66
almost every day Lake George goes through a
pattern of events similar to those experienced by
a temperate lake during a year.
These remarkably constant, warm conditions seem to allow the blue-green algae (BGA)
( Microcystis spp.) to maintain an overall dominance. Although >150 species of algae have
been recorded amongst the phytoplankton in
the Lake George, the BGA comprise >70 % of
the algal biomass all throughout the year. When
the water is calm, rafts of Microcystis can be
seen at the surface. Consequently, light does
not penetrate beyond about half a metre down
into the water and many of the algae spend
much of the day in darkness.
It was thought for many years that animals
could not digest BGA, such as Microcystis , and
that even if fi sh swallowed them they passed
through the gut unharmed and that nourishment
was obtained from bacteria and other algae taken
at the same time. One of the most interesting
aspects of the Lake George community is that
it is dominated by herbivores that can and do
utilise Microcystis as food. The small copepod
Thermocyclops hyalinus is one, and it dominates
the zooplankton. The others include two species
of fi sh, the tilapia ( Oreochromis niloticus ), which
is harvested commercially, and the much smaller
cichlid ( Haplochromis nigripinnis ) which comprises c 60 % of the total fi sh biomass.
The herbivorous fi sh Oreochromis niloticus
(formerly called Oreochromis nilotica ) occurs in
many African lakes. It feeds on BGA. It has a
daily digestive rhythm of acid secretion in the
stomach to cope with feeding during the day.
Although it is surrounded by a thick soup of
algae, it is not a nocturnal feeder. The stomach is
empty by dawn and there are only dead brown
algae left in the intestine. It begins to feed and
the passage of live algae into the stomach stimulates the secretion of strong acid. This helps
digestion of algae from green to brown as they
pass into the intestine. After feeding ends at
dusk, digestion continues through night until the
stomach is empty (Moriarty 1973 ). Lake George
was said to contain a large population of hippopotamus which was thought to be responsible
for the fertility of the lake. It is said that even
nitrogenous waste excreted by about 3,000 hippos
seems to be not enough as compared to that
required by the dense population of algae. It
seems more likely that it is the constancy of the
environment in Lake George which allows the
algal population to persist. Longer-lived animals, such as fi sh, do not usually seem to show
seasonal succession of species. Also, the lack of
diversity in the fi sh fauna of Lake George is,
probably, due to the relatively uniform structure
of the habitat. The lake is also relatively young
and is supposed to have originated about 3,500
years ago. This also reduces the scope of evolutionary diversifi cation.
The adult hippo, perhaps, has no natural
enemy. However, the young are vulnerable to
lions and crocodiles. There are, perhaps, no
crocodiles in Lake George. Certain other animals
take the advantage of the hippo’s bulk, e.g. the
little cyprinid fi sh ( Labeo velifer ) grazes on algae
growing on its skin; terrapins use half-submerged
hippo as basking platform; and birds like agrape
frequently perch on hippo for preying on aquatic
animals. Thus, hippo epitomises the interdependence of a lake and its surroundings.
6.1.5 Great Height Lakes in Africa
6.1.5.1 Lake Bunyonyi in Africa
This would be dealt with under High Altitude
Lakes.
6.1.6 Lakes in Indonesia
Ruttner, in course of his work with the German
Limnological Expedition of 1928–1929, made a
number of observations on the macrophytic
vegetation of lakes in Java, Sumatra and Bali.
The fl ora of the deeper water usually included
Hydrilla verticillata , which had occurred in 9 out
of 12 lakes studied. This was generally an abundant fl ora and was one of the most widespread
aquatic angiosperms encountered. This plant
could reach a depth of 8 m in Lake Singkarak in
Central Sumatra. It, however, reached a lesser
depth in other more transparent lakes of the
6 Lakes of the World
almost every day Lake George goes through a
pattern of events similar to those experienced by
a temperate lake during a year.
These remarkably constant, warm conditions seem to allow the blue-green algae (BGA)
( Microcystis spp.) to maintain an overall dominance. Although >150 species of algae have
been recorded amongst the phytoplankton in
the Lake George, the BGA comprise >70 % of
the algal biomass all throughout the year. When
the water is calm, rafts of Microcystis can be
seen at the surface. Consequently, light does
not penetrate beyond about half a metre down
into the water and many of the algae spend
much of the day in darkness.
It was thought for many years that animals
could not digest BGA, such as Microcystis , and
that even if fi sh swallowed them they passed
through the gut unharmed and that nourishment
was obtained from bacteria and other algae taken
at the same time. One of the most interesting
aspects of the Lake George community is that
it is dominated by herbivores that can and do
utilise Microcystis as food. The small copepod
Thermocyclops hyalinus is one, and it dominates
the zooplankton. The others include two species
of fi sh, the tilapia ( Oreochromis niloticus ), which
is harvested commercially, and the much smaller
cichlid ( Haplochromis nigripinnis ) which comprises c 60 % of the total fi sh biomass.
The herbivorous fi sh Oreochromis niloticus
(formerly called Oreochromis nilotica ) occurs in
many African lakes. It feeds on BGA. It has a
daily digestive rhythm of acid secretion in the
stomach to cope with feeding during the day.
Although it is surrounded by a thick soup of
algae, it is not a nocturnal feeder. The stomach is
empty by dawn and there are only dead brown
algae left in the intestine. It begins to feed and
the passage of live algae into the stomach stimulates the secretion of strong acid. This helps
digestion of algae from green to brown as they
pass into the intestine. After feeding ends at
dusk, digestion continues through night until the
stomach is empty (Moriarty 1973 ). Lake George
was said to contain a large population of hippopotamus which was thought to be responsible
for the fertility of the lake. It is said that even
nitrogenous waste excreted by about 3,000 hippos
seems to be not enough as compared to that
required by the dense population of algae. It
seems more likely that it is the constancy of the
environment in Lake George which allows the
algal population to persist. Longer-lived animals, such as fi sh, do not usually seem to show
seasonal succession of species. Also, the lack of
diversity in the fi sh fauna of Lake George is,
probably, due to the relatively uniform structure
of the habitat. The lake is also relatively young
and is supposed to have originated about 3,500
years ago. This also reduces the scope of evolutionary diversifi cation.
The adult hippo, perhaps, has no natural
enemy. However, the young are vulnerable to
lions and crocodiles. There are, perhaps, no
crocodiles in Lake George. Certain other animals
take the advantage of the hippo’s bulk, e.g. the
little cyprinid fi sh ( Labeo velifer ) grazes on algae
growing on its skin; terrapins use half-submerged
hippo as basking platform; and birds like agrape
frequently perch on hippo for preying on aquatic
animals. Thus, hippo epitomises the interdependence of a lake and its surroundings.
6.1.5 Great Height Lakes in Africa
6.1.5.1 Lake Bunyonyi in Africa
This would be dealt with under High Altitude
Lakes.
6.1.6 Lakes in Indonesia
Ruttner, in course of his work with the German
Limnological Expedition of 1928–1929, made a
number of observations on the macrophytic
vegetation of lakes in Java, Sumatra and Bali.
The fl ora of the deeper water usually included
Hydrilla verticillata , which had occurred in 9 out
of 12 lakes studied. This was generally an abundant fl ora and was one of the most widespread
aquatic angiosperms encountered. This plant
could reach a depth of 8 m in Lake Singkarak in
Central Sumatra. It, however, reached a lesser
depth in other more transparent lakes of the
6 Lakes of the World