The Baltic Sea Environment
Session 2
Life in the Baltic Sea
CONTENTS
1. Introduction
1.1 Marine ecosystems
1.2 The special conditions of the Baltic
1.3 The natural history of the Baltic
2. Elements of ecology
2.1 What is ecology?
2.2 Main groups of living organisms in marine ecosystems
2.3 The food web
2.4 Energy turnover
3. Coastal ecosystems in the Baltic
3.1 The coast as a high production area
3.2 The green algal belt
3.3 The Fucus belt
3.4 The blue mussel belt
3.5 Shallow soft bottom ecosystems - Important for fish production
3.6 Flads - shallow water bodies slowly turning into land
4. The open sea, the deep softbottoms and Kattegat
4.1 The open water system - the pelagic community
4.2 Deeper softbottom ecosystems
4.3 The Kattegat coast - a comparison
5. Fish, birds and mammals
5.1 Fish and fishing
5.2 Mammals - threatened species
5.3 Birds
6. A life in stress - natural and man made
6.1 Life in winter
6.2 Adaptation to changing salinity
6.3 Pollution sensitivity of the Baltic
7. Summary
8. Glossary
9. Literature references
1
1. INTRODUCTION
1.1 Marine ecosystems
The main part of the surface of earth, 70 %, is covered by water, and most of this is salty, with a stable salt content of
about 3.5 % (35 %o, permille). These environments are called marine, as opposed to the fresh water systems called
limnic systems, mainly lakes and rivers, with essentially no salt.
Life once originated in water, and water is still the element essential to all life on earth. Many marine animals still
depend heavily on the aquatic environment for reproduction and distribution of their offspring and have free living
larval stages. Organisms, both animals and plants, are able to live in the water without the specialized supportive
structures needed by organisms that inhabit the land. At the same time, many mobile marine organisms are streamlined
which enables them to move efficiently through the water.
Water is one of the most peculiar natural compounds. Many substances are soluble in water, others can be suspended in
water like oil drops and solid particles. In this way water and aquatic environments function as transporting media for
nutrients, polluting substances, dead organic matter and living organisms. The organisms living in water are more or
less permeable to dissolved substances both nutrients, essential elements and polluting substances.
Substances not soluble in water. e.g. oil and fat and many other organic compounds, stay on the surface of the water
and sometimes form a thin cover. In fact practically all lake and sea surfaces has at least some such substances that
form an extremely thin, sometimes monomolecular, covering, a film, of water-nonsoluble substances. Many of these,
such as pollutants from oilspills, are toxic to life.
In northern latitudes water freezes during winter. Luckily the solid form of water, ice, is lighter that the liquid and
floats. Thereby the rest of the water becomes insulated and the freezing slows down. An even more unusual property of
the water further helps It to stay liquid during winter. Water has a density maximum at plus 4 °C. Colder heavier water
con-tlnuously sinks to the bottom while the warmer water goes to the surface and is cold off and thus the entire
wolume has to be chilled off before freezing can start.
Of the many special properties that marine animales have is the capacity to extract dissolved oxygen from the water.
Oxygen can be dissolved in a concentration of about 10 mg oxygen per liter of water. Animals are thus able to "breath"
In the water, but of course if the oxygen content decreases they may suffocate and die. Oxygen is normally not limiting
in the upper part of the water column but may be so at larger depths.
Marine organisms represent all kinds of life forms. Sea grasses and algae, have a role in the sea that corresponds to that
of the green plants on land. Microscopic organisms, e.g. bacteria and plankton, are basic to the systems. A multitude of
invertebrate animals are found. The dominating group among vertebrates are fishes, but the largest now living
vertebrates, i.e. the whales are also found in marine environments.
Marine ecosystems are not uniform. On the contrary they are at least as varied as the land environment with which we
are more familiar, with many different niches for animal and plant species to occupy (Fig. 1). In open water, the so
called pelagic systems special environmental conditions prevails and other living communities are found than at
bottoms, called benthic systems. Soft bottoms are again different from rocky or hard bottoms. All these communities
further have marked changes with increasing depth. These physical differences combined with a wide range of
salinities and temperature regimes occuring along the coasts, in shallow and in open water as well as on deep
softbottoms present a varied environment for living organisms to occupy.
The marine environment set many restrictions to man's exploitation passed for example in the form of overfishing and
pollution.
1.2 The special conditions of the Baltic
The Baltic Sea is one of the largest brackish water areas in the world. It Is also a relatively young sea, created after the
Ice Age, with the present low salinity conditions largely unaltered for only some 3,000 years. In evolutionary terms this
is a short time.
As compared to fully marine environments the Baltic Sea with Its brackish water has a very poor both flora and fauna
(Fig. 2). The number of marine species found start to decrease dramatically as one goes through the Danish straits into
the Baltic Proper and then continues to decrease as we continue up to the Finnish Bay, through the Archipelago Sea
Into the Bothnian Sea and finally the Bothnian Bay. Of the 1 500 macroscopic animals off the Norwegian coast, only
some 70 species are still to be found in the central Baltic. Of the approximately 150 macroscopic algae on the
Norwegian coast only 24 remain at .the Finnish coast (Fig. 3).
Since the number of species in the Baltic is low compared to fully marine systems, the number of "stand-ins" capable
2
of filling the niche of an excluded species is small or even zero. The risk that whole life functions may be excluded
because of environmental disturbances is therefore great. The continuously decreasing salt concentrations, the salinity
gradient, is the main reason for this poverty of life forms. Only very few of the marine organisms, adapted to a salt
content of 35 %o, are able to regulate the water content in their cells when the osmotic pressure is changing due to the
diminishing salt.
In the Baltic Proper the water is brackish with a salinity around 5—6 %o (Fig.3). In the Bothnian Sea the salinity is
about 4 %o and in the Bothnian Bay there is close to freshwater conditions. The variation in salinity is large in
Kattegat, between 15-30 %o. In the Baltic itself this variation is much smaller and in the Bothnian Sea the salinity Is
almost stable.
Temperature also influences life in the Baltic. Thus the salinity gradient is parallelled by a productive season of only 45 months in the far north of the Bothnian Bay caused by up to six months of ice cover, and a productive season in the
southern sounds and Kattegat of 8-9 months, with sea ice only in cold years. Thus, in some instances temperature
rather than salinity may be the factor limiting any further penetration of marine organisms into the Baltic. Surface
water temperatures show about the same range from -2 °C in the winter period to around +25 °C during warm summer,
but the length of the warm water period and growth season increases from the north to the south. The salinity gradient
and the length of ice free periods in different areas are shown In Fig 3.
Another ablotic factor is the fluctuating water level. On the North Sea coast the daily tidal difference Is about 20 cm
while in the Baltic it is almost negligible. Changes in weather, however, cause large irregular differences in the water
levels in all the coastal areas, depending on the air pressure, of direction and strength of wind and time of the year. The
water level can vary by up to 1 m along the coasts. In spring there are often quite long periods of low water, while in
autumn long periods of high water often prevail.
Kattegat is biologically quite different from the Baltic Sea south of the Danish straits. Kattegat has a rather intense
exchange of water both with the North Sea and the Baltic Proper. At the coast of Kattegat the water is almost estuarine,
especially in the bays of the inner archipelago. Compared to the fully marine water of the Atlantic, ca. 32 %o, the
salinity may thus vary by between 15 to 30 %o during the year. It therefore more or less functions as an estuary to the
North Sea, and many typically marine species - sea anemons, sea cucumbers, sea stars and so on -are found along its
coasts. There is much more of biological diversity in Kattegat. Thus this part of the Baltic region will be treated
separately below.
1.3 The natural history of the Baltic Sea
Since the Inland Ice of the last glaciation started to receed from Scandinavia some 15,000 years ago the Baltic Basin
has changed several times between freshwater and marine conditions. The first large water body had no connections to
the Atlantic and was thus a large icewater lake, the Baltic Ice Lake. At about 10,000 years ago a water breakthrough to
the Atlantic occurred in middle Sweden at the height of Stockholm. The first brackish water sea, the Yoldia Sea,
appeared. But only after another 2,000 years land uprise closed the connection and during a new period of Isolation the
Ancylus Lake was formed.
Again by land uplift the surface of this lake raised to the extent that the Danish islands were flooded. In this manner the
present connection through the Danish straits was established 7,000 years ago. Again marine conditions was
Introduced. Since then the salinity of the Baltic Sea has decreased during several thousand years. The present
conditions of low salinity has prevailed for some 3,000 years.
The life forms of the Baltic are all Immigrants from the neighboring regions. They have arrived to the Baltic during
different periods and then either survived or disappeared due to the changing conditions. Three such different types of
immigrants can be distinguished.
First we have the marine organisms from the estuaries, shallow coastal areas, in the Atlantic and North Sea. These are
adapted to fully marine (salt concentration in excess of 30 %o) or brackish water. To this group belong the algae
bladder wrack, the blue mussel, and the fishes sprat and cod. The second group of immigrants consists of the
freshwater fauna and flora from lakes in continental Europe, e.g. the filamentous green algae Cladophora glomerata.
and the fishes perch and pike.
These first two groups are poorly adapted to cold water conditions which results in a low number of species on deeper
bottoms which is opposite to the situation in marine waters where the species richer communities are normally found in
the deeper areas.
The third group is small and consists of glacial immigrants that arrived from different directions. The arctic marine or
brackish water species that arrived from east and north contain e.g. large isopod species Soduria entomon [with an
older name Mesidothea entomon) and the freshwater shrimp Myste relicta. Another few glacial immigrants apparently
came from west. They are also arctic marine organisms, but since they do not survive in fresh water they must have
come through the westernly route. Examples are the brown algae Sphacelaria arctica and the mussel Macoma
calcarea. The latter group of glacial relicts tend to occur below the summer thermo-cline, e.g. in the deeper colder
3
water in the Baltic.
Recently we have found that some of the Baltic species are close relatives to North American species, such as the
common mussel Mytilus. This will probably lead to changes in our view of the biogeographical history of the Baltic
Sea in the future.
