
Why are oceans getting darker? – DW – 06/06/2025
In the past 20 years more than a fifth of our oceans have been growing darker. What is causing this and how worried should we be? To mark World Ocean Day on June 8, we've repackaged a deep dive that will take you beneath the Baltic Sea to explore how ocean darkening is changing the marine ecosystem, plus the steps we need to take to protect our oceans.
Interviewees:
Claas Wollna, fisherman from Stralsund
Oliver Zielinski, director of the Leibnitz Institute for Baltic Sea Research in Warnemünde
Florian Hoffmann, biologist with the World Wildlife Fund in Stralsund
Dag Aksnes, marine ecologist at the University of Bergen
Maren Striebel, biologist at the Institute for Chemistry and Biology of the Marine Environment in Wilhelmshaven
Listen and subscribe to Living Planet wherever you get your podcasts: https://pod.link/livingplanet Got a question for us? Email livingplanet@dw.com. And, if you like the show, leave us a rating and review on whichever podcast platform you use – and tell a friend!
Transcript:
Claas Wollna: That's perch, flounder, pike, zander and whitefish. That's fine, I had worse catch.
It's a good morning for fisherman Claas Wollna. He has just come back from the gillnets and now heaves four boxes of catch onto the jetty. Some of the fish are still wriggling.
Wollna is the last permanent fisherman in the region of Stralsund, a harbor town on the German coast of the Baltic Sea. Most of his colleagues have given up. Fishing no longer earned them a sufficient income. And that's because their most important fish, the herring, is almost gone. It doesn't reproduce sufficiently.
To save the herring from extinction, Wollna is only allowed to catch a meagre 1.3 tons a year.
Claas Wollna: I am not against protection of herring at all. I understand that if the stock is poor, it needs to be saved. But that needs to be done in a way that people can survive.
Claas Wollna's struggle runs way deeper than mere fishing regulations. And the missing herring is just a symptom of a much a bigger problem. Something's fundamentally wrong with the water ... no, with the sea itself, all along the coast. Not just in the Baltic Sea, but at many coasts around the world. Something has changed.
Dag Aksnes: And then we saw that the water down there was very dark.
Florian Hoffmann: We could see about an arm's lenght. It wasn't even a meter.
You're listening to Living Planet, I'm Neil King. And this deep dive is a literal one. We're about to explore the phenomenon of coastal ocean darkening, also known as coastal browning or brownification. Although 'known' seems to be a bit of an overstatement. A fairly small community of researchers around the world has only just begun to understand where this darkening comes from and how it messes up pretty much everything from seaweed to fishery and even the oceans' ability to help us protect the climate.
To get started, we head to another harbor town on the German Baltic Sea coast, Warnemünde, to meet Oliver Zielinski a renowned expert on Coastal Ocean Darkening.
Oliver Zielinski: Ocean darkening and specifically coastal ocean darkening refers to how much light gets into the depths of the ocean to a plant or a fish sitting at the bottom of the ocean. It refers to the light within the ocean itself, not as seen from above. From a bird's eye view, an ocean surface can shine brightly and yet be very dark under water.
Oliver Zielinski is the director of the Leibnitz Institute for Baltic Sea Research. He has studied the darkening in the Baltic Sea and North Sea for years. However, the problem is a global one. Researchers found the darkening happening in the waters around New Zealand, the US, Singapore, China, Japan and in the Medditerranean Sea.
Oliver Zielinski: …that's where we measured darkening of coastal waters. This is where we have long series of measurements and where human interaction with the ocean has been strong.
'Human interaction with the ocean' - we'll save this little bit of what Zielinski just said for later when we talk about what – or who – causes the darkening.
Before that, we're taking a little walk with Zielinski to the harbor quay. He wants to show us something. A tool.
Oliver Zielinski: I now lower the disk into the water and watch it slowly drift down from the surface.
The disk that Zielinski puts in the water, is a Secchi disk, named after its inventor, Pietro Angelo Secchi, a 19th century priest and scientist in Italy. It is white and 30 cm in diameter. A bit like a large pizza plate on a cord. Zielinski is holding the cord with both hands, letting the disk sink down centimeter by centimeter into the water just off the quay.
Oliver Zielinski: It is getting harder to see the disk now as the murky water covers it. The important thing is that I now figure out the exact depth at which I can hardly see the disk. If I lift it up a little now, I can see the disk again. That's it. And that is the depth I'll write down.
The disk is roughly 2.5 meters under the sea surface. And according to Zielinski, light, as a rule of thumb, reaches down into the water three times as deep. That's ... 7.5 meters then. That sounds pretty deep actually, but it used to be much more. Since 1900, the depth of visibility, or Secchi depth, on the coasts of the Baltic Sea and North Sea has decreased by 3 to 4 centimeters per year. The question is: why did this happen?
Oliver Zielinski: The water itself obviously didn't change over the past 100 years. So it's the substances that got into the water, three of them: algae, dissolved matter and sediments.
Okay, so here is what we've got so far. Coastal waters around the globe have been getting darker for decades. Meaning that the depth to which light can reach into the water has decreased. And that's because of algae, dissolved matter and sediments. Let's explore what that exactly means – and what harm it does.
We're in a speedboat, heading out to a lagoon in the Baltic Sea, east of Stralsund.
We're not here on our own of course.
Florian Hoffmann: I am Florian Hoffmann, I am a biologist and have been working with the World Wildlife Fund in Stralsund for 10 years.
Florian has kindly agreed to take us out for a diving trip. We arrive at the lagoon. But if you're thinking of crystal-clear, turquoise water now, you won't find that here. Quite the opposite. But that's why we're here after all.
