The sea that is losing its breath

Algal blooms, beach closures, collapsing fisheries and ‘dead zones’ are all symptoms of an environmental crisis in the Baltic. But beneath the surface, VOTO’s work is shedding light on the root causes.

On a sweltering morning in July, families arrive on the wide, white sands of a beach on the west coast of Gotland in the middle of the Baltic Sea. They are expecting the simple summer joy of a cooling dip, but instead they are met by daunting messages: Do not swim. Harmful algae present.

The water is streaked green; dregs of stinking biomass collect along the shoreline, smelling like a sewer.

Every summer, these closures make evening news across Sweden, Germany, Poland and the Baltic states. Reporters interview disappointed swimmers. But the invisible crisis driving these closures goes deeper than a ruined day out. It is a slow suffocation of the sea itself, accelerated by human actions and climate change.

“The oxygen crisis is the headline issue here,” says Martin Mohrmann, a researcher at VOTO. “And in the Baltic region, it becomes very personal. You see it every summer.”

A sea that traps its problems

The Baltic Sea is unlike most parts of the world’s oceans. It is a shallow, highly stratified sea, layered like a cake. Lighter, fresher water sits at the surface. Beneath it lies colder mid-depth water, and below that a dense and salty bottom layer formed during winter inflows from the North Sea. These layers are separated by sharp-density gradients, known as thermoclines and haloclines, which resist mixing. Only a narrow connection through the Danish Straits links the Baltic to the North Sea, allowing limited inflow of saltier Atlantic water.

What turns this natural stratification into a crisis is the scale of human pressure layered on top of it. Shallow, enclosed, and ringed by heavily populated nations, the Baltic is fed by around 200 rivers, from a catchment area which is home to 85 million people.

Everything that happens on land eventually flows downhill into the water. Fertilisers from agriculture; nutrients from wastewater treatment plants; runoff from cities and industry. All of these carry nitrogen and phosphorus into the sea. In a more open, better-flushed ocean these inputs might be diluted. In the Baltic, they accumulate.

This excess of nutrients is known as eutrophication. Those surplus nutrients cause explosive blooms of biomass near the surface of the sea, fed by sunlight and fertiliser-rich water. When that organic matter dies, it sinks. Bacteria break it down – and in doing so, consume oxygen. Because the Baltic’s density layers act like sealed lids, that oxygen is not easily replaced. Paradoxically, ‘more’ life — in the form of short-term blooms of opportunistic algae and bacteria — leads to less life by burning through the Baltic’s oxygen supply as that biomass decays. The European Environment Agency identifies nutrient runoff from agriculture and wastewater as the principal driver of eutrophication and oxygen loss in enclosed seas like the Baltic.

“Once those layers form, it’s very hard for oxygen from the surface to reach the deep water,” Mohrmann explains. “That’s the physical reason the Baltic’s bottom layers lose oxygen and don’t easily recover.”

Year after year, the process repeats, ratcheting oxygen levels lower and expanding zones where fish, invertebrates and many forms of marine life struggle to survive. According to the Stockholm University Baltic Sea Centre and intergovernmental assessments by HELCOM, oxygen-poor zones now cover an area comparable to or greater than the size of Denmark.

What families encounter on the beach each summer is simply the visible edge of this process. “The beach closures you see are basically the surface symptom of the same underlying problem,” Mohrmann says.

An accelerating decline

By comparing VOTO’s observations with 40 years of government measurements from Sweden and Germany, Mohrmann can place today’s conditions in a concerning historical context.

“The Baltic is about two degrees warmer now than in the 1980s,” he says – roughly 0.5°C per decade. “And the strongest warming is actually at depth.” Oxygen levels in the deep basins continue to decline relative to the long-term average.

The consequences ripple outwards. Cod, once the backbone of Baltic fisheries, are exquisitely sensitive to temperature and oxygen. Spawning grounds that sustained stocks for centuries have become inhospitable.

“In Germany, cod catches dropped from around 300,000 tonnes a year to under 1,000,” Mohrmann says. “That’s catastrophic.”

Coastal fishing communities have collapsed. Industrial trawlers still operate at scale, catching fish largely for animal feed. Small-scale fisheries that once were central to regional food cultures are disappearing.

At the same time, public health advisories warn pregnant women not to eat Baltic fish due to accumulated pollutants. Algal blooms close beaches. The sea acquires an image problem.

“People talk about it as a dirty, over-eutrophicated pond,” Mohrmann says. “That emotional disconnect makes it harder to care for it.”

Sanna Thimmig-Johansen, VOTO’s CEO, argues that a mindset shift is needed. She says: “We’ve learned to adapt to symptoms of a sick sea in small, practical ways – like finding other places to swim on a day when a certain beach is closed. But adaptation must not turn into acceptance. We shouldn’t accept a degraded Baltic as the new normal. We should be addressing the causes, not adapting to the symptoms.”

A vital proving ground

This is where VOTO’s work comes into focus. Beneath the waves, the foundation’s fleet of autonomous underwater gliders patrols the Baltic, collecting constant high-resolution vertical profiles of the sea measured every few tens of centimetres, every 15 minutes, day and night.

“What we have is essentially an interactive atlas of the water column,” Martin says. “With the underwater gliders, you can actually see cause and effect. A bloom pulse, then the organic matter sinking, then oxygen being consumed below. Ship-based sampling once a month almost always misses that.”

By maintaining continuous, high-resolution observations, VOTO makes invisible processes visible. It allows scientists, policymakers and the public to see not just that the Baltic is changing, but how and why.

“Our data feeds into climate studies, ecosystem research, weather forecasting,” Martin explains. “The physics – density gradients, stratification, mixing – underpin everything else. Fish stocks, algal blooms, oxygen loss, even local weather are all linked to the same processes.”

There are precedents for recovery. When countries banned PCBs and DDT in the 1970s and 1980s, grey seal populations rebounded dramatically in the Baltic. Sea eagles have returned from near-extinction. Conservation, when followed through, worked.

The Baltic today faces a different, more complex set of pressures – climate change layered atop eutrophication, pollution and overfishing. But the very extremity of the Baltic is what makes it a vital proving ground. It is fragile, but still alive.

“If you can save the Baltic, you can save any sea,” says Martin. “The first step is to stop the decline.”

FROM DEPTH TO UNDERSTANDING

Voice of the Ocean’s underwater gliders help reveal what is happening beneath the surface of the Baltic — from algal blooms and oxygen loss to the physical processes shaping one of the world’s most fragile seas.

By making invisible change visible, VOTO helps turn ocean data into knowledge, and knowledge into action.