Marine nature is a sensitive entity upon which human impact varies widely

The organisms of the Baltic Sea live under intense pressure. The sea is surrounded by dense populations, which affect the sensitive marine environment. A century ago, hunting caused the collapse of seal populations, after which the marine environment began to be plagued by toxic substances. Today, the sea is particularly affected by eutrophication.

Eutrophication has changed the habitats and living conditions of marine species. The consequences of eutrophication are reflected in the species of the Baltic Sea: plankton and benthic communities have changed, and there have also been changes in fish stocks and the populations of nesting birds.

Of particular concern is the decline of certain key species. Bladder wrack, seagrass, as well as other species, are of paramount importance to many organisms because they provide both shelter and food. Changes in key species are reflected throughout the entire marine food web.

Many pressures cause stress and change in marine communities

In addition to eutrophication, marine nature is burdened by many other issues, such as the construction of coastal and marine areas, dredging, and alien species. These can directly affect communities or change their habitats.

Tutkijan kämmenelle mahtuva, ruumiiltaan reilun sormenlevyinen, jalkoineen reilun kahden sormen levyinen rapu.
Originally from North America, the estuarine mud crab is an invasive species that has established itself in Finnish marine areas.

The most recent concerns include underwater noise, marine littering, and climate change. However, their effects are not yet sufficiently known.

Together, all of these different pressures cause a considerable total strain on the marine environment. This is evident in both underwater communities and surface aquatic marine life. The state of marine nature is monitored using several different indicators.

Marine mammals are doing both well and poorly

Three marine mammal species live in Finnish marine areas, i.e. the grey seal, the ringed seal, and the harbour porpoise. Their populations have fluctuated widely over the last century.

The condition of marine mammals has been particularly affected by both hunting and environmental toxins. Hunting collapsed seal populations in the early 20th century. After that, seals began to be plagued by chemicals originating from the forest industry, in particular. These weakened the reproduction of seals so much that the populations continued to decline.

It was not until the 1990s that seal populations began to recover as the concentrations of harmful substances decreased. Despite this, numbers of both grey and the ringed seals remained low until the early 2000s. Environmental toxins may also have affected porpoise populations, although their drowning in fishing nets has been a more significant threat.

The status of marine mammals is monitored by several indicators. The abundance of animal populations, the distribution of species, and the condition of the population are used as indicators of their status.

The status of the grey seal is stable, but the ringed seal is particularly threatened by climate change

Today, more than 30,000 grey seals live in the entire Baltic Sea area. Of these, approximately 13,000 individuals occur in Finnish sea areas, and this population continues to grow moderately. Using all indicators, the condition of the grey seal is assessed as good in Finland.

Grey seals on a rocky islet.

In the Baltic Sea, the ringed seal population is particularly concentrated in the Bay of Bothnia. This is because the ringed seal is dependent on ice and snow, especially during the breeding season. The latest census result for the Bay of Bothnia counted almost 13,000 ringed seals. The inter-annual variation in these population count results is particularly affected by the ice conditions at the time of the calculation.

Both the Archipelago Sea and the Gulf of Finland have separate ringed seal populations. The number of ringed seals is extremely low in both areas and their populations have not shown any signs of recovery. Climate change threatens the survival of the ringed seal, especially in these more southern areas.

The harbour porpoise is extremely rare in Finnish sea areas

The harbour porpoise is the only whale species found in the Baltic Sea. Until a hundred years ago, porpoises were found in all Finnish sea areas. Today, only a few are detected per year. The porpoise occurs here at the extreme limits of its distribution.

The main reason for the decline of harbour porpoises is considered to be the intensification of fishing; porpoises are drowned in various nets, especially in the southern Baltic Sea.

In the past, i.e. more than a hundred years ago, porpoises were also hunted, which could have caused the decline of the population. Today, harbour porpoises are thought to suffer from, among other things, shipping, as well as underwater noise.

The status of seabirds has undergone major changes in recent decades

Finland's fragmented coastline and archipelagos offer good living conditions for many seabirds. However, there have been major changes in the composition and abundance of the species over the last 30 years. The are many reasons for such changes.

A total of 29 bird species nest in Finnish sea areas. Many are also found in inland waters. Some species only found at sea include the razorbill, eider duck, and the black guillemot.