Many of the organisms In the Baltic, coming both from fresh water and marine areas, are not able to grow to the same
size as in their original environment. A typical example is the Baltic herring which is a small form of the Atlantic
herring. Also blue mussels in the Baltic never reaches the same size as those found along the coast of Kattegat.
Similarily, the freshwater snail Theodoxus is smaller in brackish environments compared to speciemens living in
freshwater. The same phenomenon is found among the plants. Almost all red algae are smaller than their relatives In
the North Sea. Still, several species of fauna and flora have been able to adapt to life in brackish water. Thus, many
marine algae living in the Baltic have thicker cellwalls, compared to the same species in more marine environments, an
aid to keep the right salinity level in the cells.
It seems unlikely that the Baltic contains species that are unique to the Baltic, so called endemic species. The Baltic
Sea has probably not existed long enough for new species to have evolved. In fact almost all of the organisms that once
were suspected to be unique to the Baltic have later been shown to be adapted variants of species found elsewhere. The
candidates are all either just subspecies, or belong to little-studied or taxonomically difficult groups. The Baltic flora
and fauna is therefore in sharp contrast to that of the Mediterranean Sea, with 40 % of the plant species considered to
be endemic.
2. ELEMENTS OF ECOLOGY
2.1 What is ecology?
The complex natural systems of which living organisms are parts are termed ecosystems. An ecosystem includes not
only organisms, but also the several non-living, abiotic, components of the environment within which the organisms
are found. Most importantly, it Includes all the interactions that bind the living and nonliving components together Into
a functioning system.
Species in an ecosystem Interact with each other in a multitude of ways. Most basic is the food web where one species
feeds on another. Thus plants are consumed by animals and they are in turn consumed by other animal species. Many
other types of interactions and couplings between species also occur. A species may function as vehicles for spreading
of spores, pollen or eggs or may be used as protection by other species. Relationships of mutual beneficial
interdependence, are called symbiotic while relationships where one species is exploited by the other is called
antagonistic. Evolutionary older ecosystems tend to be more complex and have more interactions and couplings
between its component species.
Different species have different functions in any community. The term most commonly used to describe the total role
of a species in the community is the ecological niche. The niche comprises all the bonds between the population, the
community and the ecosystem in which it is found. These bonds include factors such as the tolerance ranges and
optima for all abiotic environmental variables e.g. salinity and temperature, the sort of organisms that can be utilized
for food by the population, as well as the organisms that feed upon it and the area in which it lives.
The environment in which an ecosystem is found is called a biotope. Softbottoms, hardbottoms and the open sea are
examples of marine biotopes. The part of the biotope which a particular species occupy is called a habitat. Example of
an habitat for small crustacean animals are the "forests" that are formed by the bladder wrack on the bottoms along the
Baltic shores.
Ecology is a key science for those concerned with both natural and man made changes in ecosystems. The intimate
couplings between the environment and the species living there will influence the balance of the ecosystem and the life
conditions for its members. It is only by knowing how the whole system works that we are able to understand how
changes In nonliving, i.e. abiotic, factors will effect a community of organisms.
2.2 Main groups of living organisms in marine ecosystems
In the marine ecosystems a great variety of animal and plant species is represented (Table 1). Below the main
characteristics of a few more Important groups are mentioned.
Table 1. Population density of the main groups of animals found in the sea near Kaliningrad
A. PLANKTON (At the water volume. Specimens per m3.)
Bacteria (open sea, water surface)
Protozoa (overall)
1.5-2.0 x 106
10-40 x 106
4
50-550 x 103
500 x 103
10-50 x 103
Crustacea (Vistula Bay)
Mollusca (Mytilus edulls larvae)
Crustacea (Copepoda, Cladocera)
B. BENTIC FAUNA (At the bottom. Specimens per m2.)
Crustacea, Ostracoda
Oligochaeta, Polychaeta
30-50 x 103
1-3 x 103
1-2 x 103
5-15 x 103
3-9 x 103
1 x 103
0.05-0.3 x 103
Bryozoa
Mollusca Gastropoda
Insecta Diptera
Mollusca (Dreissena)
Mollusca (Mytilus, Macoma, Mya)
The main groups of marine plants are algae. These are cryptogams, which do not have flowers and root systems but are
instead attached to the bottom with a holdfast. Depending on the dominating colour of their pigment the algae may be
divided into green, brown and red algae. The algae, like all plants, are photosynthetic and depend on sunlight for their
growth. A specialized group of algae are the charophytes, having root-like structures anchoring them in the sediment.
Seagrasses together with other higher submerged aquatic plants are phanerogams which have flowers and roots but less
supportive structures in their stem compared to land living phanerogams. (See next page.)
Facts on Plants
ALGAE LIVING ON HARD BOTTOMS:
Fucus vesiculosus;
Brown algae
Fucus serratus;
Brown algae
Cladophora glomerata;
Green algae
Laminaria digitata;
Brown algae
Enteromorpha;
Green algae
Laminaria saccharina;
Brown algae
Ceramium;
Red algae
Furcellaria
Red algae
CHARACEANS ANDPHANEROGAMS LIVING ON SOFT BOTTOMS
Chara;
Characea
Zostera marina;
Phanerogam
Potamogeton;
Phanerogam
Najas
Phanerogam
Facts on Animals
ANIMAL SPECIES LIVING ON SOFT BOTTOMS
Macoma baltica; Cardium;
Mollusk
Mollusk
Mya arenaria;
Mollusk
Amphiura and Ophiura;
Echinoderms
Nereis;
Polichaetes
Saduria entomon
Crustacean
ANIMALS LIVING ON HARD BOTTOMS
Asterias rubens; Balanus improvisus;
Echinoderm
Crustacean
Idothea;
Crustacean
Mytilus edulis;
Mollusk
Gammarus;
Crustacean
Carcinus meanas
Crustacean
Mollusks: Snails, slugs, clams, oysters and squid are all mollusks. Mollusks are soft-bodied animals, but most are protected by a
hard shell made of calcium carbonate. One of the groups within mollusks are bivalves. These have shells divided Into two halves.
The parts of the shell are hold together by a powerful muscles, thereby protecting the soft body. Snails belongs to the largest groups
of mollusks, the gastropods. Most gastropods are protected by single, spiraled shells into which the animal can retreat when
threatened.
Crustaceans: Lobsters, crayfish. crabs and shrimps are all relatively large crustaceans called decapods. They all have an outer
calcerous skelton. The isopods are mostly small marine crustaceans, with many of Its members found in the algal belt. Among the
small crustaceans, the copepods are one of the most numerous of all animal groups. They are Important members of the plankton
communities that form the basis of marine food chains. Barnacles are sessile crustaceans, are common on rocky shores. They feed
by using their feet to paddle food towards the mouth.
Echinoderms: Sea stars, sea urchins and most other echinoderms, from the grek echin, spiny and derm, skin, are bottom living
animals with radial symmetry. Echinoderms are typically found in truely marine systems. Unique to echinoderms is the water
vascular system, a network of hydraulic canals that branch into extentions called tube feet that function in locomotion, feeding and
gas exchange.
Polychaetee: Most potychaetes are marine and belong to the annelids, i.e. worms with segmented bodies. Each segment of a
5
polychaete has a pair of paddlelike or ridgelike structures called parapodia ("almost feet"). These functions both as gills and for
locomotion. The main part of potychaetes live to tubes and catch particles from the water.
Bryozoans: These are small sessile, i.e. attached, animals growing as thin covers on algae or rocks. They form centimeter-sized
colonies of thousands of individuals.
The main groups of marine animals are invertebrates, animals without backbones. Among these we find the largest and
most varied animal groups. Here we will only describe five groups with Important representatives in the Baltic. (See
Facts on animals.) The most important not marine invertebrate group is larvae of Insects found mainly in the almost
freshwater parts of the Baltic. The vertebrates are better know and Include fish, amphibians like frogs, some mammals
and birds.
Bacteria are simple unicellular life forms. As they are devoid of a well organized cell nucleus they are calls
procaryotes. Bacteria are present everywhere in the marine environment as well as in other living organisms.
Aparticular case is the cyanobacteria often called blue-green algae. Some of the blue-green algae are able to fix
atmospheric nitrogen, a property only present in bacteria.
We will also introduce three concepts that do not refer to systematic groups but rather to llfestyles. These are plankton,
necton and benthos.
The plankton comprises all those marine organisms which drift passively or whose power of locomotion is insufficient
to enable them to move contrary to the motion of their inhabited water mass. Some plankton species have organs,
flagella or antennas with which they can move within the water mass and perform vertical migrations during different
time of the day.
Free floating small algae are called phytoplankton. Among these we have the diatoms, which are small silicate
unicellular algae, and dinoflagellates, which are small unicellular algae with flagella enabling them to move In the
water. The free floating animals zooplanktons, count groups like the copepods, rotifers and the unicellular ciliates.
Necton, on the other hand, comprises all swimming consumers that are able to remain in, zones of high productivity In
the face of currents which would otherwise carry them away. Their swimming capacity also permits them to migrate
from one area to another and thereby select favourable habitats In which to spend different periods of their lives. This
group is dominated by fish species. Species like the eel in the Baltic may be capable of exploiting all the different types
of aquatic habitat during its lifetime. Its larval life is spent in the deep sea. In the surface water of the open ocean and in
coastal waters, when adult, it inhabits the freshwater environment, except for the final spawning migration.
Benthic, are those organisms that are attached to bottoms. The attachment site, the substrate, might be stones or rocks
(I.e. hard bottoms), or sand or mud (i.e. soft bottoms). Benthos refers to the entire ecosystems at bottoms.
2.3 The food web
Organisms can use energy in several forms. The majority of plants, i.e. phytoplankton, algae and macrophytes, obtain
their energy directly from sunlight using green chlorophyll and sometimes additional pigments like the brown and red
algae. Plants are the primary producers of the eco-system producing organic matter from water, carbon- dioxide and
nutrients. Animals living on plant material are called grazers or herbivores. They fill the role of consumers in the
ecosystem. Those animals that live by catching other living animals are predators or carnivors. Finally, bacteria, and in
general organisms living on dead organic matter, detritus, are called decomposers. They return the nutrients to mineral
form by decomposing the organic matter.