We drop anchor near a small island, where many plants are supposed to grow on the seabed, like seaweed or crested pondweed, green and lush. The water is not that deep, only 2.5 meters, as the echo sounder on board indicates. You don't need big diving gear for this depth. So, we're already dressed with a wetsuit, now it's time for the flippers and the belt with heavy weights.
Florian Hoffmann: I'll start with two weights. I'll go down and then tell you how much you take with you. Because the wetsuit leads to a little buoyancy.
Time to spit on the diving goggles, rub the spittle so that the goggles don't fog up, adjust the mouthpiece of the snorkel - and we're ready to go.
Hoffmann jumps first.
Florian Hoffmann: Visibility is terrible.
Let's go take a look for ourselves.
The seabed is just a few strong pulls away. Gliding above it, we can only see about an arm's length, beyond that it gets dark. Not dark in the sense of black of course, we are in shallow water after all, more like a fog, a dense and murky mixture of brown and green.
What we can see directly in front of us are patches of sand and then again patches of seaweed and other plants. Their stalks seem to pop up out of nowhere and sweep across our arms and face.
To be honest, it's a bit of a confusing and uncomfortable environment at first, but after gaining some orientation, much of the murkiness seems to come from masses of small green particles. They float around in the water weightlessly, like artificial snow in one of those kitschy snow globes.
Back on the boat, Florian Hoffmann explains what we just saw. The green particles, he says, that's phytoplankton, the basis of all life in the seas and producer of half the oxygen we breathe. So having phytoplankton in the seas is essential. But when there's too much of it, it makes the water foggy as it dies and slowly sinks to the ground.
Florian Hoffmann:This means that the light zone decreases. ... We know from older literature that the depths to which you could look down into the water here used to be up to 8 meters. It now has decreased to three or two meters. And that of course makes it a lot harder for sea plants to grow on the seabed here.
Hoffmann pulled out a handful of seaweed and brought it back on board. Some of the stalks are covered with small brown stains. The remains of phytoplankton shield the living plants from light even in death.
The reason for this mess in the water lies on land, Hoffmann says.
When farmers spread too much fertilizer on their fields, it doesn't only make their crops grow.
Florian Hoffmann:That overuse of fertilizer leads to increased supply of water bodies with nutrients. That's nitrogen, which is important for photosynthesis, and phosphorus, a component of the DNA, which is also an important building block for life.
Florian Hoffmann has brought a clipboard with him, with sheets of paper, showing charts and numbers.
Florian Hoffmann:So, these are figures from the Federal Ministry of Environment from 2021. Agriculture accounted for 78% of nitrogen inputs in the Baltic Sea and 51% of phosphorus inputs, while point sources such as sewage treatment plants accounted for another 10% of nitrogen inputs and 20% of phosphorus inputs.
Apart from these nutrients and water of course, it's light that makes photosynthesis possible and lets plants grow. Imagine if someone switched off the sun. How long would life on earth survive? The trees, the bushes, insects, birds, mammals, all life.
This is a dark thought experiment. But looking at the foggy water underneath our boat out there on the lagoon, the threat seems real enough. If the seaweed doesn't get enough light, it dies. Like here in the lagoon. Spanning more than 500 km² big there used to be giant underwater seaweed lawn here just 70 years ago. Today, the plants have retreated to the shallow edges of the lagoon.
This has effects on the whole food chain in the water and beyond. Small fish that are at the start of the chain use seaweed to hide and to spawn. In this part of the Baltic Sea, it's the herring that essentially depends on it as it lays its spawn in the seaweed. The herring population has massively collapsed over the last 10 years. In an unfavorable combination, the fish migrated to the lagoon earlier due to warmer waters – but then failed to find sufficient seaweed there.
The problem with that is that the herring is at the beginning of the local marine food web. Less herring means less food for bigger fish, ducks, and less catch for fisherman Claas Wollna, whom we heard at the beginning of this episode.
Healthy seaweed is also a real climate superhero. One square kilometer of seaweed captures twice as much CO2 as terrestrial forest and it does this 35 times faster as well. The same goes for other plants in coastal waters. Researchers in New Zealand looked at the health of kelp in a lagoon that had strong inflow of nutrients from agriculture and the city of Auckland. The found that the darkening in that lagoon caused local kelp forests to degrade and fix up to 4.7 times less carbon than they usually would.
To mention it a bit in advance: we're not doomed because of Coastal Ocean Darkening. That being said though, our take-away from the diving trip with Florian Hoffmann is that Coastal Ocean Darkening does both harm biodiversity and the climate.
Except for one life form that seems to handle the dark waters quite well. As Hoffmann speaks, a handful of common jellyfish floats past the boat, just a little below the sea surface. Jellyfish who consist of 95% water are particularly transparent and small, about the size of a saucer. They have a distinct advantage in the murky water.
Dag Aksnes: Jellyfish don't need light to feed. It's a so-called tactile predator.
That's Dag Aksnes.
Dag Aksnes: I'm a marine ecologist at the University of Bergen. And I have studied mostly fjords but also the ocean.
You've likely seen pictures of Norway's majestic fjords. Long and narrow, placed between steep cliffs and several hundred meters deep. At first glance the fjords appear to be lakes, but they are in fact saltwater inlets from the North Sea, mixed with some freshwater from land that gets into the fjords via mighty waterfalls. But we're moving away from the topic...
After decades of research, Dag Aksnes has come to know the fjords around Bergen like the back of his hand, both above and below the water's, in the living and non-living world. Until he and his colleagues from the University of Bergen went to a fjord called Lurefjord to check up on the fish population there. They let the trawl net down into the water, started the boat's engine and set off.
Dag Aksnes: We would expect, like, at maximum in half an hour to get like 100 kilograms of this fish, which is very abundant also. But, when we trawled in this field, then we actually had to cut the trawl in the water and destroy it because we couldn't have it all up on deck.
