61
Monomictic lakes could again be of two types,
namely,
(i) Warm monomictic : This kind of lake occurs
in the warm subtropical regions. Here, the
water temperature is never <4 °C. There is
only one circulation period during the
winter.
(ii) Cold monomictic : This kind of lake occurs in
the polar regions. Here, the water temperature never goes >4 °C. Seasonal overturns
occur during the summer months.
5.3.2 Dimictic Lakes
In this type of lakes, there are generally two periods of mixing or overturns.
5.3.3 Oligomictic Lakes
These are thermally stable tropical lakes, in
which mixing occurs very rarely and slowly.
5.3.4 Polymictic Lakes
These are high altitude equatorial lakes, in which
circulation occurs more or less continuously.
Stagnation period, if any, is very brief.
5.3.5 Amictic Lakes
Lakes in which there is practically no mixing
being sealed off by ice from most of the changes
at the earth’s surface are called amictic lakes.
These lakes are sometimes found in the Antarctica
and in high mountains.
5.3.6 Meromictic Lakes
These are chemically stratifi ed lakes which
become permanently stratifi ed by the intrusion of
saline water or salts liberated from sediments,
thereby creating a permanent density difference
between the surface and bottom waters and
leading to the formation of a ‘chemocline’
between the circulating and non-circulating
layers. ‘Meromictic’ denotes partly mixed. Free
oxygen and aerobic biota could be absent in the
bottom waters of such lakes.
5.3.7 Holomictic Lakes
Lakes in which surface and bottom water mix
wholly and periodically are called holomictic
lakes.
5.4
Classifi cation of Lakes on the
basis of Nutrition
On the basis of nutrient status and productivity,
lakes could be classifi ed into the following types
(Thienemann 1925 ):
5.4.1 Oligotrophic Lakes
These are deep, steep-banked lakes with the following features:
1. Volume of epilimnion relatively small as compared to hypolimnion. Water is quite transparent with poor nutrient status. Permanent
abundance of oxygen at all levels. Sediment
low in organic matter. Scanty littoral vegetation and phytoplankton generally poor with
rare blooming of blue green algae (BGA)
5.4.2 Eutrophic Lakes
These are shallow lakes with broad littoral zone
and other features given below:
1. Greater epilimnion–hypolimnion ratio, water
less transparent with abundance of nutrients
and calcium, hypolimnial oxygen depletion
occurs during summer, sediments rich in
organic matter, abundance of littoral vegetation; phytoplankton generally quantitatively
rich with frequent blooms of BGA
Conditions in deep lakes differ from those of
shallow lakes although they could be in the same
5.4 Classifi cation of Lakes on the basis of Nutrition
Monomictic lakes could again be of two types,
namely,
(i) Warm monomictic : This kind of lake occurs
in the warm subtropical regions. Here, the
water temperature is never <4 °C. There is
only one circulation period during the
winter.
(ii) Cold monomictic : This kind of lake occurs in
the polar regions. Here, the water temperature never goes >4 °C. Seasonal overturns
occur during the summer months.
5.3.2 Dimictic Lakes
In this type of lakes, there are generally two periods of mixing or overturns.
5.3.3 Oligomictic Lakes
These are thermally stable tropical lakes, in
which mixing occurs very rarely and slowly.
5.3.4 Polymictic Lakes
These are high altitude equatorial lakes, in which
circulation occurs more or less continuously.
Stagnation period, if any, is very brief.
5.3.5 Amictic Lakes
Lakes in which there is practically no mixing
being sealed off by ice from most of the changes
at the earth’s surface are called amictic lakes.
These lakes are sometimes found in the Antarctica
and in high mountains.
5.3.6 Meromictic Lakes
These are chemically stratifi ed lakes which
become permanently stratifi ed by the intrusion of
saline water or salts liberated from sediments,
thereby creating a permanent density difference
between the surface and bottom waters and
leading to the formation of a ‘chemocline’
between the circulating and non-circulating
layers. ‘Meromictic’ denotes partly mixed. Free
oxygen and aerobic biota could be absent in the
bottom waters of such lakes.
5.3.7 Holomictic Lakes
Lakes in which surface and bottom water mix
wholly and periodically are called holomictic
lakes.
5.4
Classifi cation of Lakes on the
basis of Nutrition
On the basis of nutrient status and productivity,
lakes could be classifi ed into the following types
(Thienemann 1925 ):
5.4.1 Oligotrophic Lakes
These are deep, steep-banked lakes with the following features:
1. Volume of epilimnion relatively small as compared to hypolimnion. Water is quite transparent with poor nutrient status. Permanent
abundance of oxygen at all levels. Sediment
low in organic matter. Scanty littoral vegetation and phytoplankton generally poor with
rare blooming of blue green algae (BGA)
5.4.2 Eutrophic Lakes
These are shallow lakes with broad littoral zone
and other features given below:
1. Greater epilimnion–hypolimnion ratio, water
less transparent with abundance of nutrients
and calcium, hypolimnial oxygen depletion
occurs during summer, sediments rich in
organic matter, abundance of littoral vegetation; phytoplankton generally quantitatively
rich with frequent blooms of BGA
Conditions in deep lakes differ from those of
shallow lakes although they could be in the same
5.4 Classifi cation of Lakes on the basis of Nutrition