Imagine floating in a once-thriving sea, only to find the water thick, lifeless and choked with algae. Across the globe, thousands of aquatic ecosystems are turning into so-called dead zones driven largely by human activity. However, the majority of people remain unaware of the quietly escalating threat they represent and the danger they pose to our future because they have never heard of them.
So, what even are dead zones? Dead zones are areas in bodies of water that occur because of hypoxia, defined as dangerously low levels of oxygen in water. It results from several factors such as nutrient runoff, industrial pollution and wastewater. All those factors travel mostly by rivers that flow into seas and feed oxygen-consuming algae and bacteria. As we all know, oxygen is vital not just for humans, but also for other living beings. That is why the term dead zones reflects the reality of these waters.
‘Dead’ means no life, which is exactly what happens with marine life due to the lack of oxygen. Imagine it like a movie. Beneath the waves lies a small ecosystem of fish and swaying plants, a quiet world in motion. When excess nutrients flow in, algae and bacteria rise like a green tide. As the tide fades, microbes draw down oxygen, and the surface warms, sealing the depths below. It is not a war of good and evil, but a story of suffocation; over time, oxygen fades, and the once-lively ecosystem grows still beneath a heavy, breathless sea.
As we have said, rivers act as conduits carrying waste and nutrients into the sea. Farmers use fertilizers to boost crop growth, but when they overuse them, piles of excess nutrients seep into the soil and rainfall washes them into rivers. Simultaneously, inadequate infrastructure in certain countries results in disposal of undertreated wastewater into rivers. The best example is the Danube River that runs through multiple countries and flows into the Black sea, feeding a region where 2022 FAO data reveal that marine life is facing severe disruption.
Another major factor that causes the pollution of seas, in addition to the entire environment, is industrial pollution. Big factories in industrial areas cause pollution of nature around them: from air pollution to discharge off waste into rivers and onto land. While some low-oxygen zones occur naturally, human-caused pollution and rising temperatures combine to create new dead zones and make existing ones larger and more frequent, amplifying their consequences.
As dead zones spread around the world, they contribute to climate change causing a positive feedback loop. Because of climate change, the increased water temperatures result in less oxygen and rainfall helps the nutrient offload, accelerating the growth of dead zones. Dead zone areas release more greenhouse gases that intensify climate change, so each side reinforces the other.
You may wonder how dead zones affect us beyond their role in worsening climate change. To address the obvious point first, dead zones harm aquatic life and disrupt the food chain. As a certain area loses its biodiversity, the natural cycle is broken, meaning that feeding and breeding of some species are disrupted, which can lead to their endangerment.
The lack of fish also causes problems for the food market. People harvest some species in specific areas, and the decrease in one or more areas can lead to higher prices, less seafood on the market and even a drop in their export. Fishermen are also directly impacted by the reduction in fish numbers. For most of them, it means losing their regular source of income or giving up on a family tradition. Besides fishermen, tourism-dependent communities struggle financially as the beaches and natural scenery lose their appeal.
Numerous examples of dead zones exist, but we will highlight just a few. Number one on the list is the most famous one: the Gulf of Mexico. The NOAA identifies the Gulf of Mexico’s seasonal dead zone, fed by the Mississippi and Atchafalaya Rivers, as the nation’s largest and the world’s second largest recurring hypoxia area. America’s second longest river, together with its tributaries, flows through 32 U.S. States collecting nutrients and all kinds of waste that end up in the Gulf, worsening the dead zone. Research shows that this specific dead zone occurs periodically every year from late spring to fall, and it has been present since the turn of the 20th century.
In Europe, the most severe cases of hypoxia occur in the Black Sea, the Baltic Sea and the northern Adriatic Sea. The Black Sea dead zone represents a growing issue because of two factors: the Danube River and the Russo-Ukrainian war[1]. As we have said, the Danube flows through several countries, carrying their waste into the Black Sea. Impacts of the war also reach the natural environment, as pollutants such as aircraft parts and military waste enter the sea, hindering its ability to cleanse and regenerate itself. One of the biggest contributions to the dead zone was the destruction of the Kakhovka Dam, releasing huge amounts of wastewater into the sea.
The Adriatic Sea is famous for its beauty, but the Po River is subsequently polluting the north part. As Italy’s longest river, it flows through heavily industrialized and agricultural areas, carrying microplastics, agricultural runoff, sewage and industrial waste into the sea.
ScienceDirect data from 2024 report that the Baltic Sea dead zone is the world’s largest human-induced dead zone. According to National Geographic data from 2023 it consists of seven out of the ten largest dead zones in Europe that make up an area of around 70 000km2. The problem with this area is that the oxygen levels depend on the inflow from the Atlantic sea through the Danish straits. Given that the straits cannot regenerate the whole area of the sea, the situation is only getting worse. Although the Baltic countries have implemented several measures, the area remains in its worst condition to date, raising the question of how much more severe the situation would be without these efforts.
When it comes to measures, we can propose various solutions but it is questionable whether they are implementable. Proposals that are already in the game are sustainable agriculture, more severe control over fertiliser use, stricter surveillance of waste disposal- untreated wastewater and industrial waters, building of riparian buffer zones etc. Sustainable agriculture entails agriculture that meets the needs of today without compromising the needs of the future. Some examples are crop rotation and composting, often used by small farms.
On the other hand, overuse of fertilisers often occurs in areas where farmers raise large numbers of livestock that require constant feeding. This creates a chain reaction: farmers rely on more fertilisers to accelerate crop growth, while fisherman struggle with declining catches as nutrient runoff fuels dead zones and depletes marine life. Stricter control over fertiliser use is essential, but developing and adapting less harmful alternatives could allow both farmers and fishermen to sustain their livelihoods.
In many countries, crumbling infrastructure poses another major problem-one that people cannot fix overnight but remains urgent because of the chain of issues it creates. With outdated or non-existent sewage systems, many communities have little choice but to dispose of wastewater and sewage into nearby rivers, worsening pollution and fuelling the expansion of dead zones.
However, innovations like riparian buffer zones- vegetated areas along rivers and wetland –are helping to reduce pollution and preserve the seas by intercepting runoff and filtering pollutants before they reach the water, so we can call it nature preserving itself.
Overall, dead zones represent a serious environmental issue to which many people unknowingly contribute. They slowly devour our seas and affect everything from the food and job market to nature’s ecosystem. Nevertheless, hope is not lost. The oceans are sending us a warning, and the power lies in our hands. With awareness, innovation and determination of working together, communities, governments and individuals can turn the tide; ensuring that our waters can sustain life for generations to come.
Every step we take- whether reducing pollution, farming sustainably or protecting rivers- can slowly bring back life to dead zones. The challenge is urgent, but we must take action to make our seas a place of hope, not despair.
[1] Kvach, Y., Stepien, C.A., Minicheva, G.G. et al. Biodiversity effects of the Russia–Ukraine War and the Kakhovka Dam destruction: ecological consequences and predictions for marine, estuarine, and freshwater communities in the northern Black Sea. Ecol Process 14, 22 (2025). https://doi.org/10.1186/s13717-025-00577-1