Instead of a bit of fish – and after just a few minutes – the trawl net was bursting with thousands of big, orange, fluorescent jellyfish.
Dag Aksnes: You can trawl them and, actually, in five minutes you can have five tons. So it's a ton per minute.
The helmet jellyfish is found all over the world and that's perfectly normal. But not in such large numbers. For the whole fjord, Aksnes estimated the population of jellyfish at 50,000 tons. Or in individuals...
Dag Aksnes: Ohhh (laughs). Well if it's 500 grams each, in a ton you will have 2000 and then you have 50,000 tons multiplied by 2,000.
Which translates into 100 million jellyfish. But almost no other life.
Dag Aksnes: And then we started wondering, why is this jellyfish here and not the fishes?
Dag Aksnes: And then we saw that the water down there was very dark.
Too dark for visual-hunting fish to see its prey. Meanwhile jellyfish don't need light but use their tentacles to sense prey. Less competition and warmer seas due to climate change help them to spread. Not only in the Lurefjord, which is now also known as the 'jellyfish fjord', but to a smaller extent also in many other fjords along the Norwegian coast.
But the reason for the dark water in the fjords is different from the nutrient overflow in the Baltic Sea.
Dag Aksnes: So actually, we got a very extended water column with coastal water containing lots of dissolved organic matter which originates from land. And then this question, of course, which we still investigate, is why has this amount of dissolved organic matter which absorbs lights increased.
Dissolved organic matter. Small particles of rotten leaves or wood. It gets into the Norwegian fjords via the rivers from all over Northern Europe. And stays in there for decades before it eventually degrades. In the case of Lurefjord, it accumulates more and more due to an exceptionally narrow exit to the sea.
The irony here is that the source of all this organic matter is something that we'd usually desire: more nature.
Dag Aksnes: The evidence now is that this is because of increased greening in Northern Europe. [...] There are more trees now than a hundred years ago. Much more. This is partly because of change in land use. [...] The other reason is, I believe, warming and also increased precipitation over Northern Europe, which also stimulates greening. [...] More green coverage in Scandinavia and Northern Europe which produces more dissolved organic matter that enters the sea sooner or later.
Think of it as a cup of tea. You pour in the water, add a tea bag and watch as the water slowly turns brown.
And now think of that cup of tea as a big barrel standing in a garage building, named like a character from a Transformers movie: planktotron.
We're at the Institute for Chemistry and Biology of the Marine Environment in Wilhelmshaven. That's another German harbor town – the last one, that's a promise – but this one is situated at the North Sea coast. The planktotrons are 12 large cylindrical tanks made from stainless steel and wrapped with hemp and black foil for insulation. The numbers 1 to 12 are taped on the foil with pink duct tape.
Maren Striebel: We can simulate the marine environment on a smaller scale here. We can manipulate nutrients or what we did in the Coastal Ocean Darkening project was to manipulate the light.
Maren Striebel is a biologist at the institute. She does research on plankton and was part of the research group of Oliver Zielinski, whom we heard earlier.
Maren Striebel: We had three different levels of intensity of input of dissolved organic matter. There was definitely shading at the beginning and an impact on the primary producers, the phytoplankton. We observed a reduction in its biomass, which had an impact on the next trophical stage, i.e. the food web in the water. But then the organic matter degraded over time and the nutrients were used up, so the system returned to its orginial state at some point.
The dissolved organic matter waseaten up, so to say, by organisms in the water, which subsequently cleared and brightend up. That's good news. But in another experiment Striebel and her colleagues also added sediment to the water in the planktotrons, or sand, taken from the local beach. And sand doesn't get used up by organism because, well, it's just sand, there are no useful nutrients in it like the organic matter. So the sand stays in the water.
Again, think of it as a cup or glass. This time you add a teaspoon of sand and give it a good stir. The water turns murky for some time, before the sand slowly settles at the bottom of the glass. But the seas are no glass of water and the sediment in it is more than a teaspoon.
Here's Oliver Zielinski again.
Oliver Zielinski: Storms carry more sediment into the water and make the water murky. We will have more storms as a result of climate change. But we also have more sediment in the water because we have coastal erosion due to coastlines and construction work [in the water]. Trawlers stir up the sea ground with their heavy nets. All these things mobilize sediments and make the water cloudy. So the effect of sediment for visibility in the water is very strong.
Nature's trick to keep sediment on the seabed is vegetation, like seaweed. It stabilizes the ground with its roots. If the seaweed retreats because of too little light, the seabed becomes even more unstable and the water even murkier. It's a vicious cycle, if you like.
Oliver Zielinski: I rather like to tell science in positive narratives. If we manage to grow seaweed again, this will also bind sediment. The water will become clearer and we may be able to go deeper to establish even more seaweed.
Speaking of positive narratives, Oliver Zielinski hardly complains as we talk. He has every reason to do so, doesn't he? I mean, people are worried about plastic in the seas or coral bleaching. But darkening water isn't getting that kind of attention.
His optimism comes from the fact that darkening is already stagnating in the Baltic Sea in particular, but also in the Mediterranean and North America. In the North Sea, the water has even been brightening again since the 1980ies thanks to regulations around fertilizer use and the ban of phosphate in washing detergents, less nutrients have been entering the North Sea.
But with global warming, rainfall and storms will become more extreme, which will lead to more organic matter and sediment being washed into coastal waters, Zielinski says. The best way to stop coastal ocean darkening would therefore be to limit global warming.
Oliver Zielinski: Measures are being taken. But the efforts need to be increased, they actually need to be doubled, because climate change is working against us. We have to make an even greater effort to get back to the situation we had [in the coastal waters] before.
