87
6.5.6 Lake Myvatn
It is a good example of a shallow lake in Iceland.
This lake (situated 65° 30′ N; 17° W; altitude
277 m; lake area 37 km
2
; lake volume 76.5 million m
3
; max. depth 4.2 m; mean depth 2.0 m;
infl ow groundwater; outfl ow River Laxa) is
famous for ‘ the number and diversity of its waterfowl populations, particularly nesting ducks’. The
lake could be broadly divided into two main
basins with a very irregular shoreline and many
islands. It is surrounded by boggy ground and a
number of small ponds and lakelets, which provide ideal breeding grounds for a variety of birds.
The main lake, however, supplies rich food
resources during the breeding season. It is situated in an active volcanic area. Many hot and
cold springs increase nutrient inputs into the lake.
The lake has a short retention time. So, nutrients
are constantly replaced. The lake generally
remains covered with ice for about 190 days during each winter. However, it receives a lot of
bright sunshine during the summer. These factors
lead to high level of primary production by both
planktonic algae and the blanket weed
( Cladophora ), which covers large areas of the
bottom. This provides substrate for small invertebrates and thereby takes over the role of macrophytes which are uncommon in Myvatn.
The bio-community is not very diverse as in
many other high altitude lakes. The benthic plants
and animals develop luxuriant populations than
the planktonic species which tend to be swept
away. The benthic animals include the small
bivalves like the Pisidium and many species of
worms. The benthic plants include the
Cladophora . The benthic midge larvae show a
spectacular display of emergence from the water
like a billow of smoke. The zooplankton is generally dominated by rotifers and benthic cladocerans. They, together with the benthic insect larvae,
provide food for the birds and fi shes, like the
trouts, chars and sticklebacks. In addition to the
ducks, many waders breed and feed in the wetland areas surrounding the lake.
The birds of this lake have been traditionally
protected since centuries. Their predators like
the arctic foxes, ravens and falcons have been
traditionally killed. This is because the farms
around the lake had been using the eggs of the
ducks as a supplementary source of income since
around 1712 AD.
6.6
Saline and Soda Lakes
Freshwater consists of only hydrogen combined
with oxygen. But even the purest highland stream
contains more than this. Many chemicals dissolve in rainwater as it comes down through the
atmosphere. More are picked up as the water
washes over plants and rocks and percolates
through the ground before running into rivers and
lakes. The amount of chemicals dissolved in the
water of a lake, thus, depends on the nature of the
rocks over which the infl ows pass.
In most FW lakes, water, with its dissolved
chemicals, leaves the lake through the outfl owing
rivers and the total quantity of outgoing water
and chemicals more or less balances what comes
in. When a lake has no outlet, the lake basin is the
fi nal destination of old water and chemicals from
its catchment area. It is called a closed or
endorheic basin. If it receives water very rich in
chemicals and also lies in very hot part of the
world, a lot of water will be lost through evaporation, but the chemicals will be trapped within the
lake, forming a very strong salt or soda solution.
The best-known examples of this phenomenon are, perhaps, the Dead Sea and the Great Salt
Lake in the USA. But the largest is the Caspian
Sea, into which the Volga River fl ows. The water
of the Volga is fresh, but the Caspian Sea has no
outlet and the salts accumulate in it. In all the
examples, the predominant chemical is the chloride (or, may be fl uoride) ion, either in the form
of common salt (NaCl) or combined with another
common element such as magnesium. A few
lakes may contain carbonates instead of chlorides. The most spectacular series of these lakes
is found in the fl ow of the Rift Valley in East
Africa. Here, specialised microscopic plants and
animals form the basis of food chains supporting
some of the most diverse and abundant communities of birds in the world. The concentration of
salts in these lakes, weather saline or soda, changes
6.6 Saline and Soda Lakes
6.5.6 Lake Myvatn
It is a good example of a shallow lake in Iceland.
This lake (situated 65° 30′ N; 17° W; altitude
277 m; lake area 37 km
2
; lake volume 76.5 million m
3
; max. depth 4.2 m; mean depth 2.0 m;
infl ow groundwater; outfl ow River Laxa) is
famous for ‘ the number and diversity of its waterfowl populations, particularly nesting ducks’. The
lake could be broadly divided into two main
basins with a very irregular shoreline and many
islands. It is surrounded by boggy ground and a
number of small ponds and lakelets, which provide ideal breeding grounds for a variety of birds.
The main lake, however, supplies rich food
resources during the breeding season. It is situated in an active volcanic area. Many hot and
cold springs increase nutrient inputs into the lake.
The lake has a short retention time. So, nutrients
are constantly replaced. The lake generally
remains covered with ice for about 190 days during each winter. However, it receives a lot of
bright sunshine during the summer. These factors
lead to high level of primary production by both
planktonic algae and the blanket weed
( Cladophora ), which covers large areas of the
bottom. This provides substrate for small invertebrates and thereby takes over the role of macrophytes which are uncommon in Myvatn.
The bio-community is not very diverse as in
many other high altitude lakes. The benthic plants
and animals develop luxuriant populations than
the planktonic species which tend to be swept
away. The benthic animals include the small
bivalves like the Pisidium and many species of
worms. The benthic plants include the
Cladophora . The benthic midge larvae show a
spectacular display of emergence from the water
like a billow of smoke. The zooplankton is generally dominated by rotifers and benthic cladocerans. They, together with the benthic insect larvae,
provide food for the birds and fi shes, like the
trouts, chars and sticklebacks. In addition to the
ducks, many waders breed and feed in the wetland areas surrounding the lake.
The birds of this lake have been traditionally
protected since centuries. Their predators like
the arctic foxes, ravens and falcons have been
traditionally killed. This is because the farms
around the lake had been using the eggs of the
ducks as a supplementary source of income since
around 1712 AD.
6.6
Saline and Soda Lakes
Freshwater consists of only hydrogen combined
with oxygen. But even the purest highland stream
contains more than this. Many chemicals dissolve in rainwater as it comes down through the
atmosphere. More are picked up as the water
washes over plants and rocks and percolates
through the ground before running into rivers and
lakes. The amount of chemicals dissolved in the
water of a lake, thus, depends on the nature of the
rocks over which the infl ows pass.
In most FW lakes, water, with its dissolved
chemicals, leaves the lake through the outfl owing
rivers and the total quantity of outgoing water
and chemicals more or less balances what comes
in. When a lake has no outlet, the lake basin is the
fi nal destination of old water and chemicals from
its catchment area. It is called a closed or
endorheic basin. If it receives water very rich in
chemicals and also lies in very hot part of the
world, a lot of water will be lost through evaporation, but the chemicals will be trapped within the
lake, forming a very strong salt or soda solution.
The best-known examples of this phenomenon are, perhaps, the Dead Sea and the Great Salt
Lake in the USA. But the largest is the Caspian
Sea, into which the Volga River fl ows. The water
of the Volga is fresh, but the Caspian Sea has no
outlet and the salts accumulate in it. In all the
examples, the predominant chemical is the chloride (or, may be fl uoride) ion, either in the form
of common salt (NaCl) or combined with another
common element such as magnesium. A few
lakes may contain carbonates instead of chlorides. The most spectacular series of these lakes
is found in the fl ow of the Rift Valley in East
Africa. Here, specialised microscopic plants and
animals form the basis of food chains supporting
some of the most diverse and abundant communities of birds in the world. The concentration of
salts in these lakes, weather saline or soda, changes
6.6 Saline and Soda Lakes