The Baltic Sea is a unique and sensitive sea where organisms live on the edge of what they can tolerate due to the low salinity. It has also been called the world’s most polluted inland sea due to eutrophication and the chemical cocktail caused by human activities.
The Baltic Sea consists of the Gulf of Bothnia, Gulf of Finland, Gulf of Riga, and the Baltic Proper. The sea has a fascinating history of being both a sea and a lake, and today it is a bit of both with its brackish water. Before the last ice age, the Baltic Sea was much saltier than it is today, explaining the coexistence of both marine and freshwater species. The high sea level at that time connected the Baltic Sea with the salty waters of the North Sea and the White Sea. During the ice age, the entire Baltic Sea was covered by several kilometers of thick inland ice. It was only around 10,000 years ago that the ice retreated, making the Baltic Sea a relatively young sea.
The transition to the current brackish water is due to significant changes the Baltic Sea has undergone since the last ice age, such as land uplift, a sinking water surface, and reduced connection to the North Sea. The Baltic Sea is also unusually shallow, with an average depth of 54 meters compared to the Mediterranean’s 1500 meters. Like the Mediterranean, the Baltic Sea is an inland sea because it is only connected to larger seas through the narrow Danish straits. This means that it takes about 30 years for the water in the Baltic Sea to be completely exchanged, allowing old pollutants to persist in the area.
Today, the Baltic Sea is characterized by significant differences in salinity, ranging from around 25 parts per thousand in the Kattegat to two parts per thousand in the northern Gulf of Bothnia. The only influx of saline water occurs through the narrow straits of the Øresund and the Belts. The low salinity in the north is due to the numerous rivers and streams that flow into that area. This influences the types of species that can thrive in different areas. The brackish water is too low in salinity for marine species while it is too saline for the freshwater-origin species in the Baltic Sea. This leaves only a small number of species that are able to handle these challenges. Some species, like blue mussels, have adapted to the low salinity, resulting in them staying small, around three centimeters, compared to up to ten centimeters on the west coast where salinity is higher.
Because there are relatively few species in the Baltic Sea, the food chain is simpler. It has taken thousands of years for the few species living in the Baltic Sea to adapt to the low salinity. This makes the Baltic Sea a particularly sensitive environment because if one species disappears, there may not be another species to take over its role, which can have significant consequences for the entire ecosystem, as different levels in the food chain influence each other. The loss of key species, such as bladderwrack and top predators like seals and pike, can have particularly large consequences.
In addition to the natural challenges faced by the Baltic Sea, humans have long polluted and exploited the sea beyond its capacity. Over 90 percent of the Baltic Sea’s marine area is in poor condition, according to the latest assessment by the Baltic Sea collaboration HELCOM. A significant portion of human impact originates from land.
At the beginning of the 20th century, the Baltic Sea was a nutrient-poor marine area. However, as the population around the Baltic Sea grew, the pressure from intensive forestry in the north and agriculture in the south increased. Today, the sea is affected by an area (catchment area) about four times the size of the Baltic Sea itself. Around 90 million people from nine countries in this area impact the sea with nutrients, pollutants, and other land-based contaminants flowing into the sea. Sweden has the longest coastline among the countries. Excess nutrients, such as nitrogen and phosphorus, lead to eutrophication, radically reshaping the ecosystem.
Eutrophication primarily leads to proliferation of phytoplankton. Phytoplankton, often referred to as blue-green algae, are actually cyanobacteria that bloom in the summer due to favorable warm and calm conditions. They are also known for producing toxins that can be harmful to animals and children. Algal blooms are a natural process, but when intensified by eutrophication, the algae increase, leading to more oxygen consumption by the bacteria breaking them down when they reach the bottom. This can cause oxygen deficiency and dead zones.
Today, 97 percent of the Baltic Sea is affected by eutrophication, and 21 percent of the seabed is oxygen-free, leaving only bacteria and fungi able to survive in these areas. This corresponds to an area more than twice the size of Småland. The oxygen deficiency that largely characterizes the Baltic Sea is partly due to it being a brackish sea where oxygen-rich and fresher surface water does not mix well with the saltier and heavier bottom water. Therefore, the Baltic Sea is already sensitive to oxygen deficiency, and in combination with external stressors like eutrophication, it can have devastating consequences.
