72
lower ground is intensely cultivated. Sheep, cattle,
llamas and alpacas graze the uncultivated land
and range to higher elevations. Local people
harvest fi sh from the lake and use the Scirpus for
construction and fodder.
The surface temperature is always low and
varies little throughout the year on account of its
tropical latitude and great altitude. Extremes of
10.5–14.8 °C with a mean of 12.9 °C for the
winter months from May to September and
11.7–15.0 °C with a mean of 13.3 °C for the
summer months from January to March. The
winds had been found to be mainly local, blowing off the lake during the day and off the land
by night. They average at least 14.4 km h
−1 during the winter. The region had been known to be
evidently windier during summer. Occasional
storms from outside the basin of the lake may
produce persistent strong winds of generally
constant direction. The open water of the lake
was found to be quite transparent.
Less than half of the lake’s water comes from
the infl ows, despite the fact that these drain an area
of 58,000 km
2 or about seven times the total area
of the lake. About 53 % of the annual water input
comes as rain which falls directly on the lake itself.
The water level of the lake fl uctuates between 0.5
and 1 m each year. However, there could be also
longer-term variations in the water level which
could range from +1.2 to −3.7 m. Such changes
are said to be typical of lakes and closed basins.
Lake Titicaca does have an outfl ow, but the proportion of water loss via this route is very small
(1.5 %) that it hardly counts. Evaporation accounts
for c 90 % of water loss from the lake each year.
The rest is said to seep away.
The fl ora and fauna which live in the lake are
not very peculiar. The open pelagic zone contains range of widespread algae which are quite
similar to those found in other tropical lakes.
During summer, the phytoplankton is dominated by BGA, namely, Anabaena sp. and
Nostoc sp. During winter, the diatoms are said
to dominate. N and P levels are generally low.
However, Anabaena sp. is said to fi x some
amount of nitrogen.
The zooplankton consists mainly of two numerically dominant copepods, namely, Boeckella sp.
and Microcyclops sp. Sometimes, there could also
be few cladocerans and rotifers. One interesting
point is that the two calanoids belong to a genus
which is typical of this in the southern hemisphere.
This shows that Lake Titicaca was colonised from
south rather than from north.
The higher fl ora of Lake Titicaca was found to
be limited to eight species of angiosperms, notably, Myriophyllum elatinoides , Lilaeopsis sp.,
Elodea potamogeton , Potamogeton strictus ,
Ruppia fi lifolia , R. spiralis , Zannichellia palustris
and Scirpus tatora. The endemic R. fi lifolia was
reported to be an ecologically insignifi cant plant.
Natural fi sh community of lake consists
mainly of one catfi sh, namely, Trichomycterus
rivulatus , and a group of about 19 closely related
species of the endemic genus Orestias. The introduced rainbow trout ( Salmo gairdneri ) also forms
a component of the lake fi shery. The scarcity of
species does provide a contrast with the diversity
found in other tropical lakes. Isolation of Lake
Titicaca basin could be one of the reasons for low
species richness.
Interestingly, in Lake Titicaca , which is situated
at an altitude of 3,815 m, MSL, Potamogeton
strictes may descend to a depth of little >11 m.
This is, perhaps, the most reliable depth up to
which an angiosperm may reach. Further, Chara
and Hygrohypnum may reach up to a depth of
14 m and 29 m, respectively, in the same lake
(Tutin 1940 ).
6.2
Lakes of Temperate Region
6.2.1 Great Lakes in North America
The fi ve enormous stretches of water which
comprise the Laurentian Great Lakes form the
largest body of freshwater on the earth. These
lakes are famous, not only for their huge size, but
also for the story of their deterioration as a result
of abuse. Their enormous basins were formed by
glacial action about 10,000–15,000 years ago.
However, they now receive drainage from a relatively small area, so that water passes through
them rather slowly. This is especially true of
three largest lakes and particularly Lake Superior.