A marine community Includes all four types in the food web. A simple food-chain, moving from phytoplankton to
zooplankton, to fishes and to seals Is illustrated in Figure 4. In each step a 90 % energy loss occur. The food web
diagrams provide useful models for illustrating the complicated interactions taking place in a community and to show
the basic pathways of energy flow in an ecosystem (Fig. 5).
•
Decomposers have a special role in marine ecosystems. At large depths, below the photosynthetic zone, there is a
continuous supply of sedimenting dead organisms, which here is the basic energy source. Many different techniques
are found among the animals consuming detritus. Suspension feeders, which filter suspended particles from the water
are most characteristic of shallow regions and areas of comparatively rapid water movements. Deposit feeders, which
consume a specific fraction or the whole surrounding sediment occur in the greatest abundance in deeper areas and in
soft sediments. Many species in both categories burrow or are buried in the bottom and thus comprise part of the
infauna, i.e. animals living down in the sediment. It Is noticeable that in the absence of predators, as in parts of the
deep sea for example, several infaunal groups are represented by species dwelling exposed, on the surface of the
sediment.
2.4 Energy turnover
Aquatic ecosystems are different from land or terrestrial ecosystem in their energy turnover and the way biomass is
6
stored. In terrestrial ecosystems a large amount of the energy produced is stored in biomass for longer periods
compared to the aquatic environment. Even if the biomass found in any single period is small in the aquatic
environment the production of the system is high.
The primary production capacity of small phytoplankton is high in the illuminated part of the water mass. In a year,
phytoplankton may produce on average 15-45 times their standing biomass, and therefore biomasses of well below 50
g drywt/m2 of surface may produce more than 1 kg dry wt/m2 a year. In many aquatic habitats this production is the
major store of energy for the whole system and hence a marked contrast with land environments where large amounts
of the plant material is accumulated as wood for many years.
Systems with a high input of nutrients are called eutrophic. Often nutrients are coming from land, but primary
production by blue-green algae is sometimes important and create eutrophic conditions. With intermediate nutrient
load the situation is called mesotrophic, and at low nutrient levels oligotrophic.This is often the case in the open sea.
Shallow bays are naturally productive and few shallow areas are truely ollgotrophie, i.e. with a low nutrient input. As a
result of large nutrient loading to many of the shallow bays along the Baltic coast, these systems have become
eutrophic characterized by very high production. Under extreme conditions with a clear overload of nutrients and
organic material a saprobic system will develop.
In marginal and other shallow regions of the marine habitats a second input of photosynthesized material is produced
by the fringing flora on shores (Fig. 5). The production by phanerogams as well as algae, both micro- and macroscopic,
is often very high, usually much higher per unit area than that of the planktonic algae.
In shallow waters, benthic and nectonic grazers can therefore subsist on a diet of algae; in deeper waters, however,
living plants will be absent and the benthic animals living in or on the soft bottoms of such areas can only be supported
by the rain of dead material from the other marine systems. The bottom ecosystems of regions below the depth of the
illuminated zone is thus mainly the site of the detritus food chain, of heterotrophic bacterial activity and of nutrient
regeneration.
3. THE COASTAL ECOSYSTEMS OF THE
BALTIC
3.1 The coast as a high production area
The coastal ecosystems are the richest systems in the marine environment. The hard bottoms close to the coast are the
most species rich, while the shallow soft bottom communities have less species. The most species poor environments
are the open water systems even though they cover the largest areas.
The shallow coastal zone has a high productivity. With the run-off from land nutrients are available for primary
production, both of phytoplankton, algae and seagrasses, and light is seldom limiting. This give rise to lots of food for
mussels and fishes living in these shallow areas. Shallow coastal areas are also important reproduction and nursery
ground for many fish species.
The distribution of plants on the sea bottom depends on several environmental factors. The vertical distribution is
connected with light conditions .The benthic vegetation reaches down to about 10 m in the Bothnian Bay, to about 1825 m in the Baltic Proper and to about 30 m in the Belt Sea area.
The horizontal, i.e. south-north, distribution of plants is influenced by several environmental factors, salinity being the
most important. The decrease in the salinity influences the morphology and reproduction of marine algae, resulting in
dwarf forms without reproduction.
Another important factor is the bottom type. Since the marine algae obtain their nutrients from the water, it merely
offers a place for attachment. Hard bottoms (rocks and stones) are therefore the most suitable substrates for green,
brown and red algae, while charophytes and phanerogams inhabit sediment bottoms (sand, clay, mud) in sheltered
areas. The shallow bottoms subjected to strong wave actions or streaming water are erosion and transport bottoms.
Owing to wave activity in exposed areas, sandy bottoms are often completlyvoid of vegetation.
The deeper softer bottoms function as accumulation bottoms .These are often loaded with more polluting substances
such as heavy metals. They are also often oxygen depleted due to high amounts of accumulated organic material. Both
factors are a threat to animals living on these environments. On the other hand, in shallow accumulation bottoms,
rooted aquatic plants are able to take up heavy metals and transport them to their shoots, ande thereby making them
available to the food chain.
A typical zonatlon is usually found on rocky shores with green algae dominating close to the surface, brown algae, and
at the greatest depths the red algae are found. At even greater depths where the light limits algal growth different
animal communites are found.
7
3.2 The green algal belt
Hard bottoms closest to the shore are during summer dominated by the filamentous green algae Cladophoraglomerata.
In this zone there is a typical annual cycle. In spring when the ice cover is gone, a heavy bloom of sessile diatoms
covering the bare rocks around the waterllne starts the annual succession. The diatoms are replaced by the filamentous
brown algae Pilayella, followed by the green algae Cladophora glomerata during summer and filamentous red algae
Ceramium and brown algae during autumn and winter. This belt looks different in different parts of the Baltic. In the
Botnian Bay a perennial, i.e. longUved, vegetation of Cladophora aegagrohila and the water moss Fontinalis starts at 2
m depth which is the normal lower limit of influence of the sea ice. In the Bothnian Sea many fresh water species are
still abundant.
The algae are the primary producers in this ecosystem. The consumers, the animals, are dominated by insect larvae,
freshwater snails and small fishes in the northern parts. Further south small crustaceans, e.g. gammarides and isopodes,
favorite food for small fish, dominate during summer. Much of the production from this zone, including the algae and
many of the small animals, get loose and drifts into sheltered bays or are deposited on bottoms further out in the sea.
3.3 The Fucus belt
Below the upper zone of green annual algae, a very conspicuous perennial belt of the bladderwrack, Fucus vesiculosus
is found on hard bottoms. The bladder wrack extends down to ca 8 m depth in the northern regions of its distribution,
while in the Danish straits it is only found close to the surface. Fucus vesiculosus is the only large marine brown algae
to reachAland, the Archipelago Sea and rocky shores of SW-Finland. This northern distribution is due to the lack of
large herbivores and other large algae competing for space. Exceptionally great is its contribution to plant biomass in
the shallow labyrinths of the outer archipelago zone.
During summer Fucus can be heavily overgrown by several species of filamentous green and brown algae. Competing
for ligth and nutrients, these are in fact a. threat to its photosynthesis and growth.
The Fucus community is inhabited by an exceptionally rich fauna including mussels, snails, crustaceans, bryozoans
and even insect larvae. This is the most species rich system in the Baltic, containing some ten species of algae and 30
macroscopic animal species (Fig. 6). Many fish species, among them herring, pass their larval stages in the Fucus belt
feeding on the rich variety of small animals. Being the only large longlived, beltforming algae on the Baltic hard
bottoms Fucus is a key species in these ecosystems, and in fact in the entire Baltic.
After stormy weather, detached drift seaweed accumulates on the shore and in this way Fucus can be used by the
farmers to improve and fertilize the poor archipelago soils. Another part of the loosened Fucus biomass is deposited on
bottoms further out. It is a major contribution of nutrients in this area.
The distribution of algal biomass, decreasing fron south to north is shown in Figure 7.
3.4 The blue mussel belt
From a depth where scarcity of light does not allow further algal growth on hard bottoms the blue mussel, Mytilus
edulis, dominates entirely. This blue mussel belt extends down to depths where the bottoms get too muddy to allow the
mussel to attatch. The mussel belt starts normally at a few meters of depth and often extend to 30 meters. A few red
algal species can still grow in the bluish light in the upper part of this zone, but otherwise only some invertebrate
species are found.
The blue mussel feeds on microscopic particles of organic matter falling to the bottoms, by filtering the water. Their
filtering capacity is very high; the total amount of mussels within a 160 km2 in the Ask6 area, northern Baltic Proper,
are able to filter the whole overlying water-mass (mean depth about 20 m) once in two and a half months. Partlculate
matter "is thus consumed In the metabolic processes and inorganic salts excreted.
The predators on the mussels are in the upper zone eider ducks, able to dive to about 10 meters of depth. On larger
depths it serves as the main food for plaice. The main predators on blue mussels in Kattegat, the brittle star Asterios
rubens and shore crab Corcinus meanas, have not been able to adapt to the low salinity in the Baltic Proper, which
explains why the blue mussel is so successful as species in the Baltic Proper.
The animal biomass in the Baltic Proper is totally dominated by the blue mussel Mytilus edulis. It accounts for more
than 90 percent of the total animal biomass The total blue mussel population in the Baltic Proper including the Aland
Sea down to 25 m depth is about 8 million tons of dry weight Including shells.