The lack of oxygen is a problem for many organisms, such as cod, which needs oxygen-rich water to reproduce. Oxygen deficiency also releases phosphorus from sediments, worsening eutrophication, known as internal loading, and creating a vicious circle of nutrient leakage.
To counteract the negative consequences in the Baltic Sea, biodiversity is crucial. It acts as an insurance, making the sea more resistant to external influences. The Baltic Sea is estimated to have around 100 fish species, 450 species of macroalgae, 1000 benthic animals, 3000 plankton species, and thousands of unknown bacteria and viruses. This unique biodiversity is threatened by both eutrophication and climate change, as it alters the marine environment. This deteriorates the living conditions for some species, such as bladderwrack, while other species, such as fast-growing filamentous algae, take over their habitats.
Harbor porpoises are one of the species currently classified as critically endangered in the Baltic Sea. Like cod and herring, harbor porpoises are threatened by fishing, as they are caught as bycatch. The porpoise, which uses sound for communication, is also disturbed by the intense traffic generating underwater noise. This has led to the protection of some of the porpoise’s important reproductive areas.
Currently, around 16.5 percent of the Baltic Sea is protected, but to fulfill Sweden’s commitments under the UN Convention on the Law of the Sea, the protected areas should almost double by 2030. By reducing the burden on the sea, scaling down large-scale fishing for species like herring and cod, and establishing new marine protected areas, one can support both individual species and the overall biodiversity in the Baltic Sea.
Despite facing significant challenges, the efforts made in the past have yielded results. For instance, pollutants like PCB and lead have decreased after being banned. This has allowed animals like sea eagles and seals to recover from the effects of these toxins. In recent decades, nutrient input into the Baltic Sea has also significantly decreased. However, due to the large amounts of nutrients stored in the water, it takes a long time for the effects to become visible.
TEXT: Lina Mattsson
In February 2026, historically low water levels were recorded in the Baltic Sea, in some places, the lowest in almost 100 years. This was mainly due to a stable high-pressure system and strong easterly winds over Scandinavia, which pushed water out of the Baltic Sea. There was hope that fresh, oxygen-rich and salty water would flow in from the North Sea to the heavily tested inland sea and its oxygen-depleted seabed – but that did not happen. Find out why in the news report.
The UK and Norway have carried out a military operation to deter Russian submarines suspected of “hostile activities” in the North Atlantic.
Two vessels suspected of belonging to the Russian shadow fleet remain off the coast of Trelleborg. In March 2026, the Swedish authorities boarded the vessels, which are believed to be transporting Russian oil under a false flag. Russia regards this type of action as piracy.
The European Commission is presenting new guidelines on how Sweden and other countries can tackle growing cormorant populations. “We are continuing to protect birds and fish, whilst providing farmers, fishermen, foresters and businesses with practical and balanced rules,” writes Environment Commissioner Jessika Roswall in a press release. ”Vi fortsätter att skydda fåglar och fisk och ger samtidigt bönder, fiskare, skogvaktare och företag fungerande och balanserade regler”, skriver miljökommissionären Jessika Roswall i ett pressmeddelande.
The Russian shadow fleet is a hot topic as the ten countries in the UK-led defence partnership JEF meet in Helsinki. ‘The shadow fleet must be tackled more rigorously,’ says British Prime Minister Keir Starmer as the meeting begins.
Keep Sweden Tidy has been working with various partners in Finland and Estonia to remove abandoned fishing gear from the Baltic Sea. At the same time, it has been raising awareness of the problem of ghost nets, amongst other things, among recreational anglers and the general public. Over the three years the project has been running, nearly 9,000 metres of ghost nets and over two tonnes of lost fishing gear have been retrieved from the sea.
It may be some time before the two vessels, which are suspected of being part of the Russian shadow fleet, are able to leave Swedish waters. ‘It will probably not happen quickly,’ says Mattias Lindholm, press officer at the Swedish Coast Guard.
