State of the Baltic Sea

one of the world’s most unique and most polluted marine environments

 

Baltic Sea and its characteristics

This shallow, semi-enclosed sea is characterised by its brackish water – a mixture of salt and freshwater. The Baltic Sea’s low salinity is due to a high inflow of freshwater from rivers combined with minimal saltwater exchange with the North Sea. Its salinity is just one-tenth that of the open ocean, creating a brackish environment that supports a variety of life adapted to both fresh and saltwater conditions. This allows for a unique mix of species that wouldn’t typically coexist in fully saltwater or freshwater environments.

The Baltic’s geography further contributes to its unique status as a semi-enclosed sea with limited water exchange with the North Sea. Underwater ridges separating the deep basins cause saltier water to settle at the bottom, which remains isolated until occasional saline pulses from the North Sea refresh it. This stratification, or separation into layers, limits oxygen movement from the surface to deeper areas. It can take as long as 20–30 years for the water in the Baltic Sea to be replaced. These geographical and hydrological characteristics create a fragile ecosystem.


Environmental challenges 
The limited exchange with the North Sea means that oxygen-rich water only sporadically reaches the Baltic’s depths. When these deep layers lose oxygen, “dead zones” develop, where marine life cannot survive. 

Adding to this problem, oxygen-poor seabeds release phosphorus – a process known as internal loading, that intensifies eutrophication. Eutrophication, an excess of nutrients in the water, promotes algae blooms and can further deplete oxygen levels, creating a cycle that worsens the problem. Currently, around one fifth of the Baltic Sea’s deep seafloor lacks visible life due to low oxygen, spanning an area approximately the size of 1.5 Denmarks. 

Figure 1: Map of the Baltic Sea – the numbers 1 to 9 identify the basins, as shown in table.

The Baltic Sea in numbers

12 000 years

418 690 km²

the Baltic Sea’s age

surface area

459 metres

8 000 km

maximum depth

shoreline length

450 km³

50 000

freshwater inflow / year

islands

0.1-0.2 %

51 metres

salinity

average depth

20 000 km³

30 years

people living in the drainage area

shipwrecks

for the entire water to change

water volume

90 million

100 000

State of the Baltic Sea

The Baltic Sea’s unique biodiversity has big ecological, economic and cultural importance for people in the region. There are multiple pressures that put the Baltic Sea’s ecosystem and hence human well-being at risk. Eutrophication is the most pervasive and serious environmental problem in the Baltic Sea. Other pressure that put the health of the Baltic Sea at risk are hazardous substances, underwater noise, the extraction of fish, physical disturbances and the introduction of non-indigenous species. 

Figure 2. Extent of hypoxic and anoxic bottom water. Source: SMHI

Eutrophication

Eutrophication is the most critical environmental challenge facing the Baltic Sea. Driven by excess nutrient loads, the phenomenon leads to severe ecological imbalances, harming marine life and human activities alike.

  • Eutrophication occurs when excessive amounts of nutrients, such as nitrogen and phosphorus, are introduced into marine ecosystems. These nutrients act as fertilizers, boosting the growth of algae and other aquatic plants. While nutrients are essential for life, an overabundance can lead to environmental imbalances. 

  • In the case of the Baltic Sea, eutrophication has been an ongoing problem for decades. Most of the nutrient inputs originate from human activities on land and on sea, reaching the sea by water and air. The main sources of nutrient pollution in the Baltic Sea include:  

    Agricultural Runoff 
    Agriculture, the largest human-caused contributor, releases significant nutrients into waterways through runoff from fertilizers and animal waste. This runoff causes eutrophication and low oxygen levels, which negatively affects marine life. Sustainable farming practices, including buffer zones and better manure management, are critical to reducing nutrient leakage into the sea. 

    Wastewater Discharge 
    Despite improvements in wastewater management, many cities and towns still discharge inadequately treated wastewater, which carries excess nutrients. Urban runoff, from areas like streets and lawns, also contains pollutants that contribute to nutrient loads. 

    Natural Background Sources 
    Natural Background significantly contributes to nutrient loading in the Baltic Sea through natural processes like soil erosion, rock weathering, and non-anthropogenic atmospheric deposition. These consistent, non-human inputs form the baseline nutrient load and must be distinguished from anthropogenic sources to effectively manage eutrophication. 

