134
Since 1964, explosive growth of BGA, e.g .
Anabaena lemmermannii , became a regular feature of the lake, causing blooms ( Legovitch
1979 ). In the 1970s the phytoplankton composition changed, with new species occurring and
some of the previous species disappearing. Most
of the new algal species belonged to the
Protococcales, and they included Ankistrodesmus
angustus and Hyaloraphidium rectum . The genus
Euglena established itself in the pelagic zone.
The diatoms Melosira italica and Peridinium sp.
had appeared. Closterium had also appeared
(Kazaryan 1979 ). Before the water level decline
in 1938, the lake bottom fl ora down to 14 m depth
was dominated by stonewort ( Chara ) and moss.
By 1976, Chara and previously common
Potamogeton natans , P. pusillus , P. densus and
Zannichellia palustris , had almost disappeared
(Gambaryan 1979 ).
On the other hand, the zooplankton composition remained stable for considerable time when
the water level started to drop in 1938. There
were ten species of rotifers, cladocerans and
copepods. The highest biomass was that of copepods, which represented about 60 % of the total.
Gradually, the species composition started to
change, which was accompanied by changes in
the biomass of the individual species. During
1974–1980, smaller zooplankton became dominant, with rotifers representing up to 80 % of the
biomass.
Concomitantly, the zoobenthos of Lake Sevan
was dominated by chironomid larvae
( Chironomus plumosus , C. markosjani , etc.),
Oligochaeta ( Potamothrix hammoniensis , Tubifex
tubifex ), leeches ( Erpobdella octoculata ,
Glossiphonia complanata , etc.), bivalves
( Pisidium nitidum and P. casertanum ), snails
( Valvata piscinalis , Lymnaea stagnalis , etc.),
mayfl ies (represented in the lake by only one species Caenis modesta ) and gammarids ( Gammarus
lacustris ) (Arnoldi 1929 ; Ostrovsky 1983 ).
Fish Fauna of Lake Sevan
Lake Sevan has an endemic fi sh fauna represented by only three species: Sevan trout ( Salmo
ischchan
Kessler),
Sevan
khramulya
( Varicorhinus capoeta sevangi Filippi) and Sevan
barbel ( Barbus lacerta goktchaicus Kessler).
However, more fi sh species were introduced in
the past in order to increase fi sh catches, namely,
Ladoga whitefi sh ( Coregonus lavaretus ladoga )
from Lake Ladoga and Lake Chud whitefi sh
( Coregonus lavaretus maraenoides ). Both species, introduced in 1924, not only established
themselves in Sevan but also crossbred producing
a new hybrid Coregonus lavaretus sevanicus
Dadikjan (Dadikyan 1986 ).
Gibel carp ( Carassius auratus gibelio Bloch)
was also inadvertently introduced in the early
1980s and established strong stocks in areas rich
in aquatic macrophytes.
While Sevan trout and Sevan khramulya were
common in the lake before the water level started
to decline, their stocks later diminished drastically. In 1984, both the species were listed in the
Red Book of the USSR ( 1984 ). Stocks of Sevan
barbel also declined and the species is now listed
in the Red Book of Armenia. On the other hand,
Khramulya stocks remained stable and coregonid
stocks have strongly increased.
Changes in Fish Stocks During
the Fall in Lake Water Level
Lake Sevan and its fi sh fauna have been facing
problems since several dams blocked the Razdan
River, the only outfl ow from the lake. The impact
of the drawdown and water-level fl uctuations
on water quality and fi sh stocks has been
considerable.
Factors Behind the Reduction
in Fish Stocks
Until the beginning of the 1960s, the lake was oligotrophic. This was followed by gradual eutrophication as a result of man-induced impacts, such as
irreversible draining of lake water for hydropower, and increasing inputs of pollutants from
the growing number of settlements around the
lake. The lake was getting shallower and water
temperature range greater with the lake surface
now being regularly covered in ice during the
winter. There was increased water turbidity, a
decrease in dissolved oxygen concentrations and
occasional deoxygenation of bottom water layers
leading to decline in aquatic macrophytes, while
6 Lakes of the World
Since 1964, explosive growth of BGA, e.g .
