78
time varies from 100 to 400 days. However, the
renewal time may be only two days during May
and June when the infl ow streams are on spate.
Wind mixing generally maintains the water
temperature uniform throughout its depth during
the ice-free period. The water temperature may
go up to maximum of about 15 °C throughout the
life. Water contains very low level of chemicals
(conductivity 10–30 μS cm
−1 ). Both the phytoplankton and zooplankton have clearly defi ned
life cycles. They also show signifi cant seasonal
changes. The benthic fauna is usually dominated
by insects ( c 74 %). It is typical of oligotrophic
lakes. Mayfl ies are generally abundant followed
by caddis fl ies and stone fl ies. Most of them are
detritivores. This indicates the importance of
allochthonous dead organic matters. The mayfl y
Leptophlebia marginata is generally abundant
when the lake is ice covered. However, the chironomids (67 spp.) are the most abundant and
diverse group of insects in the lake.
In this lake, the volume of water in the lake
basin is so small in comparison to the volume of
infl ow during the spate that it can be totally and
rapidly fl ushed out. Hence, the seasonal variation in solar radiation affects the seasonal cycles
in the lake.
6.2.8 Lakes in Denmark
There are quite a large number of lakes in Denmark.
6.2.8.1 Eutrophic Lakes in Denmark
Several eutrophic lakes in Denmark (namely,
Fureso, Fermumso, Bastrupso) had been studied
by many workers (Boye Peterson 1917 ;
Chirstensen and Andersen 1958 ; etc.) particularly with emphasis on aquatic macrophytes
(AM). All these lakes tended to have a girdle of
Phragmites australis. Lakeward of these, there
may occur Typha angustifolia , etc. Sharp zonation had been found to be best developed where
the depth increases moderately fast. Potamogeton
natans , Nymphaea alba , etc. were generally
found in somewhat deeper water. The submerged
fl ora consisted mainly of Ceratophyllum demersum , etc. The extent of submerged weed beds is
much variable. In most Danish lakes, unvegetated
areas could exist due to low transparency.
Concomitant to above, Lake Esrom in
Denmark had been studied for more than 100
years. The bottom fauna of this lake are, perhaps,
better known than any other benthic community.
The lake has an area of 17.3 km
2 , 60 % of which
has water which is >10 m deep. The lake has a
maximum depth of 22 m. Its bottom is covered
with a thick layer of rich organic mud. The benthic community is very diverse, but the number
of species vary with the type of substrate and
depth of water.
At the edge of the lake, there are about 150
bottom-dwelling species. These increase to
about 300, approx. 100 m from the shore in the
zone of submerged macrophytes. Further out,
under about 7–8 m depth of water, they are usually 50 species, reducing to 20 in the deepest
parts of the lake. Herbivores are represented only
in the shallow water. Then, detritivores dominate
the community.
Benthic animals, particularly the chironomid
larvae, are important food for many benthic fi sh.
The benthic fi sh feed at higher depths. So, they
cannot fi nd their prey by sight. Many of them
feed and taste the food with their barbels.
6.2.9 Alpine Lakes of Europe
The magnifi cent lakes of Europe and Southern
Alps were formed by glaciers which fi lled parallel
valleys running down from a long ridge of mountains. The European Alps also run roughly northeast to southeast with the lakes of Switzerland and
Austria and those of Italy to the south. A lake is
deepened by a moraine dam at the lower end in
many cases. A morainic material forms a barrier
across the centre of a valley in some cases, thus
dividing the lake into two basins. The Swiss
Brienzer Sea and Thuner Sea are good examples
of such lakes with the town of Interlaken in
between. There had been a tendency for the
glaciers to fan out at lower elevations. This is mirrored in the lakes which now occupy the valleys.
Lake Como is, perhaps, the best example, but the
effect may also be seen in Lake Constance.
6 Lakes of the World
time varies from 100 to 400 days. However, the
renewal time may be only two days during May
and June when the infl ow streams are on spate.
Wind mixing generally maintains the water
temperature uniform throughout its depth during
the ice-free period. The water temperature may
go up to maximum of about 15 °C throughout the
life. Water contains very low level of chemicals
(conductivity 10–30 μS cm
−1 ). Both the phytoplankton and zooplankton have clearly defi ned
life cycles. They also show signifi cant seasonal
changes. The benthic fauna is usually dominated
by insects ( c 74 %). It is typical of oligotrophic
lakes. Mayfl ies are generally abundant followed
by caddis fl ies and stone fl ies. Most of them are
detritivores. This indicates the importance of
allochthonous dead organic matters. The mayfl y
Leptophlebia marginata is generally abundant
when the lake is ice covered. However, the chironomids (67 spp.) are the most abundant and
diverse group of insects in the lake.
In this lake, the volume of water in the lake
basin is so small in comparison to the volume of
infl ow during the spate that it can be totally and
rapidly fl ushed out. Hence, the seasonal variation in solar radiation affects the seasonal cycles
in the lake.
6.2.8 Lakes in Denmark
There are quite a large number of lakes in Denmark.
6.2.8.1 Eutrophic Lakes in Denmark
Several eutrophic lakes in Denmark (namely,
Fureso, Fermumso, Bastrupso) had been studied
by many workers (Boye Peterson 1917 ;
Chirstensen and Andersen 1958 ; etc.) particularly with emphasis on aquatic macrophytes
(AM). All these lakes tended to have a girdle of
Phragmites australis. Lakeward of these, there
may occur Typha angustifolia , etc. Sharp zonation had been found to be best developed where
the depth increases moderately fast. Potamogeton
natans , Nymphaea alba , etc. were generally
found in somewhat deeper water. The submerged
fl ora consisted mainly of Ceratophyllum demersum , etc. The extent of submerged weed beds is
much variable. In most Danish lakes, unvegetated
areas could exist due to low transparency.
Concomitant to above, Lake Esrom in
Denmark had been studied for more than 100
years. The bottom fauna of this lake are, perhaps,
better known than any other benthic community.
The lake has an area of 17.3 km
2 , 60 % of which
has water which is >10 m deep. The lake has a
maximum depth of 22 m. Its bottom is covered
with a thick layer of rich organic mud. The benthic community is very diverse, but the number
of species vary with the type of substrate and
depth of water.
At the edge of the lake, there are about 150
bottom-dwelling species. These increase to
about 300, approx. 100 m from the shore in the
zone of submerged macrophytes. Further out,
under about 7–8 m depth of water, they are usually 50 species, reducing to 20 in the deepest
parts of the lake. Herbivores are represented only
in the shallow water. Then, detritivores dominate
the community.
Benthic animals, particularly the chironomid
larvae, are important food for many benthic fi sh.
The benthic fi sh feed at higher depths. So, they
cannot fi nd their prey by sight. Many of them
feed and taste the food with their barbels.
6.2.9 Alpine Lakes of Europe
The magnifi cent lakes of Europe and Southern
Alps were formed by glaciers which fi lled parallel
valleys running down from a long ridge of mountains. The European Alps also run roughly northeast to southeast with the lakes of Switzerland and
Austria and those of Italy to the south. A lake is
deepened by a moraine dam at the lower end in
many cases. A morainic material forms a barrier
across the centre of a valley in some cases, thus
dividing the lake into two basins. The Swiss
Brienzer Sea and Thuner Sea are good examples
of such lakes with the town of Interlaken in
between. There had been a tendency for the
glaciers to fan out at lower elevations. This is mirrored in the lakes which now occupy the valleys.
Lake Como is, perhaps, the best example, but the
effect may also be seen in Lake Constance.
6 Lakes of the World