    Other Sources: 

    • Forestry 

    • Stormwater runoff 

    • Private sewage systems 

    • Industrial processes, particularly pulp and paper production 

    • Fish farming (causing localised nutrient loading) 

    • Shipping activities (contributing to overall ecological stress and nitrogen deposition) 

    Addressing eutrophication in the Baltic Sea requires a comprehensive approach targeting these various sources, with a particular focus on sustainable agricultural practices and improved wastewater management. 

  • The effects of eutrophication in the Baltic Sea are far-reaching, impacting the ecosystem, economy, and human well-being.  

    • Algal Blooms
      One of the most visible effects is harmful algal blooms, especially cyanobacteria. These blooms can cover vast areas of the sea, reducing light penetration and depleting oxygen levels in deeper waters. Some of these blooms are also toxic, posing risks to human health, tourism, and fisheries. 

    • Hypoxia and Dead Zones
      Eutrophication also leads to hypoxic (low oxygen) conditions, as the decay of excess organic material consumes large amounts of oxygen. In severe cases, this results in anoxic areas, so called "dead zones", where oxygen levels are so low that most marine life cannot survive. Currently, around 20 percent of the Baltic Sea’s deep seafloor, an area approximately 1.5 times the size of Denmark, is anoxic. This makes the Baltic Sea home to seven out of the world’s ten largest marine “dead zones”. 

    • Loss of Biodiversity 
      Eutrophication disrupts marine ecosystems, favoring fast-growing species like algae while diminishing sensitive species, leading to biodiversity loss and habitat degradation.  

    • Impact on Fisheries  
      The decline in oxygen levels and the degradation of habitats, particularly spawning grounds for fish species, negatively affect fish populations, including commercially important species like cod. This leads to reduced catches, threatening the livelihoods of fishing communities around the Baltic.  

    • Economic and Social Costs
      The economic costs are significant, with the tourism industry suffering from algae-covered beaches and the fishing industry impacted by declining fish stocks. 

  • To combat eutrophication, HELCOM (Helsinki Commission) an intergovernmental organisation dedicated to protecting the Baltic Sea environment. Established in 1974, it brings together the nine countries bordering the Baltic Sea and the EU to collaborate on environmental protection efforts. HELCOM developed the Baltic Sea Action Plan (BSAP), which provides a comprehensive strategy for reducing nutrient loads and restoring the health of the Baltic Sea. The BSAP sets specific targets for reducing nutrient inputs, protecting marine species, and promoting sustainable use of the sea.

    A key part of this strategy is the concept of Maximum Allowable Input (MAI), which sets science-based limits on the amount of nutrients that can safely enter the Baltic Sea without causing eutrophication. Since the 1980s, the annual phosphorus inflow has decreased by over 40,000 tonnes. However, an additional reduction of 5,500 tonnes is still required to reverse the trend. 

    By establishing MAI limits, the BSAP encourages countries around the Baltic to implement measures such as improved wastewater treatment, sustainable agricultural practices, and effective nutrient management policies to reduce nutrient runoff and atmospheric deposition. This coordinated approach aims to achieve a Baltic Sea with balanced nutrient levels, preventing the harmful algal blooms and hypoxia that result from excessive nutrient loads.  

    Through coordinated regional efforts like the HELCOM Baltic Sea Action Plan, there is hope for reducing nutrient pollution and restoring the Baltic Sea’s ecological balance. 

Green algae blooms swirling around the Baltic Sea. Photo: European Space Agency

Figure 3. Source: PLC 8 Results

Other Baltic Sea pressures and key industries

  • Hazardous substances

    The Baltic Sea is among the world’s most polluted seas, with hazardous substances such as heavy metals, persistent organic pollutants (POPs), and emerging contaminants like pharmaceuticals posing serious risks. These pollutants threaten marine life, human health, and regional economies. They originate from human activities and enter the sea through wastewater treatment plants, surface runoff, and airborne deposition. Addressing this issue requires stricter regulations, improved wastewater treatment, and sustainable industrial practices to reduce pollution and protect the fragile ecosystem of the Baltic Sea for future generations.

  • Marine litter

    Marine litter originates from various land- and sea-based sources, accumulating in marine and coastal environments. It primarily consists of plastics, including packaging, fishing gear, and microplastics. This pollution poses a serious threat to the Baltic Sea’s ecosystem and human health. Wildlife can ingest or become entangled in debris, causing injury or death, while microplastics enter the food web, affecting marine life and humans consuming seafood. Addressing this issue requires stricter waste management, improved regulations, and public awareness to reduce plastic pollution and its harmful impacts.