The blue mussel has an important role in the recycling of nutrients, as revealed by community metabolism studies with
plastic enclosures in the Baltic Proper. The nitrogen and phosphorus extreted by the mussels fill the entire nutrient
demands of the large algae in the area where they grow. On top of that the Mytilus population also furnish 5 percent of
the nitrogen and 20 percent of the phosphorus needed to maintain the total phytoplankton biomass in the same area
(Fig. 8). In total the blue mussels in the entire Baltic Sea excrete to the trophic zone roughly 250,000 tons of inorganic
8
nitrogen, 80,000 tons of inorganic phosphorus and 100,000 tons of organic nitrogen per year. These figures are easily
as large as the total landbased inputs to the Baltic of these substances. However, it should be noted that the mussel
contribution is a recirculation while the land contribution is a new addition of nutrients to the ecosystems.
3.5 Shallow soft bottom ecosystems - important for fish production
The largest part of the sea floor consists of muddy and sandy sediments. In the shallow/protected bays on the coast the
freshwater influence is obvious. The organic sediment is often covered with reed belts, Phragmites communis. Scirpus
species and submerged vegetation consisting of flowering plants Myrfophyllum and Potamogeton species, mixed with
the marine brown algae Chorda Jilum and single Fucus plants growing on stones. The marine eelgrass Zostera marina
may cover sandy bottoms rich in organic matter especially off the southern coasts of the Baltic proper but never
reaches the same dominance and productivity as in fully marine waters.
The fauna of the shallow bays has a corresponding freshwater character where insect larvae of dragon flies and
mayflies are found together with water bugs and freshwater fish like bream, rudd, tench, pike and perch. The same
animal species dominate the shallow softbottoms from the Sound to the southern Bothnian Sea, namely stickleback, the
common goby and the sand goby of the mobile fauna and cockles [Cardtum spp}. mud snails (Hydrobidae] rag worms
(Nereis) and crustaceans (Corophium uolutator) of the macroscopic infauna (animals residing in the bottom).
The production of this area is high, as the system is running on both solar energy and particulate organic matter
transported from land. There is a production outburst in spring when the shallow water is rapidly warmed up and the
microflora blooms, stimulating secondary production. At this time many fish spawn here since potential larval food is
abundant. In fact much of the production goes to larval food for fish. Due to its high production the shallow
softbottoms function as nutrient traps and much is stored as plant biomass.
Except for sport-fishing of pike and perch, man utilizes little of the production of the softbottom coastal areas. Many of
them are used intensively as small harbours or marinas and here the soft-bottom ecosystems have been destroyed.
3.6 Flads - shallow water bodies slowly turning into land
The archipelagos in the northern Baltic are shallow areas where the land gradually rises up from the sea as thousands of
islands to form a hugh maze with a variety of sounds and bays. The land uplift can in some cases lead to the isolation
of a water bodies with soft bottoms. These gradually become shallow enough to allow growth of aquatic plants, and
lose contact with the sea. This Isolation shows a distinctive topographical and botanical succession where four different
phases can be seen.
A juvenile stage is formed when the first aquatic plants colonize the most sheltered parts of a water body surrounded
by land and shallow sills. These plants contribute to the land uplift each year, by building up nutrient rich sediments.
This process leads to the formation of a so called flad, a shallow basin completely surrounded by land, leaving one or
a few narrow openings to the sea.The number of species is highest in the beginning of this stage. The following gloflad stage has connections to the sea only when the sea level is high. Competition and unfavourable environmental
conditions now has reduced the number of species to a couple, mainly the phanerogam Ncy'os marina and the
characean Chara tomentosa. The final exclusion will happen in the glo stage, where the decreasing salinity will lead to
a new succession of freshwater plants, unless the glo is overgrown by marsh or land vegetation.
Environmental factors act in parallel with competition and affect the succession of plant species and biomasses. Flads
warm up faster than the archipelago waters outside the flad, which make them favourable nursing grounds for young
fish. More important to the nutrient dynamics is however the oxygen concentration, which to a great extent control the
flux of dissolved nutrients from the sediment. Nitrogen and phosphorus are released from the sediment in anaerobic
conditions at the sediment surface, because of chemical reduction and bacterial activity. Low oxygen levels and
anaerobic conditions become more frequent as the Isolation goes on. The nutrients are used by microalgae in the spring
especially In the glo flad stage.
The succession from the juvenile stage to a glo, a specific brackish water phenomenon, is completed in some hundred
years, geologically a very short time. The total area of flads in the archipelago is small, but they are Important foraging
grounds for many organisms and resting sites for emigrating waterfowl during springtime.
4. THE OPEN SEA, THE DEEP SOFTBOTTOMS
AND KATTEGAT
9
4.1 The open water system - the pelagic community
In the open waters of the Baltic the primary producers are the microscopic phytoplankton. These are consumed by the
larger zooplankton, which in turn serve as food for the pelagic fish, most importantly herring and sprat. On top of these
predators like cod and salmon are part of the pelagic food web. Large amounts of dead plankton sinks to the bottoms
where it serves as food for the benthic communitites.
The pelagic zone down to at least 20 m depth is a important producer system, fixing carbon dioxide using solar energy.
The level of the annual pelagic primary production is estimated to be 150 g Carbon/ m2/year for the coastal area of the
northwestern Baltic and decreases north, being only some 10 % of the values for the Baltic Proper in the Bothnian Bay
(Fig. 9A).
This annual production is a sum of several intensive so called algal blooms during the year, varying from about nine in
the hydrographlcally "noisy" belts in the south to three in the northern Baltic Proper and to one in the Bothnian Bay.
Here the spring and autumn blooms have merged together, which is a typical polar feature.
The spring bloom In the northern Baltic proper starts in late February or early March (Fig. 10). With increasing light
and abundant nutrients, phytoplankton blo-masses rapidly increase, mainly composed of dinoflagellates, e.g.
Gonyaulax catenata and diatoms Thalassiosira baltica and Skeletonemacostatum. The fixation of solar energy is
intense primary production reaching values of 1.5 g Carbon/m2/day. The bacterial population rapidly increases and
serve as food for pelagic dilates, which promptly increase. These in turn are consumed by the rotifers Synchaeto, which
have also a high potential for explosive growth and propagation.
The rapidly sinking of diatoms, in combination with the absence of large herbivores, causes that as much as 40 percent
of the organic matter synthesized during the spring bloom sinks out of the pelagic zone and settles on the soft bottoms
below. In fact this constitutes as much as half of the annual requirement of food of the soft bottom communities.
In summer the water is devoid of nutrients and only low biomasses of both phyto- and zooplankton persist for awhile.
After some time, production increases again, maintained by small forms of monads and dinoflagellates. Larger
zooplankton forms become common in the open sea, mainly the copepods Temora longicomis and Pseudocalanus
minutus elongatus. Most of the sedimented material during this period consists of fecal pellets from zooplankton, of
low nutritional value for the soft bottom organisms.
In July-August a conspicuous bloom of blue-green algae, especially Nodularia spumigena, dominates the pelagic zone
in the Baltic proper. This nitrogen-fixing freshwater cyanobacterium produces gas vacuoles and sometimes in large
quantities float to the surface where winds and currents arrange the thread-bundles in dense patches. In the usually
nitrogen-starved Baltic waters, the process of nitrogen fixation is important and the estimated 100,000 tons of nitrogen
fixed in the northern and central Baltic proper are certainly important for the input of potential energy into the Baltic
through stimulated fixation of solar energy.
The decline of the blue-green bloom releases nutrients, especially nitrogen, to the water, and initiates an autumn bloom
of dinoflagellates, green algae and diatoms. The large zooplankton forms are now at their maximum, and pelagic fish
like herring and sprat graze heavily, storing up fat reserves for the winter.
The open sea ecosystem provides the basic food chain for the major fish species of the Baltic.The fish production of
the Baltic is difficult to estimate but fish catch provides a lower estimate of close to 100.000 tons yearly. Obviously the
Importance of the open sea ecosystem to man can not be exaggerated.
4.2 Deeper softbottom ecosystems
The deep softbottoms dominate entirely the sea floor of the Baltic Sea. For the major parts of the sea the depth varies
between 50-150 meters.
The main Baltic area has been classified as aMocomacommunity from the dominating marine mussel Macoma baltica.
Macoma, together with the amphipods Pontoporeia affinis and Pontoporeia femorata, the polychaete Harmothoe
sarsi and the sipuncu-lold Halicryptus spinulosus make up more than 90 percent of the total macrofauna blomass in the
northern Baltic Proper (Fig. 11). In the Bothnian Bay and the central part of the Bothnian Sea a species poor
community of the isopod Saduria and crustacean Pontoporeia dominates. The deep basins of the Baltic proper has a
greatly impoverished fauna, with almost no macroscopic animals.
Primary production and animal biomass decreases from south to north, mainly caused by the salinity decrease (Fig. 9
Band C). The scatter of data for the same latitude mostly reflects different depths and the inhomogeneity of the bottom
substrate, which present differening nutritional potentials for the fauna. The low values in the Bothnian Bay are mainly
a result of the exclusion of the marine mussels. The annual production of the total soft bottom fauna in a coastal area of
the northern Baltic Proper has been roughly estimated at 7 g Carbon/ m2.•
In the absence of filter feeding mussels in the Bothnian Bay the decomposition of organic material is taken over by a
variety of small animal species, lesser than about 1 mm, living in the sediment. This results in a longer and therefore
10
less efficient food chain before the macrofauna level is reached.
4.3 The Kattegat coast - a comparison
Kattegat is quite different from the Baltic Sea as its higher salinity allows a more typical and species-rich marine flora
and fauna with no single algae or animal species dominating as in the Baltic.
A narrow zone of barnacles shows the mean water level. In the uppermost part of the shore crustforming bluegreen and
filamentous green algae (e.g. Calothrix - Lyngbya - Prasiola -BUdingia} cover the rocks. Below this zone, which is
similar to the Baltic, a more species rich perennial brown algal belt exists, consisting of three Fucus species on
sheltered sites growing together withAscophyllumnodosum. In the vertical zonatlon Fucus spirolis the first fucoid
species in the Kattegat area followed by Fucus uesiculosus and Fucus serratus. (Fig. 13). The different algal species are
often covered with bryozoans and hydroids as well as newly settled barnacles. On the coast of Jutland, another Fucus
species, Fucus distichus has been a permanent member of the algal flora for long time. Fucus disttchus originates from
the Arctic Ocean and grows there during the summer months.
