ESTONIAN ENVIRONMENTAL REVIEW 2009

the density of reed shoots has increased in the last two
years. The concentration of new reed areas is also great
on the north coast of Lake Peipsi, which was previously
free of reeds. The area under reeds in the Estonian part of
the lake has increased by several hundred hectares over
the last few decades due to rising trophic level.
The abundance and biomass of organisms living on the
bottom of water bodies (macrozoobenthos) has remained
high in Lake Peipsi and the biotic community continues
to be biologically diverse. On the area close to shore with
a harder bottom, the amphipod Gmelinoides fasciatus is
profilerating. Introduced in the 1970s from Lake Baikal for
the purpose of increasing the fish food supply; the species
has nearly completely driven out the local amphipods
Gammarus lacustris and Pallasea quadrispinosa.
The pollution of water near the shoreline of Lake Peipsi
has increased. This can be seen in the sudden rise in the
vegetation on the surface of the water.
Due to the proliferation of algae, fish spawning grounds
have become muddy and as a result, fish reproduction
conditions have worsened. For fish that inhabit cold,
clean water such as vendace, whitefish, burbot and
smelt, the Peipsi is no longer a suitable habitat. A shift
has taken place toward more dominant pike-perch and
bream populations as they prefer eutrophic, warm and
turbid water.
Due to intense fishing, the number of large predator
fish is too low and thus the fish populations are not held
in balance. Pike-perch is caught too early. Young fish
predominate in the pike-perch catch (1–2 generations)
and they cannot yet function as predators. The Peipsi fish
populations are out of balance. The abundance of pikeperch
is not in conformity with stocks of its primary prey
– plankton-eating smelt and vendace. The smelt stock
is almost depleted in the lake. The loss of equilibrium
in the fisheries affects the entire ecosystem negatively
through food chains, changing the proportions of other
habitat groups, above all the relationship of phyto- to
zooplankton. The rotan (Perccottus glenii), an undesirable
alien species that has appeared in the Narva river
and reservoir, has not yet reached the Peipsi.
Lake Võrtsjärv
There are no certain data regarding changes in the
balance of species in the Lake Võrtsjärv; but a number
of species found previously in 1995–2001 were not seen.
Of the quantitative changes the most important were
the continuing development of a belt of common reed
(Phragmites australis) encircling the entire lake, and the
earlier supplantation (1965–1966) of massive underwater
plants in the southern part of the lake such as Chara
contraria and Canadian pondweed (Elodea canadensis)
with whorled water-milfoil (Myriophyllum spicatum)
communities, which is now seen over a broader area
and in other parts of the lake. The common hornwort
(Ceratophyllum demersum) has also become widespread
in the southern part of the lake, another indicator of
eutrophication.
With regard to the biomass of zoobenthos (not including
family Unionidae) only one species has absolute
dominance – the buzzer midge (Chironomus plumosus).
The introduced zebra mussel (Dreissena polymorpha)
has grown rare in Võrtsjärv. The fish are under strong
fishery pressure and there have not been major changes
in recent years.
For more about the condition of aquatic biotic communities,
see chapter Water, sections 6.4.3–6.4.5.
8.3.3. Reasons for reduced
biological diversity
Large lakes
The ecosystem in water bodies are impacted by malfunctions
in waste water treatment facilities in settlements,
contamination with nutrients, toxic chemicals,
petroleum products as well as by overfishing and introduction
of alien species. The movement of fish in rivers is
impacted by weirs and dams and other obstacles erected
on rivers.
The ecosystem is also impacted by natural fluctuations
in the water level and temperature, extreme weather
conditions and ice conditions, etc. The state of many
lakes (including Peipsi and Võrtsjärv) becomes especially
bad when the high temperatures coincide with low water
level. When water level is low, the wave action impacts
the bottom of the lake as well. This is hazardous for living
creatures as the waves stir up nutrients from the sediments
on the bottom, which promotes water blooms. Winter
brings with it the risk of anoxia and fish kills happen if
water levels are low.
In the case of Võrtsjärv, it can be said that the impact
of nutrients from agriculture and waste water from settlements
has been less in the 2000s than in years past
and exerts less of an impact on the lake than does the
natural fluctuation of lake water levels. Still, the increase
in bacteria and ciliates shows that eutrophication is continuing
in Võrtsjärv. Due to the abundance of nutrients,
both phyto- and zooplankton and biological diversity
of zoobenthos is decreasing, and the areas inhabited by
shoreline and aquatic plants is increasing.
The Peipsi ecosystem’s equilibrium is jeopardized above
all by excess phosphorus content, which makes algae
proliferate. This leads to nighttime oxygen deficiency
as the oxygen is used by decomposition processes in the
algae and fish may asphyxiate. Large predator fish and
especially pike-perch should not be caught too early
in their life cycle. To protect fish stocks, the fishing for
undersize fish should be restricted and the size of the
eyelets in nets should be increased.
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