The hidden dead zones spreading across the Baltic Sea floor

Bornholm is a strategically located, remote island in the heavily polluted Baltic Sea. Now the windswept Danish island finds itself at the centre of an environmental and geopolitical crisis.

The tourist season on Bornholm is short. When I step off the bus the large harbour is empty except for a handful of yachts and a detachment of Danish soldiers. The fish-processing factory on the quay is silent and apparently abandoned, and over its shoulder peeks a new estate of holiday homes.

The harbourmaster's office is nonetheless still busy, one of its walls lined by old sepia photographs of the harbour in busier days. "I have worked here for nearly 27 years," Tom Nielsen, the harbourmaster, says. "We used to have 55 boats at one time, and now we have one left… You could walk across the harbour from one fishing boat to another. It was absolutely full. So many ships, so many people in the industry, in the factory, as mechanics, electricians. There were three people on land for every one person at sea.

"It was a shock when the fishing disappeared… and there are still no fish."

Commercial cod fishing around Bornholm has been banned since 2019 owing to the collapse of local cod stocks. In 2024, the 141-year-old fishermen's association in Bornholm closed down.

It may take more than 400 years for the maritime environment to recover from factors such as overfishing, oxygen depletion and rising sea temperatures. Some believe it may not happen at all. For under the Baltic Sea's waters, an invisible enemy is on the march. 

Areas of the sea floor with little or no oxygen, known as "dead zones", appear to be creeping closer to Bornholm's beaches. This is due to human pollution from fertilisers and sewage creating huge algal blooms, which, when they die, sink to the sea floor and cover it. Their decomposition uses up the available oxygen, kills the living organisms that depend on it, and – as a result – creates dead zones.

As if this wasn't enough, the Baltic Sea is facing a new threat to its survival. Russia's 2022 full-scale invasion of Ukraine created a "ghost fleet" of ships to bypass Western sanctions and the price cap on its oil exports.

Today there are growing fears of an accidental massive oil spill from one of the allegedly poorly maintained merchant ships of Russia's ghost fleet that would further devastate the fragile Baltic Sea environments, a claim which Russian authorities deny. 

Towering over the harbour at the far end of the quay in Tejn is the tall corrugated-iron factory that used to make ice for the island's fishing fleet. Now it is home to a local environmental education charity named Ivandet (Danish for "in the water"). 

"After the industry collapsed no one wanted to talk about what happened," says Marie Helene Miller Birk, a marine biologist and co-founder of Ivandet. "We found that the most important thing right now was to get people in the water and get them talking."

Ivandet replaced the machinery with a café, a mezzanine office space and a balcony with 180-degree views out over the Baltic Sea. The walls and pipes of the factory remain as a memorial to the fishing industry. The organisation created an artificial lagoon out into the sea.

"In the summertime, families come down and spend time with us," says Birk. "We have a marine biologist, usually me, with waders, fishing nets and water binoculars. The kids start getting curious, pull their parents in, and then the conversations begin.

"It's like the parents have needed someone to ask about all these things that they've heard in the media about the pollution," she adds.

Co-founder Magnus Heide Andreasen, a PhD student in marine ecology at the National Institute of Aquatic Resources in Copenhagen, and now co-founder of habitat restoration startup Redox, which is working on a new commercial technology which aims to reverse the oxygen depletion of the sea floor and restore its polluted sediments.

He's can't tell me much more, though. "It's still secret." 

Looking out across the Baltic Sea from the balcony, it's hard for me to believe that the sea is so polluted.

"That's the biggest problem the sea faces," says Birk. "It looks so beautiful." 

Under the sea's surface, it can be a very different story. 

"The Baltic Sea is a small, semi-enclosed sea with a unique set of characteristics," says Rüdiger Strempel, Executive Secretary, Baltic Marine Environment Protection Commission (HELCOM) over a video call. "It's........

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