Deeper down Fucus species are replaced by the brown algal species Lamtnaria. From about 5 m depth a very rich red
algal zone extends down to a maximum of 20 meters depth. In the deeper part of the red algal zone only red
crustforming algae are found together with many different animals species, among them the colourful sea urchins, sea
stars and sea anemons. The Laminaria forest with understory red algae and a diverse animal community, can be
regarded as the marine "rain forest".
A new invading species, originally from Japan, with a high capacity to outcompete some of the native fucoid species,
Sargassum muticum, is now commonly found growing in shallow areas at low salinities. In many small harbours
Sargassum is growing to several meter height and start to be a problem for some native algae.
Deeper hardbottoms are covered with a multitude of animals and the blue mussel, Mytilus edulis Is restricted to areas
with less predation. Below the limits of algal growth different animal communities are covering the rocks.
Dominating animal groups are sea urchins, sea stars, sea cucumbers, crabs, mussels, and at the deepest rocks marine
fungi Is the dominating component of the community.
Fladen is the greatest outground of Kattegat and is known as a good fishing area. The very flat hardbottom rises up to 9
meters and there Is always a strong current. The current usually moves northwards but occasionally southwards. With
the northgoing current, water from the Baltic moves over Fladen. Due to the strong current the bottom which varies
from sand to rocks and stones, is free of sediment. The vegetation of blade-forming seaweeds goes down to 26 meters
and the calcareous red algae can go further down to 30 meters. On Fladen we have a large forest of Laminaria digitata
at 10 meters although single individuals can reach much deeper.
The most shallow softbottom community is dominated by several mussel species, Mya, Cardium and Macoma baltica.
Between 15 and 20 m depth a community of briue stars ОрЫига and the mussel Abra alba with thin shells, a popular
food for flat fish, are found (Fig. 12). In the deeper softbottoms of Kattegat the animal community is dominated by the
polychaete Haploops but this has now decreased in abundance due to eutrophocation.
In the eastern part of Kattegat and northern part of the sound a Amphtura community is found from 20 m depth down
to ca. 100 m. Many species have disappeared from this community due to increased amounts ofsedimenting organic
matter. Still the total biomass has increased, caused by the much larger number of Amphiurafiliformis in later years.
Amphiura Jilifomus lives in the sediment with its arms sticking out into the water catching sedimenting organic
particles. Along the Kattegat coast this community is found from the shore line to ca 10 meters depth.
In the species rich communities in Kattegat many different species belong to the same functional groups, e.g.
filterfeeders and predators. Thus, if one species disapperas another might be able to fill the empty niche. This ecosystem is therefore belived to be more stable than those in the Baltic proper and therefore less vulnerable to stress
caused by man.
5. FISH, BIRDS AND MAMMALS
5.1 Fish and fishing
The fish fauna of the Baltic Sea exhibits a mixture of species of freshwater and marine origin, similar to the rest of the
flora and fauna. Again the salinity decrease causes the Baltic to be poor in species. The number of marine fish species
in the North Sea is 120, in the Kiel Bay 69, in the southern and middle parts of the Baltic Sea 41, in the Aland Sea,
Gulf of Finland and Gulf of Bothnia 20.
The lifecycle of fish consists of a larval stage, often at shallow coastal areas, an adult phase dominated by feeding and
wintering spent in the open sea, and the return to the coasts for spawning. Some species, in particular marine, migrate
long distances between the different periods in their life. The fish that spawn in the open sea are usually marine.
11
The freshwater fish often dominate the coastal areas, especially the littoral zone, but hardly penetrate into the open sea,
with the exception of some whitefish in Bothnian Bay with its almost fresh water. In the inlets and archipelago of the
Bothnian Bay, the fish fauna consists of mainly perch, roach, pike, whiteflsh and vendace. Most of the freshwater fish
species do not travel far. They often seek their spawning grounds in the river mouths and inlets, where the salinity and
temperature are more favourable. The Fucus belt cover extensive areas and constitutes important nursery grounds and
foraging areas for fish like pike, perch, eel, bream, Cisco and gobles.The freshwater fish in the Baltic Sea have usually
only a short growing period in the summer. In pike-perch for example, growth is restricted to the time between July September.
The fish in the open sea are dominated by three marine species, herring, sprat and cod. Herring migrates during the
year to different parts of the Baltic ecosystem to search for food and reproduction areas. Winter time is dominated by
decomposition processes. Due to a lack of food herring is forced to feed at the bottom on amphipods, mysids and
polychaetes, successively moving to shallow areas where they stay until spawning in May. By staying in the coldest
part of the watermass, sheltered from hydrodynamic disturbances, the fish minimize their energy losses, and schooling
decreases the impact of predators.
At least in the northwestern parts of the Baltic Proper the herring prefer to deposit their roe on filamentous red algae as
Furcellaria or other filamentous brown algae like PUayella. A couple of weeks old, the fry return to the open waters.
The herring thus utilizes all three subsystems during the year, the pelagic zone in summer, the soft bottom in winter
and the seaweed system in spring, and acts as a transport link of organic matter between them. This is another example
of how the different systems are coupled together, demonstrating the fact that if some plant or animal population
disappears or increase, this may disturb the whole ecosystem.
The sprat has a life cycle very similar to that of herring. However It spawns in the open sea and has pelagic roe like the
cod, although spratroe stays in less deep water with lower salinity. Compared to herring sprat is a more specialized
zooplankton feeder.
The pelagic fish in the surface layers benefit from moderate eutrophication and herring and sprat have greatly increased
their populations in the Baltic for many years. The potential fish food on the soft bottoms above the primary halocline
has increased up to sevenfold since the 1920s, most probably due to eutrophication. There was a drastic increase in the
catches of sprat from the mid 1950s to mid 1970s (Fig. 14). Since this period catches have decreased significantly,
probably due to a combination of heavy predation by cod and over-fishing.
The reproduction of cod in the Baltic is restricted to areas with salinities around 10 %о and higher, which are found
mainly in five deep basins. The resulting increased primary production causing oxygen depletion in deep waters,
decreased the reproductive success of cod.
The population of salmons in the Baltic belongs to the Atlantic salmon species, Solrno solar. It has been isolated from
the populations living in the North Atlanic and some genetic changes have been reported. After hatching the young
salmon may remain up to 5 years in the river, before migrating in May-June to spend up to 4 years in their feeding
areas, mainly in the Baltic basin. In September-November they return to to spawn in the rivers. The Swedish stocks of
Baltic salmon are mainly native to the big rivers in the northen part of the country. The building of dams in connection
with power plants from the 1940s has drastically changed and reduced the salmon populations. By establishing
compensatory hatcheries at each river and mass rearing ofsmolts which are relased into the river it has been possible to
keep reproduction at a high level.
The releases of hatchery-reared saloms to the Baltic increased during the 1980s while the recruitment of naturally
produced smolts has continued to decrease (Fig 15). Since fishing is concentrated on the feeding areas, where naturally
and cultivated stocks are mixed, the recruitment of smolts to the present stock of naturally spawning salmons continues
decreasing. In this way the genetic variability of the stock decreases further.
In different parts of the Baltic the fishery varies. Freshwater fish are especially important in the catch from the northen
and eastern parts of the Baltic Sea. The whiteflsh fishery is greatest in the vendace fishery. Pike are fished intensively,
particulary by amateur fishermen. The growth conditions for commercial fish living and feeding in deeper areas are
less favorable in the Baltic than in the North Sea. Cod and flounder grow less, and the stagnant water in the deep basins
have made the former foraging areas in the southern Baltic uninhabitable and unproductive. However, single yearclasses may still be successful and give rise to intensive fishing such as occurred with cod during 1979-80. (Fig. 14.)
Table" 2. Freshwater fishes. Species composition and distribution.
1. Regularly found in the Baltic Sea.
Salmonidae: Vendace (Coregonus albula); Whiteflsh (Coregonus lavaretus); - Migratory whiteflsh (Coregonus
lavaretus); - Sea-spawntag whtteflsh (Coregonus nasus); Arctic char (Saluelinus alptnus)
Thymalidae: Grayling (Thymollus thymallus)
Osmeridae: Smelt (Osmerus eperianus)
12
Esocidae: Pike (Esox lucius)
Cyprinidae: Bream (Abromte broma); Silver bream (Abramis ballerus); Bleak (Alburnus alburnus); Asp (Aspius aspius);
White bream (Blicca bjoerkna); Crusian carp (Carassius carassius); Gudgeon (Gobio gobio); Dace (Leuciscus
leuciscus); Ide (Leuciscus idus); Chub (Leuciscus cephalus); Chekhon (Pelecus cultratus); Minnow (Phoxinus
phoxinus); Roach (Rutilus rutilus); Rudd (Scardinius erythrophthalmus); Tench {Тinса tinсa)
Cobitidae: Spined loach (Cobitis taenia L.); Stoneloach (Nemacheilus barbatulus)
Siluridae: Sheatflsh (Silurus glanis)
Gadidae: Burbot (Lota lota)
Gasterosteidae: Three-spined stickleback (Gasterosteus aculeatus); Ten-spined stickleback (Pungitius pungitius)
Percidae: Perch (Perca fluviatilis); Ruff (Gymnocephalus cernua); Pike-perch (Stizostedion lucioperca)
Cottidae: Miller's thumb (Cottus gobio); Mottlefoot sculpin (Cottus poecilopus)
2. Introduced species occasionally found In the Baltic Sea
Salmonidae: Rainbow trout (Salmo gairdneri); Lake trout (Salvelinus namaycush); Peled whitefish (Coregonus peled)
Cyprinidae: Carp (Cyprinus carpio)
Ictaluridae: Brown bullhead (Ictalurus nebulosus)
3. Anadromous and catadromous species
Atlantic salmon (Salmo safar); Sea trout (Salmo trutta); Vimba (Vymba vimba); Eel (Anguilla anguilla} and Lamprey (Lampetra
fluviatilis).