That's the big picture. Back in the harbor, marine biologist Florian Hoffmanns thinks that very specific, local action is needed too.
Florian Hoffmann: Well, trying to use less fertilizer and specifically for what you need. And possibly trying to keep water in the landscape, not pumping it directly into the sea, but letting it flow through a reedbed area where the washed-away nutrients can separate.
Today's episode of Living Planet was researched and written by Jonas Mayer. It was narrated and edited by me, Neil King. Our sound engineer was Thomas Schmidt.
To download this and past episodes of Living Planet, go to Apple podcasts, Spotify or wherever you get your podcasts. If you like what we do, make sure to hit the subscribe button.
We're also available on DW's website, that's dw.com.
You can also find this and other great podcasts on our YouTube channel DW podcasts.
Thanks for listening and sharing Living Planet with your friends and family.
Living Planet is produced by DW in Bonn, Germany.
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Why are oceans getting darker? – DW – 06/06/2025
In the past 20 years more than a fifth of our oceans have been growing darker. What is causing this and how worried should we be? To mark World Ocean Day on June 8, we've repackaged a deep dive that will take you beneath the Baltic Sea to explore how ocean darkening is changing the marine ecosystem, plus the steps we need to take to protect our oceans. Interviewees: Claas Wollna, fisherman from Stralsund Oliver Zielinski, director of the Leibnitz Institute for Baltic Sea Research in Warnemünde Florian Hoffmann, biologist with the World Wildlife Fund in Stralsund Dag Aksnes, marine ecologist at the University of Bergen Maren Striebel, biologist at the Institute for Chemistry and Biology of the Marine Environment in Wilhelmshaven Listen and subscribe to Living Planet wherever you get your podcasts: Got a question for us? Email livingplanet@ And, if you like the show, leave us a rating and review on whichever podcast platform you use – and tell a friend! Transcript: Claas Wollna: That's perch, flounder, pike, zander and whitefish. That's fine, I had worse catch. It's a good morning for fisherman Claas Wollna. He has just come back from the gillnets and now heaves four boxes of catch onto the jetty. Some of the fish are still wriggling. Wollna is the last permanent fisherman in the region of Stralsund, a harbor town on the German coast of the Baltic Sea. Most of his colleagues have given up. Fishing no longer earned them a sufficient income. And that's because their most important fish, the herring, is almost gone. It doesn't reproduce sufficiently. To save the herring from extinction, Wollna is only allowed to catch a meagre 1.3 tons a year. Claas Wollna: I am not against protection of herring at all. I understand that if the stock is poor, it needs to be saved. But that needs to be done in a way that people can survive. Claas Wollna's struggle runs way deeper than mere fishing regulations. And the missing herring is just a symptom of a much a bigger problem. Something's fundamentally wrong with the water ... no, with the sea itself, all along the coast. Not just in the Baltic Sea, but at many coasts around the world. Something has changed. Dag Aksnes: And then we saw that the water down there was very dark. Florian Hoffmann: We could see about an arm's lenght. It wasn't even a meter. You're listening to Living Planet, I'm Neil King. And this deep dive is a literal one. We're about to explore the phenomenon of coastal ocean darkening, also known as coastal browning or brownification. Although 'known' seems to be a bit of an overstatement. A fairly small community of researchers around the world has only just begun to understand where this darkening comes from and how it messes up pretty much everything from seaweed to fishery and even the oceans' ability to help us protect the climate. To get started, we head to another harbor town on the German Baltic Sea coast, Warnemünde, to meet Oliver Zielinski a renowned expert on Coastal Ocean Darkening. Oliver Zielinski: Ocean darkening and specifically coastal ocean darkening refers to how much light gets into the depths of the ocean to a plant or a fish sitting at the bottom of the ocean. It refers to the light within the ocean itself, not as seen from above. From a bird's eye view, an ocean surface can shine brightly and yet be very dark under water. Oliver Zielinski is the director of the Leibnitz Institute for Baltic Sea Research. He has studied the darkening in the Baltic Sea and North Sea for years. However, the problem is a global one. Researchers found the darkening happening in the waters around New Zealand, the US, Singapore, China, Japan and in the Medditerranean Sea. Oliver Zielinski: …that's where we measured darkening of coastal waters. This is where we have long series of measurements and where human interaction with the ocean has been strong. 'Human interaction with the ocean' - we'll save this little bit of what Zielinski just said for later when we talk about what – or who – causes the darkening. Before that, we're taking a little walk with Zielinski to the harbor quay. He wants to show us something. A tool. Oliver Zielinski: I now lower the disk into the water and watch it slowly drift down from the surface. The disk that Zielinski puts in the water, is a Secchi disk, named after its inventor, Pietro Angelo Secchi, a 19th century priest and scientist in Italy. It is white and 30 cm in diameter. A bit like a large pizza plate on a cord. Zielinski is holding the cord with both hands, letting the disk sink down centimeter by centimeter into the water just off the quay. Oliver Zielinski: It is getting harder to see the disk now as the murky water covers it. The important thing is that I now figure out the exact depth at which I can hardly see the disk. If I lift it up a little now, I can see the disk again. That's it. And that is the depth I'll write down. The disk is roughly 2.5 meters under the sea surface. And according to Zielinski, light, as a rule of thumb, reaches down into the water three times as deep. That's ... 7.5 meters then. That sounds pretty deep actually, but it used to be much more. Since 1900, the depth of visibility, or Secchi depth, on the coasts of the Baltic Sea and North Sea has decreased by 3 to 4 centimeters per year. The question is: why did this happen? Oliver Zielinski: The water itself obviously didn't change over the past 100 years. So it's the substances that got into the water, three of them: algae, dissolved matter and sediments. Okay, so here is what we've got so far. Coastal waters around the globe have been getting darker for decades. Meaning that the depth to which light can reach into the water has decreased. And that's because of algae, dissolved matter and sediments. Let's explore what that exactly means – and what harm it does. We're in a speedboat, heading out to a lagoon in the Baltic Sea, east of Stralsund. We're not here on our own of course. Florian Hoffmann: I am Florian Hoffmann, I am a biologist and have been working with the World Wildlife Fund in Stralsund for 10 years. Florian has kindly agreed to take us out for a diving trip. We arrive at the lagoon. But if you're thinking of crystal-clear, turquoise water now, you won't find that here. Quite the opposite. But