  • Non-indigenous species

    Non-indigenous species are organisms that have spread or been introduced into environments where they did not naturally exist, often through maritime transport. These species can disrupt ecosystems by altering their structure and dynamics, potentially harming native biodiversity. Between 2016 and 2021, thirteen non-indigenous species were recorded for the first time in the Baltic Sea, highlighting a growing trend. As global trade and climate change continue to facilitate their spread, monitoring, preventive measures, and effective management strategies are crucial to reducing their impact on marine ecosystems. Photo: Chinese mitten crab © Matz Berggren

  • Maritime

    The Baltic Sea is one of the most heavily trafficked seas worldwide. Up to 2000 larger vessels like cargo ships, tankers, and passenger ferries sail in the Baltic Sea at any given time impacting the environment through air and water pollution, invasive species, oil spill risks, and underwater noise. Nitrogen oxide emissions and discharges from wastewater, food waste, and scrubber wash water contribute to eutrophication. Ballast water releases introduce invasive species, while underwater noise disrupts marine life. Oil spills remain a significant threat to the region's biodiversity and water quality.

  • Agriculture

    Agricultural runoff is a large source of nutrient pollution to the Baltic Sea. Sustainable farming practices, such as buffer zones, precision agriculture, improved manure management, and efficient fertilizer use, are essential to mitigate this impact. These methods help reduce nutrient leakage into waterways, preventing excess nitrogen and phosphorus from harming marine life. Additionally, techniques like crop rotation, cover cropping, and wetland restoration improve soil health and water retention. Strengthening policies, farmer incentives, and public awareness further support the shift toward more sustainable agriculture.

  • Fisheries

    Overfishing has led to severe declines in fish stocks, particularly herring, with around 90% of catches used for fishmeal rather than human consumption. This depletion results in economic losses for commercial fisheries and disrupts marine ecosystems, impacting predators that rely on fish as a food source. To address these issues, experts recommend implementing ecosystem-based management, reducing catch quotas, and protecting spawning areas to allow fish populations to recover. Additionally, promoting alternative protein sources, strengthening enforcement of regulations, and supporting sustainable fishing practices can help ensure long-term marine biodiversity and food security.

  • Energy

    The Baltic Sea's expanding offshore wind and marine renewables sector must carefully select sites, avoid disrupting sensitive habitats and marine life seasons, and enhance cross-border cooperation to balance energy development with ecosystem protection. Conducting thorough environmental impact assessments, using nature-inclusive designs, and applying mitigation measures can help minimise harm to biodiversity. Additionally, investing in innovative technologies, habitat restoration, and long-term monitoring programs will support a responsible transition to renewable energy while preserving the region’s fragile marine ecosystem.

  • Wastewater treatment plants

    Essential for protecting the Baltic Sea ecosystem by reducing harmful nutrient and pollutant discharges that contribute to eutrophication. Modern WWTPs in the region can remove over 96% of phosphorus and more than 90% of nitrogen from wastewater, significantly improving water quality. Despite these advancements, challenges persist, including varying treatment standards, increased capacity pressures due to population growth, and knowledge gaps among operators. As more WWTPs implement advanced technologies, their positive impact on the Baltic Sea ecosystem is expected to grow, aiding in the reduction of eutrophication and improving overall water quality. 

Planetary Boundaries and the Baltic Sea

A problem of global scope
The concept of planetary boundaries outlines nine key thresholds critical for maintaining Earth's stability. These boundaries define the safe operating space for humanity. Crossing them risks pushing ecosystems toward irreversible changes.

As of the latest Planetary Boundaries assessment, humanity has exceeded six of the boundaries. Among these are areas like biochemical flows of nitrogen and phosphorus, climate change and biodiversity loss — issues that are particularly relevant for the Baltic Sea.

What affects the Baltic Sea on a regional level is interconnected with the Planetary Boundaries and environmental issues on a global level. That is why efforts to restore the Baltic Sea’s ecological balance, like the reduction of nutrient loads, are critical for safeguarding the environment regionally and globally.