5.2 Mammals - threatened species
Predators other than fish are air-breathing vertebrates such as sea birds, seals, and whales. In the Baltic three species of
seals are living, the grey seal, the ringed seal and the harbour seal. The grey seal is the largest and the ringed seal is the
smallest species. Seal populations have been greatly reduced in recent years.
The number of ringed seal is estimated to 5,500-6,000. The ringed seal is mainly found in the Bottnian bay, Bothnian
Sea , Gulf of Finland and Riga bight. The ringed seal is closely related to the Arctian populations and is thought of as a
glacial relict. It is seldom found in the southern parts of the Baltic.
The number of Baltic grey seals living in the archipelagos along the Baltic coast were around 2,000 animals in 1990.
Along the southern coast 500-300 animals, In the northern archipelago of Stockholm 1,150 animals were counted and
in Aland Sea, the Bothnian Bay and Bothnian sea 420-520 animlas were seen. During the ice free period of the year the
seals are found on shallow rocks and can at the period when they change their fur be counted.
The harbour seal inhabits only the southernmost part of the Baltic Sea were the estimated number is 200 individuals.
There are at least three populations of harbour seals, at Maklappen, at Kalmar Sound and a small group at the southern
part of Gotland. In Kattegat a much larger populations is found. During the spring 1988 many of harbour seals along
the coast of Kattegat and Skagerack were killed by a virus. The virus also reached the harbour seal populations at
Maklappen in 1988 and no pups survived in that year.
Living at the highest level in the food web of the marine ecosystems the seals are exposed to many polluting
substances. Another significant factor is drowning in fishing gear by young pups. Seal have also been hunted for many
years. The seals are feeding on different fish species. The harbour seal, for instance mainly consumes small schooling
fishes which they catch in the pelagic system and on soft bottoms. The total seal populations of the Baltic have
declined greatly, and the approximately 20,000 seals remaining can be assumed to eat about 4 kg offish a day each.
This totals 3 x 1031 Carbon dry mass of fish a year, less than 1 % of the estimated fish production.
The only whale previously common, the harbour porpoise (Phocoena phocoena), is now virtually extinct in the Baltic.
5.3 Birds
The Baltic region is rich in bird species. Some 340 species are found regularely in the region. Many of them are water
fowls living In the Baltic. Others, such as waders, live on the coast or in surrounding wetlands. Compared to oQier
marine environments the Baltic is rich In species, by its combination of marine and freshwater birds. This is a sharp
contrast to the underwater animal communitltes.
Most of the birds species migrate, between winter grounds and nesting grounds for spring and summer. The migratory
movements are especially pronounced in the Baltic region since the northern summer, with long hours of light and rich
access to food, is very attractive for nesting, while during winter the ice conditions of the Baltic Sea are severe. Thus
most birds leave the Baltic Sea and winter further south. However large number of longtalled duck, from northern
Russia, stay in winter in southern Baltic as do also tufted duck, mute swan, Canadian goose and herring gull. Large
numbers of elder ducks winter in Danish waters.
13
Of the several distinct biotops in the Baltic the archipelagoes are extensive and rich in birds. The many islands offer
excellent protection for waterfowl during their nesting period, and the shallow waters are rich in food. The
breedingwaterfowl community consists of marine as well as freshwater species. The eider duck, great black-backed
gull, tumstone and all three species of auks -guillemot, razorbill and black guillemot - are elsewhere almost exclusively
found in marine environments. But in the brackish Baltic Sea they breed together with typical freshwater species such
as great crested grebe, velvet scoter, stufted duck, goldeneye, mute swan and blackheaded gull.
A remarkble biotope, the birdcliffs, otherwise typically found along the oceans, are present on the west coast of
Gotland, with thousands of birds.
The eider duck is the most numerous off all waterfowl in the Baltic. It numbers today at least some 800,000 breeding
individuals. It is very wide spread, being absent only from the inner parts of the Bothnian Bay and the Gulf of Finland.
Here the salinity is close to zero and its main food, the blue mussel, does not occur. The eider duck almost exclusively
places its nest on islands where it is safe for mammalian predators. Thus it is neither found in southeastern Baltic
where Islands are mostly lacking.
A different biotop constitute the freshwater wetlands in the Baltic region. These have decreased dramatically in
Sweden and Dan-mark, but are still wide ranging in the Baltic States and in Poland. They are very rich in bird species
and some more remarkable birds, like the black storch, are found only here. The Wisia river valley, being largely
unregulated and thus a wetland area, is also a rich bird sanctuary.
The water bird populations have in general increased during later decades. We belive that it is benefitting from an
increased production of macroinvertebrates and fish caused by increased amounts of nutrients in the Baltic.
Particularely notably Increases have been observed for eider duck, oyster catcher and cormorant. Conservation
measures have been valuable to protect some rare species.
A case of decreasing population concerns the sea eagel. Today the Baltic harbors about 200 nesting pairs of this
magnificient bird, mostly in Finland and Sweden. Being more common during last century this species was decreasing
up to the 30:ies when it became protected in many places. During the 1950:ies it again started to decrease, this time due
to pollution. As a top predator it was severly affacted by organic pollutants. Only 25 % of the pairs (normal rate is 75
%) were successfully breeding during the period 1965-85. Several measures were made to save the sea eagel, such as
placing non-poisoned cadavers as food on small islet. Today the species is on its way to be saved to the Baltic fauna.
The most dramatic manifestation of the bird fauna in the Baltic are the migrations. There is a massive passage of
migrating birds, especially during the autumn movements. The main direction is southwestern. For many passerine
species coming to the eastern coast the open water functions as a barrier and the birds follow the coast southwards.
Ducks, geese and waders pass the sea on a broad front or make their way over the large islands. When reaching the
Swedish coast they are linked southwards.
In this way a concentration of migrants are generated on both sides of the Baltic Proper. An important flyway leads
over Schleswig-Holstein to the North Sea. There are many important resting localitites for the migrants where they
stop and feed for the continued journey. Marsalu in Estonia, the river deltas in Poland, the east coast ofOland in
Sweden, and the Rugen area are localities full of birds during migration times.
6. A LIFE IN STRESS – NATURAL AND MAN MADE
6.1 Life in winter
The spring time is the period of growth and reproduction in the northern Baltic archipelagos. Rich algal growths and
density of animal populations reflects favourable conditions for marine life. In the winter the situation is different - a
season of harsh environment for most plants and animals - even scientists!
The water temperature decreases to zero, and the metabolic activity of the water organisms slows down severalfold in
the low temperature. The short days does not give much light for photosynthesis for the algae, not even during daytime
since very little light penetrates the ice when it is covered by snow. Near the mainland the freshwater outflow from
rivers accumulates under the ice and the decreasing salinity is a further difficulty for marine organisms.
Some organisms are adapted to live in these conditions, in particular the glacial relict crustaceans. Thus the large
isopod Saduria entomon, the bottom-dwelling Pontoporeia amphipod and the mysid shrimp Mysis relicta even breed
under the winter ice. In this way they Improve the possibilities for their juveniles which are hatched during the
productive peak of the pelagic ecosystem in early spring.
The adapted species have several special properties that ensure their survival in the cold and dark winters. These
properties are Inherited. The mechanisms range from anatomic features, such as exceptionally good isolation of the
body; physiological features, such as increased light sensitivity of algal cells; the presence of substances in the blood of
Invertebrates that make body fluids less viscous in the cold; to biochemical adaptations such as special structures of the
biomolecules, in particular enzymes.
14
Organisms living normally in warmer climate have features that allow a short term survival in colder weather.
However longer periods of cold might lead to unacceptable losses of energy and thus diminished growth and even
failure to reproduce. It Is clear that changed genetic properties are needed if these species are to spread into the colder
parts of the Baltic region. Such adaptations are only the result of prolonged genetic changes, i.e. evolution, for which
there has not been time enough.
6.2 Adaptation to changing salinity
The interior of cells of living organisms all have a salt concentration of about 9 °/oo. The salts present in the cells are
mostly potassium chloride and very little of sodium chloride. Thus powerful biochemical mechanisms exists to keep
the interior of the cell in the right salt condition. For ourself the salt of the body fluids are regulated by the kidney to
about 9 %o, and the balance of ions inside the cells are upheld by special enzymes in the cellular membranes.
Living organisms have been able to adapt to all kinds of salinities.
The extremes are organims living In fresh water with 0 %o salt and those living in salt lakes where salinities can
exceed 200 %о. In these cases a considerable part of the energy consumption of the organism is used to keep the right
internal conditions of the cells. The difference in salt concentration creates an osmotic pressure that will either shrink
the cell (if more salt outside) or lead to its rupture (if less salt outside) unless the cellular structure, in particular cell
walls, prevents it.
Many marine organims are adapted to a precise salinity of about 35 %о. Others are more flexible, in particular those
that live in the coastal zones of the oceans, the estuaries, where salinity might vary between some 15-35 %о. Migration
into the Baltic Sea requires that the organims are able to handle the brackish water with much lower salinity.
Comparatively few organims have been able to do so, as we have seen, and the Baltic Sea is quite poor in biological
diversity. In most of its ecosystems a single or a few species dominate.
The marine plants and animals that have been able to adapt to the Baltic and push their distribution limits into the
brackish water are mostly from the atlantic estuaries. The marine algae along the Finnish coasts were shown to have
relatives in the Atlantic that live in the tidal zone. and thus were adapted to dramatically changing conditions, before
they moved into the Baltic Sea. In the Baltic, however, they live below the immediate coastline in the sublittoral. They
are both &amentous green and red algae, and the brown algae Fucus. However most of those living at constant salinity
levels in the sublittoral zone on the Atlantic coast have not been able to adapt to the brackish conditions of the Baltic
Sea.
