​​The Baltic Sea is a unique marine ecoregion and home to a vibrant mix of marine, brackish, freshwater, and non-indigenous species. Nestled between nine countries, it is an economic and cultural cornerstone for millions, but it is also one of the most heavily impacted marine ecosystems globally. The newly updated  Baltic Sea ecoregion – Ecosystem overview - highlights this region's delicate balance and underscores the urgent need for coordinated action to tackle its key pressures. Leading the work for the updated overview has been Carolynn Faithfull, Swedish University of Agricultural Sciences, and Riikka Puntila-Dodd, Åbo Akademi University, Finland. We spoke to them about the main pressures in the region.

Unpacking the key signals

Covering approximately 420,000 square kilometres, the Baltic Sea's shallow, semi-enclosed structure sets it apart. With an average depth of only 60 meters, limited water exchange with the North Sea, and strong vertical stratification, the sea acts as a sink, trapping nutrients and pollutants for many decades. These characteristics amplify human impacts in a region where the catchment area is three times the size of the sea itself and spans nine nations.

Nutrient runoff from agriculture, forestry, and wastewater discharge dominates as the most significant pressure. Nutrient enrichment causes eutrophication, triggering harmful algal blooms, depleting oxygen levels, and creating hypoxic and anoxic zones, especially in deeper basins.

A related and compounding issue is contamination from shipping, industrial activities, and emerging pollutants like microplastics and persistent organic compounds. The semi-closed nature of the Baltic Sea means contaminants accumulate and can have widespread, long-lasting effects on marine biota.

Fisheries, traditionally a dominant pressure in other marine ecosystems, play a smaller but still vital role here. Declining cod stocks due to overfishing and ecological imbalances have left ripple effects across the food web, with non-commercial species like stickleback and invasive non-indigenous species like round goby increasing, which may affect fish recruitment and exacerbate eutrophication.

Offshore wind energy development is an emerging pressure, one which promises cleaner energy but raises questions about its ecological footprint. “The biggest challenges here are balancing renewable energy goals with preservation of critical habitats and commercial interests such as fishing and shipping", says Faithfull. "There is a lot of talk about multiuse areas, so potentially windfarms could be combined with nature protection or aquaculture for example".

Puntila-Dodd sees the overall impact of windfarms being a combination of potential positive impacts of additional spatial protection (a de facto MPA effect) and potential negative impacts to organisms under and above water. “There are still many unknowns with large-scale offshore wind production in the Baltic Sea which causes challenges for management".

Finally, climate change is amplifying many of these pressures. Rising water temperatures, declining salinity, and increased marine heatwaves are reshaping species distributions and productivities, and intensifying hypoxia in shallow waters.

A persistent problem

Eutrophication remains the most pervasive issue. The excessive nutrient input, primarily nitrogen and phosphorus, stems from a catchment area three times the size of the Baltic Sea itself. Algal blooms and oxygen-deprived dead zones harm biodiversity and disrupt ecosystems. Despite decades of action, progress has been slow due to the system's complexity.

“The primary measures to reduce eutrophication are reducing nutrient concentrations in runoff and in wastewater", says Faithfull, "There are no established methods to combat such a large internal nutrient load". Puntila-Dodd continues that people seem to have difficulty accepting that something that took decades to mess up, will not be reversed so easily. "Management measures have already reduced loads, which has already slightly improved the state of the Baltic Sea. The improvement will in time lead to reduced eutrophication if we ensure that load reductions will continue. But we have to be patient and also remember to put effort into curbing climate change".

Socio-economic ripple effects

The economic and societal impacts of these pressures are profound. Coastal tourism, valued at an estimated €33.7 billion annually, is the largest blue economy sector in the region. Yet, degraded environmental conditions threaten these benefits, with losses projected at €9 billion annually due to pollution and ecosystem degradation.

Fisheries, although a smaller sector, have been severely impacted. Cod reproduction has been hampered by poor growth, and herring stocks have also been suffering. In coastal systems introductions of round goby and increases of opportunistic sticklebacks pose new challenges. When asked how managers can ensure the long-term viability of Baltic fisheries in light of these ecological and economic challenges, Faithfull says that the fishery needs to take an ecosystem-based perspective to management. "That is, considering the entire food web and its interactions as well as external pressures such as nutrient enrichment and climate change when setting quotas". Puntila-Dodd adds that in the future, we have to accept that the fishery may target different species than before.

Ecosystem shifts and cascading effects

The Baltic's key features, such as shallow depth, strong stratification, and limited water exchange exacerbate its susceptibility to change. Climate-driven shifts in phytoplankton and zooplankton composition have cascading effects on the food web. For example, smaller zooplankton species currently dominate, reducing food quality prey and energy intake for planktivorous fish.

The decline of larger predatory fish has released predation pressure, allowing species like sprat and stickleback to flourish. These shifts disrupt the food chain, hamper the recruitment of piscivorous fish, and promote the growth of filamentous algae, further fueling eutrophication.

When talking about how to manage restoring balance to the food web, Puntila-Dodd notes that marine ecosystems function as entities where species interactions are important. "We seem to have forgotten this in the past". Faithful adds, "If ecosystem-based fisheries management was implemented, these interactions would be considered when setting quotas for fish stocks. Additionally, further considerations might be made to preserve larger fish which have an important function in the food web".

Balancing renewables with conservation

Offshore renewable wind energy is growing rapidly in response to green energy demands. While promising a cleaner future, this development complicates maritime spatial planning. The placement of wind farms must consider existing fisheries, shipping lanes, and marine protected areas.

"Maritime spatial planning needs to consider the delivery of ecosystem services to a wider population rather than favouring short-term economic gain for a few", says Faithfull, "Inviting more stakeholders to the table for discussions and taking a wider societal perspective could result in fairer maritime spatial planning practices".

Addressing climate change

Puntila-Dodd highlights that we need to buffer our management actions to take into account climatic drivers. Climate change accelerates many of these challenges, with warmer summers intensifying hypoxia and altering seasonal cycles. The shrinking extent of sea ice affects species dependent on it, from microorganisms to seals. “There are no quick fixes here", adds Faithfull, "We need to reduce our carbon output to the atmosphere dramatically, this needs to be done on a global scale and it needs to be treated as an emergency.

Collaboration for the future

Despite these challenges, initiatives like HELCOM's Baltic Sea Action Plan and stricter fisheries quotas have shown promise. Reducing nutrient input and improving spatial planning is key to securing the Baltic's future. “That will lead to benefits to all visitors and citizens in the Baltic Sea", says Puntila-Dodd. However, the recovery will require patience and persistence.
“The Baltic is a unique and beautiful ecosystem that continues to face multiple challenges", says Faithfull, “ We have made great strides in reducing nutrient inflows, which will have positive long-term effects. There is still a lot of work to be done, but with an ecosystem-based society-inclusive approach to management there is potential for substantial improvements!".

The updated ICES Ecosystem Overview makes clear that the Baltic's future depends on holistic, international cooperation that integrates ecosystem-based management with socio-economic considerations.

View and download the Baltic Sea ecoregion – Ecosystem overview along with the Report of the Workshop for the revision of Ecosystem Overviews of the Baltic Sea Ecoregion (WKBALEO) from our online library.

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