The Planetary Boundaries over time 

Climate change and the Baltic Sea

Global warming is causing water temperatures to rise - even faster in the Baltic Sea than in most other seas. Already today, this profoundly affects the sea’s ecosystems, its water quality, and the coastal communities. The effects of climate change are expected to worsen in the coming years and the need for stronger actions for the health of the Baltic Sea is bigger than ever. 


Increases eutrophication 
Higher average precipitation will increase nutrient run-off from agricultural and urban areas. Nutrient discharge and rising water temperatures again promote algae growth and accelerate eutrophication in the sea. 


Increases the risk of biodiversity loss  
Warmer temperatures, decreasing ice cover and possible changes in salinity can alter the distribution and behavior of marine species profoundly. This can disrupt the function of the food web and the marine ecosystem leading to changes in productivity and resilience.

Solutions for a healthy Baltic Sea

The Baltic Sea's health is one of the most urgent environmental challenges for the nine surrounding countries, affecting diverse sectors from fishing to tourism. Addressing these issues requires robust international cooperation and immediate action. 

The sea is essential for millions who depend on it for livelihoods and recreation. However, the ecosystem's deterioration threatens these benefits. Revitalising the Baltic Sea can enhance its biodiversity and boost its role in absorbing greenhouse gases, crucial for mitigating climate change. We know enough to act – it's time to focus on concrete solutions.

Human activities put intense pressure on the Baltic Sea. With 90 million people depending on it for industries like shipping, tourism, and fishing, the sea is essential for local economies. However, this reliance comes at a cost to the ecosystem, requiring sustainable management to balance economic growth with environmental protection. 

Investing in the Baltic Sea's restoration not only benefits the environment but also presents significant economic opportunities. A healthier Baltic could generate €32 billion annually and create up to 550,000 jobs by 2030.  

  • The global blue economy is valued at around $2.5 trillion annually, ranking it as the world’s 7th largest economy . It includes industries such as fisheries, aquaculture, maritime transport, and offshore renewable energy. The blue economy is expected to double in size by 2030 compared to 2010 levels.

    Beyond its economic worth, the blue economy also supports over 260 million jobs worldwide in fisheries and aquaculture, contributing $100 billion annually to the global economy.

    Blue economy is an important part of the economy in the Baltic Sea Region, directly employing more than 700,000 persons. By promoting a sustainable blue economy, stakeholders in the Baltic Sea Region can work towards restoring the sea's health while simultaneously fostering economic growth and innovation. This approach offers a promising path for balancing environmental protection with economic development, making it a key solution for the long-term sustainability of the Baltic Sea.

  • In the leaflet Key measures for the Baltic Sea researchers and experts from the Stockholm University Baltic Sea Centre present some key measures to improve the environment of the Baltic Sea, from the Bothnian Bay to the Kattegat.

Funding

If your municipality or start-up wants to work on improving the water quality and ecological status of the Baltic Sea, you can access various grants and funding sources that offer the best potential.  

The Swedish Agency for Marine and Water Management (SwAM/HaV) provides a comprehensive platform for grants, calls, and funding opportunities aimed at improving marine and freshwater environments. Whether you are working on local water quality projects, reducing nutrient runoff, or restoring ecosystems, SwAM offers tailored support to help you achieve impactful results. Visit SwAM dedicated page to explore funding options, application guidelines, and current calls for proposals. 

In addition, the brochure "Var finns pengarna?" (Where’s the Money?) from the County Administrative Board of Sweden provides an overview of funding opportunities for environmental and water-related projects. It is aimed at municipalities, organisations, landowners, and businesses working to improve water quality and ecosystems.

What can I do to help?

Everyone can contribute to improving the Baltic Sea’s health: 

  • Reduce Water Pollution: Use eco-friendly cleaning products and minimise fertilizer use. 

  • Conserve Water: Lowering water usage reduces wastewater entering the sea. 

  • Support Sustainable Seafood: Choose certified sustainable seafood and avoid overfished species. 

  • Participate in Cleanups: Join local beach cleanups and volunteer for restoration projects. 

  • Raise Awareness: Engage in discussions about the Baltic Sea’s challenges and share information on sustainable practices.  

  • Baltic Sea Day: engage in an annual day of celebration in honour of the sea.  

By making environmentally conscious choices and advocating for sustainable policies, individuals can play a crucial role in protecting the Baltic Sea for future generations. 

Sources

Page updated: May 2025