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What are Ocean Dead Zones?

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By Kara Anderson, UK Copywriter

What are dead zones?
What are the effects of eutrophication on the environment?
What causes dead zones and harmful algal blooms?
Is there a link between climate change and dead zones?
What can be done to reduce dead zones?
What's the situation in 2025?
What about Greenly?

Ocean dead zones sound exactly like what they describe - they're areas of the Earth’s oceans and lakes where the oxygen level is so low that almost nothing can survive. The impact on marine and aquatic ecosystems can be extreme, with biodiversity loss and numbers of fish and other animals declining as a direct result. And unfortunately, these dead zones are on the rise thanks to human activity and climate change.

In this article, we’ll explore:

What dead zones are and why they form
The key causes driving their growth around the world
Examples of some of the largest and most concerning dead zones today
The wider environmental impacts and why they matter
Practical steps being taken — and what still needs to be done to reduce them

What are dead zones?

Dead zones are areas of low oxygen in the world’s oceans and lakes. The lack of oxygen leads to something termed 'hypoxic conditions' - an environment in which few organisms can survive - hence the term ‘dead zones’.

“Dead zones form as a result of a process known as eutrophication. Eutrophication is where a body of water (or sections of it) becomes too rich in minerals and nutrients such as nitrogen and phosphorus. ”

The process of eutrophication sets off a chain reaction in the ecosystem. Organisms, including algae, phytoplankton, and seaweed, that feed on these nutrients start to increase, creating an algal bloom, forming scum on the surface of the water.

The formation of scum on the surface of the water means that sunlight is unable to break through. This is a problem because many plants and organisms rely on sunlight in order to create energy (a by-product of which is oxygen). When plants are unable to carry out this process, the oxygen levels in the water begin to drop - something that spells bad news for a whole variety of organisms and creatures.

Oxygen levels in the water drop even further when the dead algae begins to decay. This is because the bacteria, feeding on the decaying organic material, use up the oxygen in the water, resulting in a drop in oxygen levels and contributing towards the creation of hypoxic conditions.

👉 Hypoxic conditions refers to low or depleted oxygen levels in the water. It is most often associated with the overgrowth of certain species of algae, which can lead to oxygen depletion when they die and decompose.

What are the effects of eutrophication on the environment?

There are a number of harmful effects on the environment as a result of eutrophication, all of which contribute to conditions that allow dead zones to form. Let’s take a closer look at the negative impacts of eutrophication:

Algal blooms

An algal bloom is the rapid growth of algae and phytoplankton resulting from excessive nutrients in the water (nitrogen and phosphorus). We’ve already talked about how they can quickly grow on the surface of the water, blocking out sunlight and preventing plants from creating energy and oxygen. However, the harmful effects of algal blooms extend beyond this.

Harmful algal blooms (also known as HABs) are the excessive growth of algae resulting in the production of toxins or harmful effects on people, aquatic life, other mammals and birds.

Harmful algal blooms (HABs) cause a variety of impacts. Here are some of the main ways they can cause harm:

Harmful algal blooms (HABs) can have very different impacts on aquatic environments and wildlife. Here are some of the main ways they can cause harm:

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Toxic trouble

Some HABs produce toxins capable of harming or killing aquatic life, birds, mammals, and even humans.

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Oxygen depletion

Other HABs aren’t toxic but remove oxygen from the water, suffocating fish and other aquatic creatures.

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Choking marine life

Certain algal blooms clog fish gills, smother corals, and damage other delicate marine ecosystems.

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Water contamination

Some HABs discolour the water and can even contaminate drinking water supplies.

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All part of HABs

Whether through toxins, oxygen loss, or ecosystem damage, these events all fall under the term harmful algal blooms (HABs).

“The most common harmful algal blooms are caused by cyanobacteria (often referred to as blue-green algae due to the colour of scum that they form). This is a freshwater algae that produces toxins when it grows out of control. Overgrowth of cyanobacteria threatens the health of underwater ecosystems, and its toxins can even harm or kill humans.”

The effects of harmful algal blooms can be extreme. Not only can it negatively impact the biodiversity of aquatic ecosystems, but it also has the potential to impact wildlife on land that rely on these environments for food.

And humans are not immune from this risk. Ciguatera poisoning (CP), caused by toxins from certain HABs, is the most common foodborne illness linked to fish consumption worldwide, with an estimated 200,000 to 1,000,000 cases every year. Shellfish and fish such as grouper, barracuda, and snapper have all been linked to outbreaks, which can sometimes cause serious illness and even death.

And even where an algal bloom doesn’t produce harmful toxins, the effects can still be devastating for an ecosystem. Decreased oxygen levels make survival difficult for many aquatic organisms, which in turn impacts the animals that rely on them for food. Birds like herons and mammals such as seals depend on healthy fish stocks, when these decline, their survival is also put at risk.

Hypoxia

We’ve already briefly talked about how algal blooms can lead to hypoxic conditions within a body of water. The lack of sunlight leading to an inability of aquatic organisms to create energy and oxygen, combined with the breakdown of dissolved oxygen in the water by bacteria, results in what’s known as dead zones - ie. areas where animals and organisms struggle to survive due to lack of oxygen.

There are several well‑known dead zones around the world, areas so depleted in oxygen that life struggles to survive:

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Gulf of Mexico

A seasonal dead zone forms here every summer, typically ranging between 5,000 km² and 22,000 km². It’s one of the most studied dead zones globally, largely caused by nutrient runoff from the Mississippi River.