Anabaena lemmermannii , became a regular feature of the lake, causing blooms ( Legovitch
1979 ). In the 1970s the phytoplankton composition changed, with new species occurring and
some of the previous species disappearing. Most
of the new algal species belonged to the
Protococcales, and they included Ankistrodesmus
angustus and Hyaloraphidium rectum . The genus
Euglena established itself in the pelagic zone.
The diatoms Melosira italica and Peridinium sp.
had appeared. Closterium had also appeared
(Kazaryan 1979 ). Before the water level decline
in 1938, the lake bottom fl ora down to 14 m depth
was dominated by stonewort ( Chara ) and moss.
By 1976, Chara and previously common
Potamogeton natans , P. pusillus , P. densus and
Zannichellia palustris , had almost disappeared
(Gambaryan 1979 ).
On the other hand, the zooplankton composition remained stable for considerable time when
the water level started to drop in 1938. There
were ten species of rotifers, cladocerans and
copepods. The highest biomass was that of copepods, which represented about 60 % of the total.
Gradually, the species composition started to
change, which was accompanied by changes in
the biomass of the individual species. During
1974–1980, smaller zooplankton became dominant, with rotifers representing up to 80 % of the
biomass.
Concomitantly, the zoobenthos of Lake Sevan
was dominated by chironomid larvae
( Chironomus plumosus , C. markosjani , etc.),
Oligochaeta ( Potamothrix hammoniensis , Tubifex
tubifex ), leeches ( Erpobdella octoculata ,
Glossiphonia complanata , etc.), bivalves
( Pisidium nitidum and P. casertanum ), snails
( Valvata piscinalis , Lymnaea stagnalis , etc.),
mayfl ies (represented in the lake by only one species Caenis modesta ) and gammarids ( Gammarus
lacustris ) (Arnoldi 1929 ; Ostrovsky 1983 ).
Fish Fauna of Lake Sevan
Lake Sevan has an endemic fi sh fauna represented by only three species: Sevan trout ( Salmo
ischchan
Kessler),
Sevan
khramulya
( Varicorhinus capoeta sevangi Filippi) and Sevan
barbel ( Barbus lacerta goktchaicus Kessler).
However, more fi sh species were introduced in
the past in order to increase fi sh catches, namely,
Ladoga whitefi sh ( Coregonus lavaretus ladoga )
from Lake Ladoga and Lake Chud whitefi sh
( Coregonus lavaretus maraenoides ). Both species, introduced in 1924, not only established
themselves in Sevan but also crossbred producing
a new hybrid Coregonus lavaretus sevanicus
Dadikjan (Dadikyan 1986 ).
Gibel carp ( Carassius auratus gibelio Bloch)
was also inadvertently introduced in the early
1980s and established strong stocks in areas rich
in aquatic macrophytes.
While Sevan trout and Sevan khramulya were
common in the lake before the water level started
to decline, their stocks later diminished drastically. In 1984, both the species were listed in the
Red Book of the USSR ( 1984 ). Stocks of Sevan
barbel also declined and the species is now listed
in the Red Book of Armenia. On the other hand,
Khramulya stocks remained stable and coregonid
stocks have strongly increased.
Changes in Fish Stocks During
the Fall in Lake Water Level
Lake Sevan and its fi sh fauna have been facing
problems since several dams blocked the Razdan
River, the only outfl ow from the lake. The impact
of the drawdown and water-level fl uctuations
on water quality and fi sh stocks has been
considerable.
Factors Behind the Reduction
in Fish Stocks
Until the beginning of the 1960s, the lake was oligotrophic. This was followed by gradual eutrophication as a result of man-induced impacts, such as
irreversible draining of lake water for hydropower, and increasing inputs of pollutants from
the growing number of settlements around the
lake. The lake was getting shallower and water
temperature range greater with the lake surface
now being regularly covered in ice during the
winter. There was increased water turbidity, a
decrease in dissolved oxygen concentrations and
occasional deoxygenation of bottom water layers
leading to decline in aquatic macrophytes, while
6 Lakes of the World