that's why we're here after all. We drop anchor near a small island, where many plants are supposed to grow on the seabed, like seaweed or crested pondweed, green and lush. The water is not that deep, only 2.5 meters, as the echo sounder on board indicates. You don't need big diving gear for this depth. So, we're already dressed with a wetsuit, now it's time for the flippers and the belt with heavy weights. Florian Hoffmann: I'll start with two weights. I'll go down and then tell you how much you take with you. Because the wetsuit leads to a little buoyancy. Time to spit on the diving goggles, rub the spittle so that the goggles don't fog up, adjust the mouthpiece of the snorkel - and we're ready to go. Hoffmann jumps first. Florian Hoffmann: Visibility is terrible. Let's go take a look for ourselves. The seabed is just a few strong pulls away. Gliding above it, we can only see about an arm's length, beyond that it gets dark. Not dark in the sense of black of course, we are in shallow water after all, more like a fog, a dense and murky mixture of brown and green. What we can see directly in front of us are patches of sand and then again patches of seaweed and other plants. Their stalks seem to pop up out of nowhere and sweep across our arms and face. To be honest, it's a bit of a confusing and uncomfortable environment at first, but after gaining some orientation, much of the murkiness seems to come from masses of small green particles. They float around in the water weightlessly, like artificial snow in one of those kitschy snow globes. Back on the boat, Florian Hoffmann explains what we just saw. The green particles, he says, that's phytoplankton, the basis of all life in the seas and producer of half the oxygen we breathe. So having phytoplankton in the seas is essential. But when there's too much of it, it makes the water foggy as it dies and slowly sinks to the ground. Florian Hoffmann:This means that the light zone decreases. ... We know from older literature that the depths to which you could look down into the water here used to be up to 8 meters. It now has decreased to three or two meters. And that of course makes it a lot harder for sea plants to grow on the seabed here. Hoffmann pulled out a handful of seaweed and brought it back on board. Some of the stalks are covered with small brown stains. The remains of phytoplankton shield the living plants from light even in death. The reason for this mess in the water lies on land, Hoffmann says. When farmers spread too much fertilizer on their fields, it doesn't only make their crops grow. Florian Hoffmann:That overuse of fertilizer leads to increased supply of water bodies with nutrients. That's nitrogen, which is important for photosynthesis, and phosphorus, a component of the DNA, which is also an important building block for life. Florian Hoffmann has brought a clipboard with him, with sheets of paper, showing charts and numbers. Florian Hoffmann:So, these are figures from the Federal Ministry of Environment from 2021. Agriculture accounted for 78% of nitrogen inputs in the Baltic Sea and 51% of phosphorus inputs, while point sources such as sewage treatment plants accounted for another 10% of nitrogen inputs and 20% of phosphorus inputs. Apart from these nutrients and water of course, it's light that makes photosynthesis possible and lets plants grow. Imagine if someone switched off the sun. How long would life on earth survive? The trees, the bushes, insects, birds, mammals, all life. This is a dark thought experiment. But looking at the foggy water underneath our boat out there on the lagoon, the threat seems real enough. If the seaweed doesn't get enough light, it dies. Like here in the lagoon. Spanning more than 500 km² big there used to be giant underwater seaweed lawn here just 70 years ago. Today, the plants have retreated to the shallow edges of the lagoon. This has effects on the whole food chain in the water and beyond. Small fish that are at the start of the chain use seaweed to hide and to spawn. In this part of the Baltic Sea, it's the herring that essentially depends on it as it lays its spawn in the seaweed. The herring population has massively collapsed over the last 10 years. In an unfavorable combination, the fish migrated to the lagoon earlier due to warmer waters – but then failed to find sufficient seaweed there. The problem with that is that the herring is at the beginning of the local marine food web. Less herring means less food for bigger fish, ducks, and less catch for fisherman Claas Wollna, whom we heard at the beginning of this episode. Healthy seaweed is also a real climate superhero. One square kilometer of seaweed captures twice as much CO2 as terrestrial forest and it does this 35 times faster as well. The same goes for other plants in coastal waters. Researchers in New Zealand looked at the health of kelp in a lagoon that had strong inflow of nutrients from agriculture and the city of Auckland. The found that the darkening in that lagoon caused local kelp forests to degrade and fix up to 4.7 times less carbon than they usually would. To mention it a bit in advance: we're not doomed because of Coastal Ocean Darkening. That being said though, our take-away from the diving trip with Florian Hoffmann is that Coastal Ocean Darkening does both harm biodiversity and the climate. Except for one life form that seems to handle the dark waters quite well. As Hoffmann speaks, a handful of common jellyfish floats past the boat, just a little below the sea surface. Jellyfish who consist of 95% water are particularly transparent and small, about the size of a saucer. They have a distinct advantage in the murky water. Dag Aksnes: Jellyfish don't need light to feed. It's a so-called tactile predator. That's Dag Aksnes. Dag Aksnes: I'm a marine ecologist at the University of Bergen. And I have studied mostly fjords but also the ocean. You've likely seen pictures of Norway's majestic fjords. Long and narrow, placed between steep cliffs and several hundred meters deep. At first glance the fjords appear to be lakes, but they are in fact saltwater inlets from the North Sea, mixed with some freshwater from land that gets into the fjords via mighty waterfalls. But we're moving away from the topic... After decades of research, Dag Aksnes has come to know the fjords around Bergen like the back of his hand, both above and below the water's, in the living and non-living world. Until he and his colleagues from the University of Bergen went to a fjord called Lurefjord to check up on the fish population there. They let the trawl net down into the water, started the boat's engine and set off. Dag Aksnes: We would expect, like, at maximum in half an hour to get like 100 kilograms of this fish, which is very abundant also. But, when we trawled in this field, then we actually had to cut the trawl in the water and destroy it because we couldn't have it all up on deck. Instead of a bit of fish – and after just a few minutes – the trawl net was bursting with thousands of big, orange, fluorescent jellyfish. Dag Aksnes: You can trawl them and, actually, in five minutes you can have five tons. So it's a ton per minute. The helmet jellyfish is found all over the world and that's perfectly normal. But not in such large numbers. For the whole fjord, Aksnes estimated the population of jellyfish at 50,000 tons. Or