remaining might have to re-colonise from outside. Fishes which can breathe air, e.g . Clarias
spp. and Protopterus sp., are well adapted to
such situations.
6.5.3 Lake Chad
In the tropics, the shallow lakes are very much
prone to drying up. They are refi lled during the
rainy season by ingress of water from the catchment. As such, Lake Chad is not unusual amongst
the shallow tropical lakes in undergoing wide
fl uctuations in water level. The shallower the
lake, the bigger the changes in the lake area.
These changes are mostly accompanied by
changes in the salinity of the water in the
endorheic lake. Further, the water eventually may
become too saline for most forms of life in those
lakes where evaporation constantly exceeds
infl ow. Lake Chad is one of the tropical African
lakes which shows spectacular changes in the
area and depth of its water. Nevertheless, this
enormous lake also has an interesting mechanism
by which the salinity is regulated and the water is
kept fresh over a wide range of water levels.
Lake Chad lies at 12–14° north of equator in
West Africa on the edge of the Sahara Desert. This
indicates that it is subjected to an extremely arid
climate. The annual average temperature is 28 °C.
6.5 Shallow Lakes
the swamp due to lack of wind. To the contrary,
its mixing effect ensures homogeneity out in the
open water. The idea of a swamp may be unattractive to many people. But thorough investigation of the ecology of the lake soon reveals that a
swamp, such as that surrounding Lake Chilwa,
has a vital role to play in the functioning of the
ecosystem as a whole.
The swamp water is shallower, clearer and
less saline and contains less oxygen than that in
the main lake. Less stirring by wind due to the
sheltered condition and less photosynthetic
oxygen liberation due to decreased light intensity
could be the reasons. These could afford reasons
for less oxygen in water. Further, the detritivores
and the decomposers use up whatever little oxygen is left behind. The water is usually devoid of
oxygen in the centre of the swamp.
Nevertheless, the swamp plants represent a
store of energy and also a considerable store of
nutrients. The swamp sediment is usually deoxygenated. Hence, no barrier layer is formed on top
of the sediment and nutrients, such as phosphates
which are released into the water. The plants
take up nutrients through their roots and incorporate them into their tissues. Nitrogen is stored in
the rhizomes while the plant is still alive. But
other chemicals in the rest of the plant remain
there and are released after the plant has died and
rotted away or been converted into faecal pellets.
The open water of the lake usually benefi ts from
this source of food and nutrients only when the
swamp water is mixed with that of the open lake.
This is caused when rivers passing through the
swamp, especially during the rainy season, fl ush
the swamp water into the lake carrying the dissolved nutrients and fi ne particles of plant detritus along with the fl ow of water. Further, wind
also blows lake water deep into the swamp and
thus assists in nutrient transfer to the open lake.
There is a big population of fi sh in Lake
Chilwa which has a signifi cant impact on the
local economy. There is rich abundance of fi sh
around the swamp margins, where there is rich
abundance of planktonic and benthic animals
being sustained by the productivity of the swamp.
20 % of the fi sh catch is generally registered from
within the swamp itself. The dominant commercial
ichthyospecies, notably Clarias mossambicus
and Barbus paludinosus , make up to c 70 % of
the catch. They usually move from the lake into
the rivers and swamps during the rainy season in
order to spawn, after which they return to the lake
as the waters recede.
Fluctuations in water level on a long-term
and wide-seasonal basis seem to be a characteristic feature of many African lakes. However,
this is particularly evident in shallow lakes and
specifi cally those without any outfl ow. It may be
noted here that the water of Lake Chilwa
becomes increasingly saline as its level draws
down. Many plants and animals die in this condition. Some animals produce resting eggs and
take refuge in the swamps and rivers. Animals
which can endure such low water condition can
only survive in such a condition of the lake. The
remaining might have to re-colonise from outside. Fishes which can breathe air, e.g . Clarias
spp. and Protopterus sp., are well adapted to
such situations.
6.5.3 Lake Chad
In the tropics, the shallow lakes are very much
prone to drying up. They are refi lled during the
rainy season by ingress of water from the catchment. As such, Lake Chad is not unusual amongst
the shallow tropical lakes in undergoing wide
fl uctuations in water level. The shallower the
lake, the bigger the changes in the lake area.
These changes are mostly accompanied by
changes in the salinity of the water in the
endorheic lake. Further, the water eventually may
become too saline for most forms of life in those
lakes where evaporation constantly exceeds
infl ow. Lake Chad is one of the tropical African
lakes which shows spectacular changes in the
area and depth of its water. Nevertheless, this
enormous lake also has an interesting mechanism
by which the salinity is regulated and the water is
kept fresh over a wide range of water levels.
Lake Chad lies at 12–14° north of equator in
West Africa on the edge of the Sahara Desert. This
indicates that it is subjected to an extremely arid
climate. The annual average temperature is 28 °C.
6.5 Shallow Lakes