ESTONIAN ENVIRONMENTAL REVIEW 2009

8.3. Inland water bodies
The diversity of Estonia’s inland water body ecosystems
is great in spite of Estonia’s small area. This is due to the
transitional zone between the maritime and continental
climate, the difference in the bedrock in northern and
southern Estonia (limestone and sandstone, respectively),
interchanging uplands and plains, high percentage of
forested land and fairly large number of marshes. The
dense network of flowing water bodies has primarily
been shaped by the great variation in precipitation and
landscapes. In terms of total area of inland water bodies,
Estonia ranks fourth in Europe, thanks to Lake Peipsi
(4.8%). The unique aspect of Estonian inland water bodies
is the broad spectrum of hydrochemical and physical
values. Estonia has water bodies with very light and
dark, and very soft and hard water. The typical Estonian
water bodies are rich in humic substances and high in
mineral content; they are unique in Europe. The status of
a number of water bodies or a part thereof has worsened
as a result of human impact. This has led to a decline in
the biological diversity of water bodies.
8.3.1. Habitats
Freshwater habitats are found in rivers, streams,
springs and lakes. Lakes fall into various types of categories
distinguished on the basis of trophic level, shape
(morphology), hardness, water thermal stratification,
colour and connection to the sea and other criteria. This
is the basis for distinguishing between habitats such as
dystrophic lakes, naturally eutrophic lakes, oligotrophic
lakes with moderately hard water, and so on. The nature
of lakes is similar from one region to the next, which is
why Estonia has been divided into seven limnological
sectors. Most of the lakes in south-western Estonia and
north-eastern Estonia are oligotrophic and dystrophic;
in upland Estonia, they are eutrophic; on the Pandivere
upland, alkalitrophic; in intermediate Estonia, dystrophic;
in Low Estonia, dyseutrophic; and in western
Estonia, halotrophic. Peipsi and Võrtsjärv are hardwater
eutrophic lakes J .
Loading of organic matter, nutrients and point pollution
sources from catchment tends to blur the differences
between lake types. Currently two types of human origin
are distinguished aside from the natural lake types. The
human-origin types are hypertrophic (with very high
nutrient content) and macrophytic (with extensive
macrophytic algae cover, overgrowing).
Impoverishment of biological diversity can be noted in
the case of all of the types. In stratified lakes, the share of
the oxygen-poor, deeper layers has expanded (often the
lower 2/3 of the water column is without oxygen in summer).
In shallow, dyseutrophic lakes (the predominant
lake type in Estonia) the loading of humus substances
and nutrients has led to the introduction of so-called
pond species, that have adapted to life in water with high
content of organic matter. Production of microalgae has
dropped. Fish diversity has also declined. Coverage of
macrophytic vegetation has increased.
The two main habitat types in Estonia’s running
water bodies are fast-flowing and slow-flowing rivers
or river segments. Fast-flowing rivers (river segments)
mainly have low water and a stony or gravely bed. Slowflowing
segments have sandy, muddy or clay bottoms.
The species in each are different. The greatest biological
diversity is seen in medium-sized river segments where
the rapid and slow-flowing parts alternate. Spring-fed
river segments are light-coloured and have minor seasonal
variations in water level. Summer water temperatures in
those segments are low and the water’s oxygen content
is high. They are suitable as habitats for cold-water and
oxygen-demanding species. River segments containing
a small percentage of groundwater are characterized by
darker water that is richer in humic substances, greater
variations in water level and a higher percentage of
warm-water species.
For more about assessment of water bodies and their
condition, see Chapter 6, Water, sections 6.4.2–6.4.5.
8.3.2. Species
Lake Peipsi
The structure of the species in phytoplankton in Lake
Peipsi has changed. The percentage of blue-green algae
has increased significantly and diatom biomass has
decreased. Even though the summer of 2008 was cool
and rainy, intensive blue-green algae blooms were seen
in all parts of the lakes in August. In the southern parts
of the lake, species of one of the most toxic blue-green
algal genera, Microcystis, have been predominant in
recent years. The concentration of algal toxins in water
has repeatedly exceeded the allowable limit for bathing
water. Water blooms and algal toxins pose risks to
the biotic communities of Lake Peipsi, the balance of
the entire ecosystem, and human health as well. Algal
toxins are apparently also a reason that the amount of
zooplankton has declined significantly since 2001. The
decline in zooplankton means that the food resources for
fry and plankton-feeding fish have dropped. In 2006, fat
duckweed (Lemna gibba) was found in Peipsi, a plant that
indicates the presence of domestic waste water.
Reed proliferation is occurring in Lake Peipsi, as a result
of which a number of more sensitive species of plants
are in retreat. These species include many rare species:
water awlwort, narrowleaf water-plantain and slenderleaved
pondweed – these should be accorded protection.
Macrophytes in Lake Peipsi are showing signs of impoverishment,
evidenced by a drop in the occurrence of 31
taxons. A northward shift in reed growth is occurring and
136
J
Järvede nimestik. Looduslikud ja tehisjärved. (List of Estonian lakes. Natural and manmade lakes.) (2006) / Compiled by R. Tamre. Tallinn : EEIC