in individuals... Dag Aksnes: Ohhh (laughs). Well if it's 500 grams each, in a ton you will have 2000 and then you have 50,000 tons multiplied by 2,000. Which translates into 100 million jellyfish. But almost no other life. Dag Aksnes: And then we started wondering, why is this jellyfish here and not the fishes? Dag Aksnes: And then we saw that the water down there was very dark. Too dark for visual-hunting fish to see its prey. Meanwhile jellyfish don't need light but use their tentacles to sense prey. Less competition and warmer seas due to climate change help them to spread. Not only in the Lurefjord, which is now also known as the 'jellyfish fjord', but to a smaller extent also in many other fjords along the Norwegian coast. But the reason for the dark water in the fjords is different from the nutrient overflow in the Baltic Sea. Dag Aksnes: So actually, we got a very extended water column with coastal water containing lots of dissolved organic matter which originates from land. And then this question, of course, which we still investigate, is why has this amount of dissolved organic matter which absorbs lights increased. Dissolved organic matter. Small particles of rotten leaves or wood. It gets into the Norwegian fjords via the rivers from all over Northern Europe. And stays in there for decades before it eventually degrades. In the case of Lurefjord, it accumulates more and more due to an exceptionally narrow exit to the sea. The irony here is that the source of all this organic matter is something that we'd usually desire: more nature. Dag Aksnes: The evidence now is that this is because of increased greening in Northern Europe. [...] There are more trees now than a hundred years ago. Much more. This is partly because of change in land use. [...] The other reason is, I believe, warming and also increased precipitation over Northern Europe, which also stimulates greening. [...] More green coverage in Scandinavia and Northern Europe which produces more dissolved organic matter that enters the sea sooner or later. Think of it as a cup of tea. You pour in the water, add a tea bag and watch as the water slowly turns brown. And now think of that cup of tea as a big barrel standing in a garage building, named like a character from a Transformers movie: planktotron. We're at the Institute for Chemistry and Biology of the Marine Environment in Wilhelmshaven. That's another German harbor town – the last one, that's a promise – but this one is situated at the North Sea coast. The planktotrons are 12 large cylindrical tanks made from stainless steel and wrapped with hemp and black foil for insulation. The numbers 1 to 12 are taped on the foil with pink duct tape. Maren Striebel: We can simulate the marine environment on a smaller scale here. We can manipulate nutrients or what we did in the Coastal Ocean Darkening project was to manipulate the light. Maren Striebel is a biologist at the institute. She does research on plankton and was part of the research group of Oliver Zielinski, whom we heard earlier. Maren Striebel: We had three different levels of intensity of input of dissolved organic matter. There was definitely shading at the beginning and an impact on the primary producers, the phytoplankton. We observed a reduction in its biomass, which had an impact on the next trophical stage, i.e. the food web in the water. But then the organic matter degraded over time and the nutrients were used up, so the system returned to its orginial state at some point. The dissolved organic matter waseaten up, so to say, by organisms in the water, which subsequently cleared and brightend up. That's good news. But in another experiment Striebel and her colleagues also added sediment to the water in the planktotrons, or sand, taken from the local beach. And sand doesn't get used up by organism because, well, it's just sand, there are no useful nutrients in it like the organic matter. So the sand stays in the water. Again, think of it as a cup or glass. This time you add a teaspoon of sand and give it a good stir. The water turns murky for some time, before the sand slowly settles at the bottom of the glass. But the seas are no glass of water and the sediment in it is more than a teaspoon. Here's Oliver Zielinski again. Oliver Zielinski: Storms carry more sediment into the water and make the water murky. We will have more storms as a result of climate change. But we also have more sediment in the water because we have coastal erosion due to coastlines and construction work [in the water]. Trawlers stir up the sea ground with their heavy nets. All these things mobilize sediments and make the water cloudy. So the effect of sediment for visibility in the water is very strong. Nature's trick to keep sediment on the seabed is vegetation, like seaweed. It stabilizes the ground with its roots. If the seaweed retreats because of too little light, the seabed becomes even more unstable and the water even murkier. It's a vicious cycle, if you like. Oliver Zielinski: I rather like to tell science in positive narratives. If we manage to grow seaweed again, this will also bind sediment. The water will become clearer and we may be able to go deeper to establish even more seaweed. Speaking of positive narratives, Oliver Zielinski hardly complains as we talk. He has every reason to do so, doesn't he? I mean, people are worried about plastic in the seas or coral bleaching. But darkening water isn't getting that kind of attention. His optimism comes from the fact that darkening is already stagnating in the Baltic Sea in particular, but also in the Mediterranean and North America. In the North Sea, the water has even been brightening again since the 1980ies thanks to regulations around fertilizer use and the ban of phosphate in washing detergents, less nutrients have been entering the North Sea. But with global warming, rainfall and storms will become more extreme, which will lead to more organic matter and sediment being washed into coastal waters, Zielinski says. The best way to stop coastal ocean darkening would therefore be to limit global warming. Oliver Zielinski: Measures are being taken. But the efforts need to be increased, they actually need to be doubled, because climate change is working against us. We have to make an even greater effort to get back to the situation we had [in the coastal waters] before. That's the big picture. Back in the harbor, marine biologist Florian Hoffmanns thinks that very specific, local action is needed too. Florian Hoffmann: Well, trying to use less fertilizer and specifically for what you need. And possibly trying to keep water in the landscape, not pumping it directly into the sea, but letting it flow through a reedbed area where the washed-away nutrients can separate. Today's episode of Living Planet was researched and written by Jonas Mayer. It was narrated and edited by me, Neil King. Our sound engineer was Thomas Schmidt. To download this and past episodes of Living Planet, go to Apple podcasts, Spotify or wherever you get your podcasts. If you like what we do, make sure to hit the subscribe button. We're also available on DW's website, that's You can also find this and other great podcasts on our YouTube channel DW podcasts. Thanks for listening and sharing Living Planet with your friends and family. Living Planet is produced by DW in Bonn, Germany.