6 Lakes of the World
lower ground is intensely cultivated. Sheep, cattle,
llamas and alpacas graze the uncultivated land
and range to higher elevations. Local people
harvest fi sh from the lake and use the Scirpus for
construction and fodder.
The surface temperature is always low and
varies little throughout the year on account of its
tropical latitude and great altitude. Extremes of
10.5–14.8 °C with a mean of 12.9 °C for the
winter months from May to September and
11.7–15.0 °C with a mean of 13.3 °C for the
summer months from January to March. The
winds had been found to be mainly local, blowing off the lake during the day and off the land
by night. They average at least 14.4 km h
−1 during the winter. The region had been known to be
evidently windier during summer. Occasional
storms from outside the basin of the lake may
produce persistent strong winds of generally
constant direction. The open water of the lake
was found to be quite transparent.
Less than half of the lake’s water comes from
the infl ows, despite the fact that these drain an area
of 58,000 km
2 or about seven times the total area
of the lake. About 53 % of the annual water input
comes as rain which falls directly on the lake itself.
The water level of the lake fl uctuates between 0.5
and 1 m each year. However, there could be also
longer-term variations in the water level which
could range from +1.2 to −3.7 m. Such changes
are said to be typical of lakes and closed basins.
Lake Titicaca does have an outfl ow, but the proportion of water loss via this route is very small
(1.5 %) that it hardly counts. Evaporation accounts
for c 90 % of water loss from the lake each year.
The rest is said to seep away.
The fl ora and fauna which live in the lake are
not very peculiar. The open pelagic zone contains range of widespread algae which are quite
similar to those found in other tropical lakes.
During summer, the phytoplankton is dominated by BGA, namely, Anabaena sp. and
Nostoc sp. During winter, the diatoms are said
to dominate. N and P levels are generally low.
However, Anabaena sp. is said to fi x some
amount of nitrogen.
The zooplankton consists mainly of two numerically dominant copepods, namely, Boeckella sp.
and Microcyclops sp. Sometimes, there could also
be few cladocerans and rotifers. One interesting
point is that the two calanoids belong to a genus
which is typical of this in the southern hemisphere.
This shows that Lake Titicaca was colonised from
south rather than from north.
The higher fl ora of Lake Titicaca was found to
be limited to eight species of angiosperms, notably, Myriophyllum elatinoides , Lilaeopsis sp.,
Elodea potamogeton , Potamogeton strictus ,
Ruppia fi lifolia , R. spiralis , Zannichellia palustris
and Scirpus tatora. The endemic R. fi lifolia was
reported to be an ecologically insignifi cant plant.
Natural fi sh community of lake consists
mainly of one catfi sh, namely, Trichomycterus
rivulatus , and a group of about 19 closely related
species of the endemic genus Orestias. The introduced rainbow trout ( Salmo gairdneri ) also forms
a component of the lake fi shery. The scarcity of
species does provide a contrast with the diversity
found in other tropical lakes. Isolation of Lake
Titicaca basin could be one of the reasons for low
species richness.
Interestingly, in Lake Titicaca , which is situated
at an altitude of 3,815 m, MSL, Potamogeton
strictes may descend to a depth of little >11 m.
This is, perhaps, the most reliable depth up to
which an angiosperm may reach. Further, Chara
and Hygrohypnum may reach up to a depth of
14 m and 29 m, respectively, in the same lake
(Tutin 1940 ).
6.2
Lakes of Temperate Region
6.2.1 Great Lakes in North America
The fi ve enormous stretches of water which
comprise the Laurentian Great Lakes form the
largest body of freshwater on the earth. These
lakes are famous, not only for their huge size, but
also for the story of their deterioration as a result
of abuse. Their enormous basins were formed by
glacial action about 10,000–15,000 years ago.
However, they now receive drainage from a relatively small area, so that water passes through
them rather slowly. This is especially true of
three largest lakes and particularly Lake Superior.
6 Lakes of the World