In summer, the water can take on the color of pea soup, and the shore carries the unmistakable smell of decomposing algae. In winter, the ice that earlier generations took for granted is increasingly absent. The Baltic Sea is changing before our eyes—and these are not cosmetic shifts, but a wholesale reconfiguration of an ecosystem toward a state in which biodiversity is harder to sustain, recreation becomes less predictable, and fisheries face growing uncertainty.
The latest regional synthesis by HELCOM (“State of the Baltic Sea 2023”, covering 2016–2021) puts it plainly: during the assessed period there was “little to no” overall improvement in the environmental status of the Baltic Sea, while climate pressure is expected to intensify. The report also stresses that “transformative” changes are needed across the sectors that influence the sea.
The Baltic is not a “typical” sea. It is shallow, semi-enclosed, and has limited water exchange with the ocean—while its drainage basin is enormous. This makes it exceptionally sensitive to what happens on land: agriculture, wastewater, industry, transport, and urban expansion. When this inherent vulnerability is combined with rapid climate warming, a self-reinforcing mechanism sets in: warmer waters, more blooms, greater oxygen consumption, weaker habitats, and stress on fish stocks.
Warming is the background process that “sets” many of the other dynamics. Public debate often focuses on one season—cyanobacteria in summer or storms in autumn—but the deeper issue is the long-term shift in temperature regimes and the weakening of stable seasonality. Warmer water supports more intense biological production (and thus blooms), yet it also makes it harder to maintain adequate oxygen conditions in deeper layers. Warmer water holds less dissolved oxygen, while biological and chemical processes that consume oxygen accelerate. This is why bottom-water oxygen deficits are no longer merely a side effect of eutrophication, but a crisis in their own right.
Eutrophication is a technical term, but the mechanism is straightforward: excessive nitrogen and phosphorus entering the sea act like fertilizer. In practice this means faster growth of algae and phytoplankton. Part of this biomass is incorporated into the food web, but a large portion dies and sinks to the seabed, where decomposition begins. Decomposition, like any form of biological “burning,” requires oxygen. If nutrient loads remain too high, the sea is effectively being fed beyond its capacity to process what it produces.
HELCOM’s indicator data suggest that nutrient inputs have declined compared with a late-1990s/early-2000s reference period, but not enough to resolve the problem across the entire region, and some key sub-basins remain above acceptable thresholds. This nuance matters: you can have “progress” at the scale of the whole sea while continuing to lose in the most vulnerable areas.
If eutrophication is the fuel of the crisis, oxygen depletion is its most dramatic manifestation. When deep waters and bottom layers are oxygen-poor (hypoxia), bottom-dwelling communities either die off or retreat; when oxygen disappears altogether (anoxia), conditions can favor the formation of hydrogen sulfide, toxic to many organisms. Long-term assessments by the Swedish Meteorological and Hydrological Institute (SMHI), which tracks oxygen conditions in the deep Baltic, show the scale of the problem: in 2023, anoxia affected about 19% of seabed areas and hypoxia (including anoxic areas) about 33%. Preliminary results for 2024 indicate a similar picture—around 18% anoxic and 34% hypoxic. SMHI also reports exceptionally high hydrogen sulfide concentrations in the basins around Gotland, including record levels in bottom waters in parts of the eastern and western Gotland basins.
Fish stocks function as a barometer of ecosystem health, and the Baltic cod has become emblematic of the system’s instability. Cod reproduction and survival depend strongly on oxygen conditions and salinity patterns in key areas. In its recent advice, ICES recommends a zero-catch level under the precautionary approach for the eastern Baltic cod stock for 2025 and 2026. The broader point is crucial: even the best fisheries management cannot restore fish populations if spawning grounds and bottom habitats remain trapped in a persistent oxygen crisis. This is not only a fisheries issue—it is a systemic ecological problem.
The Baltic Sea’s value is not limited to nature conservation. It is also a space of work, tourism, and everyday well-being. HELCOM emphasizes that the cost of inaction can be very high, including substantial losses linked to recreation. It is easy to fall into the false choice of “economy versus environment.” In reality, ecosystem degradation eventually sends a bill—first to coastal municipalities and local industries, then to taxpayers and society at large.
What, then, can be done? There is no single magic measure. Instead, there are levers that reliably shift the balance when applied consistently. First, nutrient reductions remain central—particularly in agriculture and wastewater systems—supported by region-wide cooperation under the Baltic Sea Action Plan. Second, it is worth recognizing the often-overlooked role of clean-air policies: reductions in atmospheric nitrogen deposition have contributed significantly to the overall decline in nitrogen inputs to the sea. Third, resilience matters. HELCOM explicitly points to the need to strengthen the ecosystem’s capacity to withstand future climate impacts—through habitat protection and restoration, limiting cumulative pressures in hotspots, and ecosystem-based management grounded in monitoring and evidence.
The hardest lesson is that the Baltic is not the responsibility of one ministry or one country. It is a system of connected vessels: agriculture across the catchment, shipping and coastal development, wastewater infrastructure, climate policy, spatial planning, and pressure on biological resources. But the direction signaled by the data is clear: improvement is possible when measures are implemented steadily and at sufficient scale—yet the effort so far has been too modest to produce an overall environmental turnaround in the 2016–2021 assessment period. The Baltic Sea has entered a new era. The question is no longer whether it is changing, but whether we will leave those changes to chance—or treat them as a shared regional project with real choices, responsibilities, and consequences.
