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What are Ocean Dead Zones?
Blog...What are Ocean Dead Zones?

What are Ocean Dead Zones?

Ecology News
Global Warming
fish swimming in water
In this article we’ll explore what ocean dead zones are, how they form, and what we can do to try and prevent them.
Ecology News
fish swimming in water

Ocean dead zones are areas of the Earth’s oceans and lakes where the oxygen level is so low that little can survive. The impact on affected ecosystems can be severe with biodiversity loss and numbers of fish and other aquatic animals declining. And unfortunately, these dead zones are on the rise thanks to human activity and climate change. 

👉 In this article we’ll explore what ocean dead zones are, how they form, and what we can do to try and prevent 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 hypoxic conditions 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 excessively 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 and forming scum on the surface of the water. 

The formation of scum on the surface of the water means that sunlight is unable to penetrate through. This is problematic because many plants and organisms rely on sunlight in order to create energy - a by-product of which is oxygen. Therefore when plants are unable to carry out this function the oxygen levels in the water begin to drop - something that is bad news for a whole variety of organisms. 

Oxygen levels in the water also begin to deplete when the dead algae begins to decay. This is because 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.

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Net zero refers to achieving a balance between the amount of greenhouse gas emissions produced and the amount removed from the atmosphere. There are two ways to achieve net zero, and they work in tandem: reducing existing emissions, while at the same time removing greenhouse gases from our environment. When what we add is no more than what we take away, we reach net zero.

👍Is net zero achievable? Absolutely, it’s a realistic target because it recognises the fact that it’s impossible to eliminate all emissions, and instead it requires that what we do emit is cancelled out by the amount removed.

The effects of Eutrophication on the environment

There are a number of harmful effects on the environment resulting from 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 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. 

HABs cover a variety of different scenarios, for example it may describe an algal bloom that produces toxins capable of harming or killing aquatic life, birds, mammals and even humans. Other HABs may be non-toxic, but deplete the oxygen levels in the water, killing off aquatic creatures as a result. Some algal blooms may also harm fish and other invertebrates by clogging their gills and preventing them from breathing, or smothering corals and other aquatic ecosystems. There are also those that discolor the water and contaminate drinking water supplies. All of these events can be classed as harmful algal blooms (HABs).

The most common and frequent harmful algal blooms are caused by cyanobacteria (often referred to as blue-green algae due to the color of scum that it forms). This is a freshwater algae that produces toxins when it is able to grow out of control. Overgrowth of cyanobacteria threatens the health of aquatic ecosystems and its toxins are even potent enough to harm, or kill, humans.

The effects of harmful algal blooms are far reaching and they can severely impact the biodiversity of aquatic ecosystems, even impacting wildlife found on land who rely on the ecosystem for food. And humans are not immune from this risk. Shellfish such as oysters have been known to cause sickness and even death when they’ve been infected by harmful microbes associated with algal blooms. 

Even where an algal bloom does not create any harmful toxins, the effects can still be devastating for an ecosystem. Decreased oxygen levels affect aquatic life within the body of water, making survival for a variety of organisms difficult to sustain. This can also have a knock on effect when it comes to the animals that rely on these organisms for food. For example birds such as herons, and mammals including seals rely on fish for survival - where fish stock declines they may also struggle to survive.

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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 result in what’s known as dead zones - ie. areas where animals and organisms struggle to survive due to lack of oxygen. 

There are several of these so-called dead zones across the world. The Gulf of Mexico for example has a seasonal dead zone that forms every year at the end of summer. It ranges in size depending on conditions but is usually between 5,000 square km and 22,000 square km.

The Baltic Sea is also home to a number of dead zones. In fact, 7 of the world’s largest dead zones are within this area. This has been partly attributed to runoff from fertilizers used in agricultural practices, alongside overfishing of cod within the region. 

Cod feed on a smaller fish known as sprat, these smaller fish feed on zooplankton, which in turn feed on algae. By overfishing cod in Baltic Sea, sprat numbers have been able to rise which means that the level of zooplankton has decreased - and since zooplankton feeds on algae, the algae has been able to bloom.

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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 3 times as much phosphorus and nitrogen entering our environment than natural emissions. 

In developed countries the primary source of these nutrients is animal manure and commercial fertilizers. Runoff from agricultural fields means that phosphorus and nitrogen is able to enter bodies of water such as streams and rivers, which eventually flow into lakes, reservoirs and the ocean. 

The dead zone in the Gulf of Mexico for example is attributed to the influx of nutrients from fertilizers and sewage that finds its way into the Mississippi river, before flowing downstream and into the Gulf of Mexico. 

In developing countries on the other hand (mainly in Latin America, Asia and Africa), phosphorus and nitrogen enters the water system via untreated wastewater. Factories and sewage treatment facilities are often less regulated which means that in some cases wastewater is simply dumped 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.

❗️ It should be noted however, that not all dead zones can be attributed to human activities. There are some that occur naturally. For example, one of the world’s largest dead zones, which can be found in the Black Sea, occurs naturally.

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The 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 does 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. 

👉 The ocean's surface layers have warmed by around 1.5 degrees celsius since the start of the 20th century.

What can be done to reduce dead zones?

The most effective way to reduce man-made dead zones is to prevent nutrients from reaching our bodies of water in the first place. This is best achieved through effective agriculture management practices that better manage runoff. 

For example nutrients found in fertilizers can be decreased where farms use less fertilizer, or implement crop covers to keep soil in place and stop rainwater runoff. Where irrigation systems are used, it is also possible to recapture nutrients from irrigation water, preventing them from entering our waterways. 

In developing countries, where waste management regulations are often more relaxed, an effective way of preventing high levels of nutrients from reaching lakes and oceans is through more strict wastewater treatment regulations and more effective treatment plants. 

And since global warming is making dead zones more likely, we can all play our part by taking efforts to cut down on harmful greenhouse gas emissions that contribute to climate change.

algal bloom in a lake

What about Greenly? 

At Greenly we can help you to assess your company’s carbon footprint, and then give you the tools you need to cut down on emissions. Why not request a free demo with one of our experts - no obligation or commitment required. 

If reading this article has inspired you to consider your company’s own carbon footprint, Greenly can help. Learn more about Greenly’s carbon management platform here.

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