DW
30-05-2025
- DW
What are the real impacts of melting glaciers? – DW – 05/30/2025
Glaciers are the planet's frozen water banks. They sustain water supply, ecosystems, and even cultural traditions. But many of these sprawling beds of ice are melting. Why does that matter? The collapse of a glacier in the Swiss Alps this week has underscored the impacts of a warming world on the ice-packed parts of planet Earth. When the melting Birch glacier on crumbled on Wednesday, it engulfed the picturesque village of Blattern in the country's southern Wallis region. Glaciers and ice sheets store about 70% of the world's freshwater reserves. High- altitude regions are often dubbed the world's "water towers" because they gradually release meltwater in the summer, sustaining towns and farms downstream. Two billion people globally rely on glacial melt for their day-to-day water needs, researchers say. Yet, as the world gets hotter, the ice is thawing. Glaciers around the world are now melting at twice the rate measured just two decades ago. Between 2000 and 2023, they lost an ice mass equivalent to 46,000 Great Pyramids of Giza. And this is impacting communities worldwide. Some regions are left with too little water while others struggle with too much. Melted ice from glaciers in the Andes contributes almost 20% of the annual water supply of Huaraz in Peru Image: Patricioh/Dreamstime/IMAGO Gl aciers as crucial freshwater resource The residents of the small western Peruvian town of Huaraz draw almost 20% of their annual water supply from melting ice. But Andean glaciers are thawing even faster than elsewhere. This poses a risk of flooding. In a decade-long lawsuit, one resident of Huaraz sued a German energy company over the potential risk to his home from a mountain lake that is filling with melt water at rapid rates. A bridge in Hassanabad village, Pakistan partly collapsed when a glacial lake burst and caused flash floods on May 7, 2022 Image: AFP Meltwater d amages infrastructure and makes mountains unstable It is not only in Peru that huge glacial lakes form when glaciers thaw. When they become too full, deadly floods can wash away buildings, bridges and wipe out fertile land, like in Pakistan, where a glacial lake burst in October 2023. The 2023 flood swept away part of the land, houses and a community hall in Hassanabad village, leaving behind steep and dangerous cliffs. Image: Akhtar Soomro/REUTERS That same month in neighboring India, a lake of melted ice overflowed and killed 179 people. Scientists estimate that globally, at least 15 million people are vulnerable to sudden flooding from thawing ice, most of them living in India and Pakistan. Since 1990, the volume of water in mountain lakes has increased by around 50%. The collapse of the Birch glacier in Switzerland caused a landslide of rock and ice that covered most of the 300-strong village of Blatten in mud. Though residents had been evacuated as a precaution, one man is missing in what scientists call the latest dramatic example of climate change's impact on the Alps. There are also now fears that a nearby river will be blocked causing flooding in the region. Swiss glacier collapse partially destroys village of Blatten To view this video please enable JavaScript, and consider upgrading to a web browser that supports HTML5 video Dwindling water supply for agriculture and electricity generation As glaciers shrink, they eventually reach a threshold — known as peak water — at which runoff declines. As a result, less melt water flows downstream with potentially far-reaching consequences. Reduced water supply has forced local farmers, who traditionally grew corn and wheat, to change both their crops and water management. Some communities in the Andes have now switched to growing a bitter potato variety that is more resilient to drought. Unstable water supply is also stalling electricity production. In Chile, 27% of power is generated by hydroelectric dams which critically depend on meltwater. In 2021, the Alto Maipo plant was shut down due to dwindling flow. Melting ice sheets increase sea levels Massive ice blocks like Thwaites Glacier in Antarctica are retreating at an alarming rate Image: Cover-Images/IMAGO It's not only glaciers in high altitudes that are melting — but also those in the ocean, like Thwaites Glacier in Western Antarctica. This frosty giant is the size of the US state of Florida and has been deemed "very unstable." Scientists say it is thawing on all sides. The melting of sea ice crucially contributes to rising sea levels. Thwaites Glacier has been dubbed "doomsday glacier" for its potential impact on what some researchers call 'abrupt' sea level rise. On the low-lying island of Fuvahmulah in the Maldives, workers are building a dyke to protect the land against sea level rise Image: Christophe Geyres/ABACA/picture alliance In the last 25 years alone, melting glaciers have caused global sea levels to rise almost 2cm (0.7 inches). This might not seem like much, but low-lying islands like Fiji and Vanuatu in the Pacific are at risk of disappearing under the waves. In addition more than 1 billion people in megacities like Jakarta, Mumbai, Lagos, Manila live within ten kilometers from the coast and protective dykes are only a temporary solution as sea levels continue to rise. Ice traditions under threat Pilgrims descend a rock face in Ocongate, Peru after a ceremony on the glacier during the annual Qoyllur Rit'i festival Image:Glaciers also hold spiritual and cultural significance. Every year, tens of thousands of pilgrims gather at one of Peru's most sacred glaciers, the Colquepunco, for a religious festival. In the past, ice blocks were carved from the glacier and carried down to local communities who believed in their healing properties. But as the glacier vanishes, this ancient tradition has come under threat. Less snowfall for Alpine ski resorts One in eight ski resorts could lose its natural snow cover by 2100, making tourists flock to higher altitudes like at Passo