Apparently the intertidal atlantic species had a good deal of genetic variability, which they used for adaption to
brackish conditions. They have coped with the situation in differentways. Some plants grow stronger cell walls. Others
have changed their biochemical properties. In some cases the changes have been such that they are no more able to
grow at higher salinities. Some red algae like Ceramium and Rhodomela grawint in the Baltic were thus not able to
cope with a higher salinity.
Some species seem to have payed a prize for the adaptation to lower salinity: In many marine animals e.g. the
adaptation to low salinity is correlated with a loss of cellular resistance to other outer stress factors such as heat,
freezing and dehydration, properties which they have in normal seawater.
6.3 Pollution sensitivity
The assumption that the species of the Baltic sea are more sensitive to stress by pollution than organisms in the North
Sea have been suggested several time. In the few experiments that have been performed this is also the case. So far we
do not know of any instance where the North Sea individuals have been more sensitive to environmental stress than the
Baltic Sea Individuals of the same species.
There are several explanations for the increased sensitivity of the Baltic species. In summary one might say that the
pollutants Interferes with a system that is already under stress, by its adaptation to the lower salinity. An example is the
interactions of toxic substances with membrane biomolecules. These membranes are already key componenets for
keeping the salt balance and the cell can not afford to lose their functions.
It is well known that the Baltic Sea has a low salinity, and it has obviously been taken for granted that the Baltic Sea
environment, like other estuaries, is also more variable than the North Sea. Although there is a clear salinity gradient
from the North Sea into the inner parts of the Baltic Sea this gradient is very stable, with the main reduction in species
numbers occurring outside the Danish sounds. Also if annual variations in salinity and temperature at different depths
are compared between the northern Baltic proper and the North Sea coast, it is clear that salinity, the factor having the
largest biological impact, varies more in the North Sea than in the Baltic, both annually and vertically in the upper
water layer. Temperature variations are about the same, but due to a lower freezing point in high saline water,
biologically detrimental low water temperatures are more likely to be reached in the North Sea. The North Sea should
thus be regarded as more variable in environmental factors, which would be the reason for the higher genetic diversity
observed in populations of North Sea, e.g. the crustacean Gcurvnarus oceanicus, and the blue mussel Mytilus edulis as
compared to the Baltic. Similarity, the broad-niched Gommarus duebeni from environmentally variable rockpools was
15
found to be genetically more variable than the more narrow-niched Gommorus oceanicus.
In all cases increased genetic diversity in a species was correlated with higher stress tolerance. This is in accordance
with other studies showing that heterozygosity in many organisms correlates significantly with certain fitness
parameters and that genetic diversity will usually yield ecological heterogeneity, and consequently higher levels of
tolerance to pollution.Thus we conclude that the Baltic Sea, also in tills respect, is probably more sensitive to pollution
than the North Sea.
The low number of species together with little genetic variation in many of the species occuring in the Baltic makes the
sea more sensitive to pollution. Since both animals and plants live under salinity and partly temperature stress some of
them almost at their border of existence further stress by different polluting substances might be even more fatal in the
Baltic than in the more marine environment such as the Kattegat and North Sea.
7. SUMMARY
• The variation in salinity is larger in Kattegat than in the Baltic Sea.
• The decreasing salinity going north is parallelled by a shorter growth period. Primary production in the pelagic and
shallow benthic communities also decrease going from south to north In the Baltic.
• The daily tidal range is negligible in the Baltic compared to about 20 cm at the coast of Kattegat.
• As compared to fully marine environments the Baltic Sea with its brackish water has a very poor flora and fauna.
• The life forms of the Baltic are all immigrants from the neighboring regions. Three such different types of
Immigrants can be distinguished, marine organisms fully adapted to high salinities, freshwater organisms from lakes
and a few flacial relicts. The first two groups are poorly adapted to cold water conditions, while the latter group tends
to occur in the deeper colder water in the Baltic.
• Many organisms are not able to grow to the same size as in their original environment.
• The low number of species in each functional group of organisms in the Baltic Sea increases the probablility that
whole functions could easily be lost in the Baltic by further man made stress e.g. pollution by toxic substances or
eutrophication.
• Due to low salinity in the Bothnian Bay, the benthic filterfeeders are lacking, and looking at the Baltic Sea, the
diversity of predators on mussels is much lower than in Kattegat.
• On rocky shores there is only one large perennial algae, Fucus vesiculosus, which if it dissappears will have effects
on the whole rocky shore animal community living in and around the FUcus zone.
• The Baltic is an evolutionary young ecosystem, with comparatively cold water and low stable salinity compared to
other marine environments. This semi-enclosed sea, with a long retention time of the water, and surrounded by densely
populated drainage area, is clearly more vulnerable to polluting substances and increased nutrient load than the more
marine coastal areas.
• Summarizing the main factors affecting life in the Baltic, it seems clear that several things render the Baltic
ecosystem more sensitive to further stress by pollution than fully marine areas, both from a physiological and an
ecological point of view.
8. GLOSSARY
autotroph An organism that can synthesize its biochemical constituents using inorganic precursors and an external
energy source. Photoautotrophs make use of sunlight through the process of photosynthesis, while chemo-autotrophs
harness some of the energy content of Inorganic chemicals through the process of chemosynthesls.
benthos The biota living on or in the surface sediment of a freshwater or marine ecosystem.
biomass The quantity of living and/or dead organic matter, usually standardized per unit area in a terrestrial ecosystem,
or per unit volume in an aquatic ecosystem.
bloom A burst of productivity of phytoplankton, resulting in a large standing crop of algal biomass and of chlorophyll,
and a restricted transparency of the water column.
competition A stress-causing interaction between organisms of the same or different taxa, caused by the need for a
common resource that occurs in insufficient supply relative to the biological demand.
decomposition The heterotrophic microblal oxidation of organic matter.
density The number of organisms per unit area, or per unit volume in aquatic studies.
16
deposition The rate of influx of material per unit area and time. Frequently pertains to influx fromthe atmosphere.
detrivore A heterotrophic microbe or animal that feeds on dead biomass.
disturbance An episodic environmental Influence, usually physical, that causes a measurable ecological change.
diversity An ecological concept that Incorporates both the number of species in a particular sampling area, and the
evenness with which individuals are distributed among the various species (the latter is sometimes expressed as the
probability of encountering an individual of a particular species).
dry weight (d.w.) The weight of a substance after water has been removed.
ecology The study of the relationships between and among organisms and their environment.
ecosystem A generic term for a system that Includes a community of organisms and their interactions with the
environment.
endemic A distinct race or species that originated locally, and that has a geographically restricted distribution.
energy budget An analysis of the inputs and outputs of energy for a system, and of the internal transformations and
storage within the system.
enrichment Enhancement of the rate of supply of nutrients to a system, causing an increase in productivity.
environment The complex of all blotic and abiotic influences on an organism or group of organisms.
environmental factor Any biotic or abiotic influence on an organism or group of organisms.
estuary The widening channel of a river where it meets the sea, characterized by spatial and temporal variations in
salinity of the water.
euphotic zone The upper portion of a water column, where light intensity Is sufficient to allow net photosynthesis to
occur.
eutrophic The characteristic of being very productive, as a result of a large rate of nutrient loading. Usually refers to
an aquatic system. See also mesotrophic and ollgotrophic.
eutrophication The process by which an aquatic ecosystem Increases in productivity as a result of an increase in the
rate of nutrient input.
evolution The gradual change over time In the genetically based characteristics of a population of organisms.
fauna The assemblage of animal taxa in some defined site or geographical area.
fecal pellet An animal excrement. In some studies of animals that are difficult to census directly, the density of fecal
pellet may be surveyed as an index of abundance.
flora The assemblage of plant taxa in some defined site or geographical area.
food chain A linear sequence of organisms that are linked by trophic interactions, as in grass-cow-people.
food web A complex assemblage of organisms that are interlinked by trophic interactions.
fry The young of various species of fish.
genetic Refers to information that is contained within the base series of the DNA of chromosomes.
herbivore An animal that feeds on plants.
heterotroph An organism that requires a source of organic matter as food. Compare with autotroph.
intertidal Refers to a marine shore zone occurring between the high water mark and the low water mark.
invertebrate Any animal that does not have a backbone.
limiting factor A metabolically essential envlromental factor that is present in least supply relative to biological
demand, and which thereby restricts the rate of productivity.
Uttoral Refers to the relatively shallow, nearshore zone of an aquatic system.
macroalga An alga with a thallus that is visible without magnification, as in seaweeds
macroscopic Visible without the aid of magnification.
microfauna The assemblage of microscopic animal taxa in some defined site or geographical area.
migration A periodic, longdistance movement undertaken by animals.
17
niche The role that an organism or taxon plays in its natural ecosystem, includinglts activities, resource use, and
interactions with other organisms.
oligotrophic Refers to a condition of a restricted supply of nutrients and small productivity. Usually refers to an
aquatic system. See also eutrophic and mesotrophic.
organic matter Carbon-containing molecules that make up or are derived from the tissue of organisms.
pelagic Refers to an open-water system.
perennial A long-lived organism. Usually refers to plants.
plankton Plants and animals that are suspended in the water column.
pollution The occurrence of substances or energy in a larger quantity than the environment can assimilate without
suffering degradation from the anthropic perspective.
predation A biological interaction in which a predator kills and eats its prey. Usually refers to a carnivore eating
another animal.
primary production Production by autotrophs. See also production.
production The quantity of organic matter that is produced by biological activity per unit area or volume. Gross
production (GP) refers to all production, without accounting for respiratory losses (R). Net production (NP) is GP - R.