In the last 30 years, about half of the nesting birds in Finnish sea areas have increased in abundance, while the rest have declined. Successful species include many large birds, such as the whooper swan, geese species, and cormorants. On the other hand, for example, duck species, such as the velvet scoter and northern shoveller, as well as the long-tailed duck that overwinters in the Baltic Sea, have declined. There has been a varied development in the abundance of wading birds and gull species.

The purple sandpiper.

Indicator species describe the status of seabirds

The white-tailed sea eagle has made an impressive return to the Finnish coast. Formerly reduced by both persecution and environmental toxins, the sea eagle population has increased to such a degree in recent decades, that its status in Finnish sea areas is now mainly assessed as good.

The white-tailed sea eagle is one indicator species for the status of seabirds. Species which are important in the food web of the Baltic Sea and which do not occur in inland waters are chosen as indicator species. Both abundance and distribution are monitored for these species.

The eider duck, i.e. Somateria mollissima, is an example of an indicator species whose population has declined sharply, especially on the south and southwest coasts of Finland. The decline of the eider duck began as early as the 1980s in the Gulf of Finland and has continued to this day.

There are more overwintering birds on the Finnish coast

The Baltic Sea is an important wintering area and migration route for Arctic birds. More seabirds are now wintering in Finnish sea areas than ever before because climate change has shrunk the ice sheet and made conditions more favourable.

However, this does not mean that these bird populations could have generally increased. For example, the number of long-tailed ducks in the entire Baltic Sea has fallen sharply. The main reason for this decline is probably found in the nesting areas of the species, i.e. in the northern tundra. However, long-tailed ducks are also experiencing distress in their wintering areas in the Baltic Sea, where they are hunted and also drowned accidentally in fishing gear.

The changes also affect the relationships between species

The barnacle goose is a new nesting species in the Baltic Sea. It first nested on the island of Gotland, off the southeast coast of Sweden in the early 1970s, before arriving in Finland about ten years later. Since then, the nesting population has grown strongly.

Unlike the barnacle goose, the cormorant is a returnee; cormorants lived on the coast of Finland until the end of the 19th century when the entire Baltic Sea population was destroyed for almost a century. Cormorant communities can benefit some other seabirds, as the rocky islets they nest upon protect their eggs being stolen by gulls, crows, and ravens.

The strengthening of sea eagle populations, in turn, has reduced the success of the eider duck, as many eiders end up as prey for eagles. However, the greatest threat to many seabirds is posed by terrestrial predators, especially mink.

The status of fish stocks varies in Finnish sea areas - migratory fish are particularly endangered

The fish stocks of the Northern Baltic Sea are composed of a unique mixture of both marine and freshwater fish species. In Finnish marine areas, the status of many fish stocks varies naturally. Also, fish stocks are affected by, among other things, fishing pressure, environmental changes, and the presence of alien species.

Fish species which occur in the sea are often divided into open water and coastal species, as well as migratory fish. The most abundant open water fish species are herring and sprat. By contrast, the coast is home to many fish which are also familiar from inland waters, such as roach and perch. The best-known migratory fish are salmon and sea trout.

Commercial fish stocks are maintained

The most important catch species in Finnish commercial fisheries are herring and sprat. Their stocks are monitored annually. Monitoring provides information and forecasts, which form the basis for deciding on country-specific catch quotas for herring and sprat within the European Union. The stocks of these two fish species have remained abundant throughout the Baltic Sea.

A shoal of herring.

Although coastal fishing is also regulated, it is done so using national fishing restrictions rather than catch quotas from the EU. In Finland, for example, the permitted gear types and minimum fish catch sizes are controlled. Such regulation aims to ensure that fishing is sustainable. Important prey fish species include perch, whitefish, and pikeperch. In particular, the stocks of perch are abundant in many places.

Eutrophication and other changes affect fish stocks

For coastal fish stocks to remain viable, fish must have good conditions for reproduction. Coastal construction, dredging, and other changes can damage or destroy important fish reproduction areas. Similarly, changes in water quality, such as acidification, also occasionally cause major problems for fish, particularly during their sensitive reproductive phase.