Tonale in Italy Image: Nikokvfrmoto/Pond5 Images/IMAGO The Presena glacier in Italy, a popular destination for skiers, has reportedly lost a third of its volume since 1990. And natural snow in the European Alps is expected to decline by 42% by the end of the century. Scientists estimate that many ski resorts worldwide won't be profitable anymore in the future. Warning systems and artificial glaciers can help Early warning systems like this weather station in the Karakoram mountain range in Pakistan can help adapt to the threats of melting glaciers Image: Akhtar Soomro/REUTERS Locals can adapt to some of these dangers. In the Pakistan village of Hassanabad, an early warning system has been installed to monitor activity at the nearby Shisper glacier. Should there be a need for a warning, it can be communicated through external speakers in the village. In the neighboring Ladakh region, researchers are experimenting with growing artificial glaciers that can mitigate water shortage in summer to meet this challenge. But these strategies can only work up to a point. Scientist say the best way to tackle receding glaciers is to slow the rising temperatures that are heating the Earth. Edited by: Anke Rasper


Local Germany
09-05-2025
- Local Germany
EXPLAINED: How Germany is preparing for a looming US 'brain drain'
Citing far-reaching job and funding cuts at US universities and research institutions, Trump's actions have endangered science and well-being worldwide, the President of the Alexander von Humboldt Foundation, Robert Schlögl, said recently. In response German research institutions and universities are making moves to work more closely with US researchers. But despite some initial fervour around the idea of attracting the US's top talent, most are taking a more cautious approach. Here's how some of Germany's leading research institutions are responding, including some of the opportunities for US researchers they are pushing. Will the US suffer a brain drain? The German Academic Exchange Service (DAAD) suspects that the situation in the USA could lead to a global shift. "Top talent from countries like India, China or Brazil, who would have previously gone mainly to the USA, are now considering whether other countries could be a better option," said DAAD spokeswoman Cordula Luckassen. Large-scale "brain drains", or the exodus of qualified scientists, have occurred many times in history. Germany suffered its own brain drain around 1933, after the National Socialists came to power when Jewish and dissident academics fled the country. Luckassen suggests that in the DAAD offices in the US, there is a growing interest in Germany as a science location among international doctoral candidates and postdocs. Many of these academics currently work or study in the US on temporary visas and in temporary positions, often financed by federal funds. What's the reaction in Germany? But while some might expect Germany's leading research institutions to move to sweep up top researchers from the US, so far they appear to be taking a softer approach. Tim Urban, press spokesman of the Leibniz Association, told DPA that he wouldn't pursue a 'targeted poaching of American colleagues,' because it 'risks weakening American science even further'. The Max Planck Society (MPG) aims to provide the so-called Transatlantic Program with more funding, which would open up options in Germany for scientists who cannot continue their research in the USA. Advertisement In response to a recent call for applications, the MPG received twice as many applications from the USA as in the previous year, President Patrick Cramer recently said in an interview with Der Spiegel . READ ALSO: 'We need dual citizenship' to support cutting edge research in Germany, says top scientist Meanwhile at German universities, the impact of the US' political shift is currently being felt. A spokeswoman for the University of Leipzig suggested there is an increase in interest among American partners for close cooperation. The Humboldt University in Berlin has reportedly received isolated direct inquiries from US scientists in recent weeks, as has RWTH Aachen University. "If the situation in the US gives rise to opportunities to strengthen the profile of Goethe University through suitable appointments, we will of course take advantage of them," said the press office of Frankfurt University. READ ALSO: 10 reasons to study in Germany Where's the funding? Of course, efforts within Germany to actively lure in top talent from the US could be expected to ramp up depending on developments on the US side – especially for researchers in fields that are being actively targeted by the Trump administration, such as climate, inequality and life sciences. But currently institutions in Germany, and really across Europe and the world, don't have the budget to support nearly as much research as the US had been supporting in recent decades. David Ho, an American oceanographer and climate researcher wrote on social media app Bluesky that, 'The US spends more on research and development than any other country in the world,' adding that if China and Japan are removed, the US spends almost as much as all other countries combined. Advertisement So for German institutions to have a real shot at attracting top researchers, they'll need to come up with significantly more funding for science. Some efforts are beginning to come together toward that end. For example, President of the Alexander von Humboldt Foundation, Robert Schlögl, announced that he would support more top researchers from the USA with appropriate financial support and grant them temporary residence and work opportunities. Also, in a guest article in Der Spiegel last month, leading German scientists called for the development of a so-called "Meitner-Einstein Program", to effectively poach outstanding researchers from the US for German universities and research institutions with financial backing from the Federal Ministry of Education and Research. With reporting by DPA.