The search for sites in Sweden where carbon dioxide can be stored beneath the seabed has been underway for three years. Two potential sites have now been identified. ‘We've received some excellent samples and the results so far are very promising,’ says Sofie Lindström of the Geological Survey of Sweden (SGU).
The Baltic Sea countries will strengthen the monitoring of underwater cables and ships. The Coast Guard will be the Swedish hub for sharing information and protecting cables. ‘Together, we will become sharper and more powerful in detecting and responding to various incidents, especially those related to cable security,’ says Lena Lindgren Schelin, Director General of the Coast Guard.
The Baltic Sea is suffering from a serious ecological crisis. Above all, it is the high levels of phosphorus that cause oxygen-depleted areas and severe recurring algae blooms. Phosphorus is an essential element and an important component in agricultural fertilisers, but when it leaks into the Baltic Sea, it has a severe impact on the sea.
This year, water equivalent to almost two Lake Vänerns has been pushed out of the Baltic Sea. At the same time, the record low water level offers some hope for the oxygen-depleted seabed of the inland sea. ‘But it would be a bit like winning five Triss lottery tickets in a row,’ says Jörgen Öberg, oceanographer at SMHI.
The Kunming-Montréal Framework was adopted at CBD COP15 in Montréal in 2022. By COP16, all countries were to develop an action plan on how to implement the targets included in the biodiversity framework, including the target of protecting 30 per cent of nature in the oceans and on land. Three years later, Sweden still does not have such an action plan.
The ship Fitburg, which was seized by Finnish police last week, will be escorted out of Finland into international waters on Monday, according to the National Bureau of Investigation.
Finland märker ingen minskning av rysk oljeexport genom Östersjön. Däremot har riskerna ökat när skeppen i ”skuggflottan” blir allt skraltigare, och sjönavigation störs ut i den trånga Finska viken. - Det värsta scenariot vore en kollision med en fullsatt Finlandsfärja, säger kommodor Mikko Simola ombord på bevakningsfartyget Turva.
For the first time in half a century, Sweden is investing in large modern warships. The new frigates cost billions and can hunt submarines, act as floating air defence and secure the vital sea route to Sweden.
Toxic bathing water, dead fish and dead seabeds – the impact of climate change on the sea is becoming increasingly clear. But the fight against marine death begins well inland. ‘2018 was a wake-up call,’ says the Swedish farmer Erika Olsson, one of many who have created wetlands.
The government wants to remove the Environmental Code's ban on nuclear power in virtually all Swedish archipelagos. This would open the way for nuclear power plants in Bohuslän, Öland, Gotland, and the Stockholm archipelago, among other places.
All parts of the oceans are under triple attack, according to the EU's Copernicus Climate Service's major annual ocean report. ‘These changes affect us all – ecosystems, society and the economy,’ says oceanographer Karina von Schuckmann.
1.6 million tonnes of ammunition are rotting on the seabed of the Baltic and North Seas. This rubbish from the Second World War is a ticking time bomb. As the casings rust, they release chemicals that slowly poison the seas.
The European Commission is clearly hesitant to change the protection status of the great cormorant, despite pressure from Sweden and other countries.
What will you remember from the inauguration of a national park in your own neighbourhood? A sunny autumn day? Absolutely. Some more or less pertinent phrases from your Minister of Climate and the Environment? Maybe, but mostly the fact that she wasn’t even born when the park was first discussed. A trumpeter and two slightly wayward sheep? Yes, the unexpected will always stick to your memory.
When researchers talk about ‘missing data,’ they are referring to replacing previously estimated values with actual, real values in various explanatory models. This can be particularly important in climate change models. Even though approximate values provide a good picture of future climate change, the more reliable the measurement points are, the more robust the model becomes.
The European Commission proposes that the fishing quota for herring in the central Baltic Sea – which was doubled this year amid widespread criticism – be kept. At the same time, it wants to reduce herring fishing in the Gulf of Bothnia.
An oil field described as the largest ever found in Poland has been discovered off the coast of Świnoujście, reports Dagens PS.