Both eutrophication and rising water temperatures affect the composition of fish communities. Some fish species can cope with and even benefit from such conditions, including pike and cyprinid fish, such as roach and bream. Many fish species that thrive in cold waters, in turn, suffer as both the climate and sea warm. These include the marine-dwelling grayling (Thymallus thymallus) and the burbot (Lota lota). In the latest endangerment assessment, the burbot is now classified as a near-threatened species.

Migratory fish are widely endangered

Many migratory fish species have been assessed as being endangered. Those species with the weakest status include the critically endangered European eel, as well as the endangered sea trout.

These threat levels are mainly because the dams built on rivers prevent fish from entering their spawning grounds. Obstacles which prevent migration, such as dams and other changes in river nature have collapsed both salmon and sea trout stocks throughout the entire coastal areas of Finland. The distress of these fish is further compounded by the silting up of the gravel spawning beds in those few rivers that have not been built upon. Moreover, climate warming also poses additional challenges.

Attempts are being made to improve the status of migratory fish by rehabilitating streams and also by restricting fishing. Indeed, the measures have proved successful in many places, and migratory fish have been able to return to their spawning grounds.

Plankton communities suffer from a lack of salty water

In the Baltic Sea, plankton communities live in brackish water. This is reflected in the species composition of the plankton, which has both marine and freshwater species. Marine species require a fairly high salinity to live, and their number in the Baltic Sea has recently declined.

The salinity of the Baltic Sea is affected by both the rivers flowing into the sea, which bring freshwater, and the occasional inflows of seawater from the Danish Strait, i.e. the so-called salt pulses. When there is hardly any saline water entering the Baltic Sea, marine species decline. Climate change is expected to further decrease the salinity of the Baltic Sea, which will be reflected in plankton communities.

A phytoplankton (plant plankton) community.

The status of plankton communities and their habitat is assessed by monitoring both phytoplankton and zooplankton species. Both indicators provide evidence of habitat eutrophication, e.g. cyanobacteria have become more abundant and zooplankton increasingly contains the small-sized species typical of eutrophicated waters.

The altered plankton community provides poorer nutrition for herring and sprat, as well as other plankton-eating fish. These changes are stronger in the Gulf of Finland than in the Gulf of Bothnia.

Eutrophication has affected the status of the sea bottoms

The seabed offers a wide range of habitats for the species of the Baltic Sea. However, eutrophication has changed the living conditions of the seafloor. This is reflected in the benthic communities.

There is a wide range of different seabed habitats found in Finnish sea areas. Bottoms composed of mud, gravel, sand, and rock each maintain their own kind of biotic communities. Where there is enough light, vegetation increases habitat diversity. By contrast, the dark zone is entirely composed of organisms from the animal kingdom. The animals that live there eat, among other things, plant waste that sinks to the bottom from above.

Species and oxygen content indicate the status of the seabed

The status of the seabed is assessed directly with the help of aquatic vegetation and benthic fauna, as well as by measuring the oxygen content of the water near the seafloor.

Oxygen content is an important indicator because it dictates an organism’s possibility for life. In a eutrophicated sea, there is a threat that oxygen will become depleted near the bottom. This is because the decomposition of dead plankton and other organic matter which sink to the bottom consumes all of the available oxygen.

Eutrophic bottoms occur along the coastline in all Finnish sea areas. Eutrophication is particularly evident in the decline of aquatic vegetation, as well as the decrease in plant biodiversity to a single species. Bladder wrack kelp and red algae are no longer found in the shallowest waters. In eutrophic areas, the proportion of sensitive aquatic plants in the community is also lower.

Bay barnacles and blue mussels.

There are large regional differences

Of all the Finnish sea areas, the Gulf of Finland and the Archipelago Sea are the most eutrophic. Deoxygenation particularly affects large areas of the bottom of the Gulf of Finland.

By contrast, the deep bottoms of the Gulf of Bothnia are oxygenated with a diverse benthic fauna. However, the gradual progression of eutrophication has caused the oxygen content of the seafloor to slowly decline in the Gulf of Bothnia also.

The habitats of the Bay of Bothnia and the Kvarken are still largely in good condition. In the Bothnian Sea, however, the status of the seabed has deteriorated, especially near the coastline.