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Baltic Sea

Home to seven of the world’s ten largest dead zones, the Baltic Sea suffers from severe oxygen depletion. Fertiliser runoff and overfishing — particularly of cod — have disrupted the ecosystem and allowed algae to bloom unchecked.

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Black Sea

Once rich in marine life, the Black Sea now has a vast area where oxygen levels are too low to support most organisms. Decades of nutrient pollution have left large regions uninhabitable at depth.

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Chesapeake Bay (USA)

Each summer, a dead zone develops here, affecting fisheries and aquatic biodiversity. Agricultural runoff and warming waters combine to create one of the largest recurring dead zones in the United States.

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Lake Erie

This freshwater lake develops a seasonal dead zone in its central basin, sometimes stretching up to 10,000 km². It impacts both aquatic ecosystems and local drinking water supplies.

What causes dead zones and harmful algal blooms?

“Human activities are the leading cause of excess nutrients such as phosphorus and nitrogen being washed into our oceans and bodies of water. This is why dead zones and harmful algal blooms are often found near inhabited coastlines or bodies of water. ”

The rise in industrial scale farming and intensive agricultural practices, general population growth, and industrial activity have increased the amount of nitrogen and phosphorus entering our air, soil, and bodies of water.

👉 Did you know? Human activities have resulted in three times as much phosphorus and nitrogen entering our environment than natural emissions.

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Developed countries

The primary sources of phosphorus and nitrogen here are animal manure and commercial fertilisers. Runoff from agricultural fields carries these nutrients into streams and rivers, which eventually flow into lakes, reservoirs, and the ocean.

One example is the Gulf of Mexico dead zone, largely caused by fertilisers and sewage entering the Mississippi River before flowing downstream into the Gulf.

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Developing countries

In many parts of Latin America, Asia, and Africa, phosphorus and nitrogen often enter the water system via untreated wastewater.

Factories and sewage treatment facilities are frequently less regulated, meaning wastewater is sometimes discharged directly into lakes, rivers, and the ocean.

Another cause of excess nitrogen is the release of nitrogen into the atmosphere via the burning of fossil fuels. The atmospheric nitrogen is able to reach bodies of water through the water cycle (ie. when it rains or snows).

“There are currently 415 dead zones that have been identified globally. These areas have increased significantly during the last few decades. In fact the number of dead zones has almost doubled since the 1960s. The majority of these can be found along the coastlines of the United States, the Baltic States, Japan, and the Korean Peninsula. ”

❗️ Note: It should be noted, however, that not all dead zones can be attributed to human activities. Some occur naturally — for example, one of the world’s largest dead zones, found in the Black Sea, is naturally occurring.

Is there a link between climate change and dead zones?

“Not only is there a strong link between human activities and the formation of dead zones, there’s also a strong link between climate change and a heightened risk of dead zones forming!”

This is because climate change is causing increasingly intense rainfall and storms, which leads to more runoff from fields into our rivers and bodies of water. This allows nutrients used in farming to reach out waterways.

Another impact of climate change is that as the Earth’s temperatures warm, so do the Earth’s bodies of water - from our rivers, to our lakes, to our oceans. Warm water is able to carry less oxygen than cooler water, and so, as temperatures increase, oxygen levels will decrease, making it much easier for dead zones to form.

👉 Did you know? The ocean's surface layers have warmed by around 1.5°C since the start of the 20th century.

What can be done to reduce dead zones?

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Smarter farming practices

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The most effective way to reduce man-made dead zones is to stop excess nutrients from entering our waterways in the first place. This means better agricultural management — using less fertiliser where possible and applying it more efficiently to reduce runoff.

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Protecting soil and water

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Planting cover crops can help keep soil in place and prevent rainwater from washing nutrients into rivers and lakes. Similarly, buffer zones of vegetation around fields act as natural barriers, trapping nutrients before they reach waterways.

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Capturing and reusing nutrients

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Where irrigation systems are used, it’s possible to recapture nutrients from irrigation water rather than letting them flow downstream. This keeps fertilisers on the land where they’re needed and out of rivers, lakes, and oceans.

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Improving wastewater treatment

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In many developing countries, weaker regulations mean untreated sewage and industrial wastewater often end up in lakes and oceans. Building more effective treatment plants and enforcing stricter standards can help stop large amounts of nutrients from entering the water system.

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Tackling climate change

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Warmer waters make dead zones more likely and harder to recover from. We can all play a role by reducing greenhouse gas emissions — from switching to renewable energy to making more sustainable lifestyle choices.

What's the situation in 2025?

The situation in 2025 shows both progress and ongoing concerns when it comes to ocean dead zones. Here are a few key insights shaping the current conversation:

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Gulf of Mexico

This year’s dead zone measured around 4,400 square miles — one of the smaller recordings in recent decades. However, the five-year average still sits at roughly 4,755 sq mi, which is more than double the long-term reduction goal.

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A growing global issue

There are now over 400 identified dead zones around the world, collectively covering an area larger than the United Kingdom.

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Funding remains a challenge

Meeting long-term targets, like shrinking the Gulf’s dead zone, would require cutting nitrogen runoff by around 45% — a goal that’s proving difficult without significant investment and stronger policies.

The data shows some positive steps forward, but the scale of the challenge makes it clear there’s still a long way to go in tackling nutrient pollution and restoring balance to our marine ecosystems.

What about Greenly?

At Greenly, we help companies understand, measure, and reduce their environmental impact through a full suite of carbon management solutions:

Our sustainability services	What it includes
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Whether you’re starting your reporting journey or looking to set ambitious sustainability targets, Greenly provides the expertise and tools to turn insights into action. Get in touch today to find out more.