Primary production is production by autotrophs. Secondary production refers to herbivores, and tertiary production
refers to carnivores.
productivity
Production standardized per unit ot time and area.
reproduction A sexual or asexual process by which organisms produce new and discrete individuals that are similar to
the parent.
respiration A biochemical process that occurs in all organisms, in which complex organic molecules are broken down
by various enzymatic reactions in order to derive energy for metabolism. The ultimate products of respiration are
carbon dioxide, water, and other simple inorganic molecules.
rocky intertidal A hard-rock habitat zone that occurs between the high water and the low water levels of the seashore.
species Populations of organisms that actually or potentially interbreed, and produce fertile hybrids. Species are named
with a latinized binomial.
stability The tendency of a system to persist relatively unchanged over time.
standing crop The quantity of biomass per unit area or volume of an ecosystem.
stress Physical, chemical, and biological constraints that limit the potential productivity of the biota. Any
environmental influence that causes measurable ecological detriment.
sublittoral Refers to an intermediate water depth that occurs between the nearshore shallow littoral zone and the
deepwater zone.
subspecies A genetically and anatomically distinct subspecific taxon that is not reproductively isolated from other such
subspecies.
succession A process that occurs subsequent to disturbance, and that involved the progressive replacement ofbiotic
communities with others over time. In the absence of further disturbance this process culminates in a stable climax
ecosystem that is determined by climate, soil, and the nature of the participating biota. Primary succession occurs on a
bare substrate that has not previously been modified by organisms. Secondary succession follows a less intensive
disturbance, and it occurs on stubstrates that have been modified biologically.
tolerance (1) Refers to a genetically based physiological tolerance of an environmental stress or combination of
stresses. (2) Refers specifically to tolerance of the stressful environmental conditions of the under storey of a closed
forest. See also shade-tolerant.
vulnerable Apt to suffer damage from some stressful environmental agent.
zooplankton Small animals that occur in the water column.
9. LITERATURE REFERENCES
Ackefors, H., Johansson, N. and B. Wahlberg. 1991. The Swedish compensatory programme for salmon in the Baltic;
an action plan with biological and economical implications. - ICES Mar. Sci. Symp. 192:109-119.
18
Anderson, A.-B., Lasslg, J. and H. Sandier. 1977. Community structure of soft bottom macrofauna in different parts of
the Baltic. - In: Biology ofbenthic organisms, pp. 7-20. Ed. by B.F. Keegan, P.O'Ceidigh and P.J.S. Boaden. Pergamon
Press, Oxford. 630 pp.
Aneer, G. 1980. Estimates of feeding pressure on pelagic and benthic organisms by Baltic herring (Clupea harengus v.
mem-bras L.). - Ophelia, Suppl. 1:265-275.
Ankar, S. 1977. The soft bottom ecosystem of the Northern Baltic Proper with special reference to the macrofauna. Contr. Asko Lab., Unlv. Stockholm 19:1-62.
Dietz, J. , Heide-Jorgensen, M.-P. and Harkonen, T. 1989. Mass deth of harbour seeals (Phoca uttulina) in Europe.
Amblo %:258-264.
Elmgren, R. 1989. Trophic dynamics in the enclosed brackish Baltic Sea. - Rapp. P.-v. Cons. int. Explor.Mer. 183:152169.
Helander, B. 1989. Survey of grey seal Halichoerus grypus and harbour seal Phoca vitulina along the Swedish Baltic
coast 1975-1984. pp 10-21 in Proceedings of the Soyjet- Swedish Symposium "Influence of human activltyes Qn the
Baltic ecosystem, eds. A. Yablokov and M. Olsson, Leningrad.
Hansson, S. and L.G. Rudstam. 1990. Eutrophication and the Baltic fish communities. - Ambio 19:123-125.
Hullfors, G. and A. Nieml. 1981. Vegetation and primary production. - In: The Baltic Sea. pp. 220-238. Ed. by A.
Voipio. Elsevler, Amsterdam. 418 pp.
Hempel, G. and W. Nellen. 1974. Fische der Ostsee. - In:
Meereskunde der Ostsee, pp. 215-232. Ed. by L. Magaard and C. Rhelnhelmer. Springer Verlag, Berlin. 269 pp.
Janke, K. 1990. Biological Interactions and their role in community structure in the rocky intertidal of Helgoland
(German Bight, North Sea). Helgol. Meeresunter. 44:219-263.
Jansson, A.-M. and N. Kautsky. 1977. Quantitative survey of hard bottom communities in a Baltic archipelago. - In:
Biology ofbenthic organisms, pp. 359-366. Ed. by B.F. Keegan, P.O'Ceidigh and P.J.S. Boaden. Pergamon Press,
Oxford. 630 pp.
Jansson, B.-O. 1972. Ecosystem approach to the Baltic problem. -Bulletins from the Ecological Research Committee.
NFR 16:1-82
Jansson, B.-O. 1978. The Baltic -a systems analysis of a semi-enclosed sea. - In: Advances in oceanography, pp. 131183. Ed. by H. Chamock and G. Deacon. Plenum Press, New York. 356 pp.
Johannesson, K. 1989. The bare zone of Swedish rocky shores: why is it there? Oikos: 54:77-86.
Kachalova, 0. and G. Lagzdins. 1974. Organisms of the bottom fauna in the southern part of the Gulf of Riga. - In:
Biology of the Baltic Sea. 1. Hydrochemistry and hydrobiology of the Gulf of Riga, pp. 258-302. Acad. Sci. Latvian
SSR., Inst. ofBiol.. "Zinatno", Riga.
Kaiser, W., Renk, H. and S. Schulz. 1981. Die Plimarproduktion der Ostsee. - Geodat. Geophys. Veroff., R IV H
33:27-52.
Kangas, P. 1976. Littoral stony-bottom invertebrates in the Krunnit area of the Bothnian Bay. - In: Bothnian Bay
Symposium 1974, Proc. Acta Univ. Oul. A. 42. 1976. Biol., 3:97-106.
Kautsky, N. 1981a. On the trophic role of the blue mussel (Myttlus edulis L.) in a Baltic coastal ecosystem and the fate
of the organic matter produced by the mussels. - Kieler Meeresforsch., Sonderh. 5:454-461.
Kautsky, N. 1982a. Growth and size structure in a Baltic Mytilus edulis L. population. - Mar. Biol. 68:117-133.
Kautsky, H.1988. Factors structuring
Stockholm unlveristy.
phytobenthic communities in the Baltic Sea. Thesis. Department of Zoology,
Kautsky, H., Widbom, B. and F. Wulff. 1981. Vegetation, macrofauna and benthic meio-fauna in the phytal zone of
the archipelago of Lulea - Bothnian Bay. - Ophelia 20:1, 53-77.
Kautsky, L. and H. Kautsky. 1989. Algal species diversity and dominance along gradients of stress and disturbance in
marine environments. -Vegetatio 83:259-267.
Larsson, P.-O. 1984. Some characteristics of the Baltic Salmon (SatmosotorL.) population. - Ph.D. Thesis, University
of Stockholm, Stockholm, Sweden. 80 pp.
Lappalainen, A., Hallfors, G. and P. Kangas. 1977. Littoral benthoc of the Northern Baltic Sea. IV. Pattern and
dynamics of macrobenthos in a sandy-bottom Zostera marina community in Tvarminne. - Int. Rev. ges. Hydrobiol.
62(4):465-503.
19
Lassig, J., Leppanen, J.-M.rn Niemi.A.andG.Tamelander. 1978. Phytoplankton primary production In the Gulf of
Bothnia in 1972-1975 as compared with other parts of the Baltic Sea. - Finn. Mar. Res., 244:101-115.
Lehtonen, H. and J. Toivonen. 1981. Fresh-water fishes. - In: The Baltic Sea, pp. 333-341. Ed. by A. Voipio. Elsevler,
Amsterdam. 418 pp.
Lindholm, T. 1978. Autumnal mass development of the "red water" ciliate Mesodinmm rubrum In the Aland
archipelago. - Mem. Soc. Fauna Flora Fennica 54:1-5.
Magaard, L. and G. Rhelnheimer. 1974. Meereskunde der Ostsee. Springer-Verlag, Berlin. 269 pp.
Mauchhne.J. 1980. The biology of mysids. - In: Adv. Mar. Biol., 18:1-369. Ed. by J.H.S. Blaxter, F.S.
Munsterhjelm, R. 1987. Flads and gloes in the archipelago. Geological survey of Finland. Special paper 2:55-61.
Nellbring, S. 1988. Quantitative and qualitative studies of fish in shallow water, northern Baltic Proper. Thesis.
Department of Zoology, Stockholm University.
Nilsson, L. 1980. Wintering diving duck populations and available food resources in the Baltic. -Wildfowl 31:131-143.
Ojaveer, E. 1981. Marine pelagic fishes. - In: The Baltic Sea, pp. 276-292. Ed. by A. Voipio. Elsevler. Amsterdam. 418
pp.
Remane, A. 1934. Die Brackwasserfauna.Verh. Dt. Zool. Ges., 36.
Rudstam, L.G., K. Danielsson, S. Hansson and S. Johansson. 1989. Diel vertical migration and feeding patterns
ofMysis rrdxta (Crustacea, Mysidacea) in the Baltic Sea. Marine Biology 101:43-52.
Theede, H. 1974. Die Tirwelt. I. Okologie. - In: Meereskunde der Ostsee, pp. 171-188. Ed. by L. Magaard and G.
Rheinheimer. Springer-Verlag, Berlin. 269 pp
Thurow, F. 1980. The state offish stocks in the Baltic. - Ambio 9:153-157.
Voipio, A. (ed.). 1981. The Baltic Sea. Elsevier, Elsevler Oceano-graphis Series, Amsterdam. 418 pp.
Waem, M. 1952. Rocky-shore algae in the Oregrund archipelago. - Acta phytogeogr. suec., 30:1-298.
Wallentinus, I. 1979. Environmental influences on benthic macrovegetation in theTrosa-Asku area. Northern Baltic
Proper. II. The ecology of macroalgae and submersed phanerogams. - Contr. Asko Lab., Univ. Stockholm 25:1-210.
Zenkevltch, L. 1963. Biology of the seas of the USSR. London 955 pp.
Zmudzinski, L. 1978. Variability of species composition and biomass of bottom fauna in the Southern Baltic area. - In:
Produktywnosc, Ekosystemu Morza Baltyckiego. pp. 143-161. Ed. by W. Mankowski. Polska Akademia NaukKomitet Badan Morza. Wroclaw. 342 pp.
20