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We all love to stare at the ocean. We are all mesmerised by its deep blue or crystal-clear waters. Yet, the reality is sometimes far from this idyllic image. In many regions of the ocean, the water is toxic, green or red and lifeless…

When it comes to water pollution, we often think of what is visible: oil spills, plastic pollution etc. But the biggest source of pollution is hidden as little molecules within the water: nutrients. An excess of nutrients due to human activities have been causing one of the biggest threats to marine biodiversity for decades: eutrophication.

Let’s dive into coastal waters to understand what is causing marine ecosystems to become lifeless and how we can remediate this critical and urgent issue. 

Creating the imbalance

First and foremost, let’s define the term “eutrophication” which we will be using throughout this article. Eutrophication happens when there is an overproduction of algae in the water due to an increased quantity of nutrients – such as phosphorous and nitrogen. 

To understand the bigger picture of how only two molecules – phosphorous and nitrogen – can degrade entire marine ecosystems, we first need to consider that each ecosystem is a dynamic balance between biotic (biological) and abiotic (physical) elements. 

In a healthy ecosystem, the number of species or individuals, and their physical environment (temperature, pH, salinity, oxygen concentration, quantity of nutrients…) can change and fluctuate, but only to a certain extent: a healthy ecosystem is able to auto-regulate and remains within a certain state to be productive and resilient.


However, passed a certain range of conditions (fewer individuals of a certain species or too many nutrients in our case), an ecosystem enters the phase of « degradation »: it becomes imbalanced and the process and activities that ensure its viability start failing. 

Although there are always nutrients such as nitrogen and phosphorous in the water, they are usually found in limited quantities. These substances are used by photosynthetic organisms, such as algae. When there is an overabundance of nitrogen and phosphorous in the water, these nutrients are not limited anymore: they are not limiting the growth of algae anymore and allow them to proliferate. These algae blooms disrupt the dynamic of the whole underwater ecosystem and cause its degradation…

But how exactly can too many algae in the water lead to the impairment of the whole ecosystem? 

An unbreathable environment

The biggest issue comes with the degradation of the algae. 


As the algae die, they fall through the water column to the bottom of the sea where they will be decomposed by bacteria. The bacteria use the oxygen from the water through respiration whilst decomposing the organic material. When huge quantities of algae are being degraded – like in the case of these blooms – the oxygen concentration of the water drop, impairing the viability of the ecosystem. 

In some regions of the ocean, oxygen levels are so depleted that life ceases to exist. These regions are straightforwardly called « dead zones » and are considered one of the biggest threats to marine biodiversity. The Gulf of Mexico, the Baltic Sea, and the Chesapeake Bay are some of the most notorious ones due to their intensity and history, but the number and severity of dead zones around the world are rapidly increasing. 

The interconnection between land, sea and humans

Unfortunately, like many issues threatening marine life, this water pollution is caused by anthropogenic activities

Some of the excess nutrients found in the water come from the wastewater from human populations: the sewage treatment is not good enough, or non-existent, increasing the discharge of the effluent from habitations to water systems. 


However, the biggest source of excess nutrients found in the water comes from agriculture. As we mentioned before, nutrients are what limit the growth of photosynthetic organisms, and this does not only apply to marine plants: the growth of terrestrial plants, such as crops, is also limited by nutrients. Today, in most conventional agricultural practices, fertilisers made of nitrogen and phosphorous are being used in large quantities in order to increase crop production. Most of these nutrients end up in the rivers or in the groundwater and find their way to coastal waters. 

Eutrophication is the perfect illustration of how much everything is connected: terrestrial and marine ecosystems, as well as human populations. The way we live on land clearly impacts the ocean, and our land-based activities can have highly detrimental consequences for marine life. 

But eutrophication does not only impact marine life: it also has dramatic consequences for coastal communities. The economy is heavily affected as tourism, shellfish production and fishing cannot take place or become impaired. It is also a huge threat to human health when some of these algae release toxins which can be deadly to those who breathe them. 

Local issues in France and Spain

France and Spain are also concerned by this water pollution, and understanding some local examples can help us to understand a process happening globally. Let’s dive into some local ecosystems where « green tides » have been happening for a while, impacting highly biodiverse coastal regions.

In France, Brittany (North West) is the region the most hit by eutrophication. Green tides have been occurring each year, causing one of the most notorious ecological issues in the country. The shallow waters with weak currents and excess nutrients allow for the proliferation of algae, transforming the clear blue water into a green soup, leading to the closure of the beaches to the public and heavily affecting local shellfish farms. The algae also release toxic fumes which are deadly for animals and humans who breathe them. 

But the culprit is not the algae itself, nor is the excessive nitrogen found in the water allowing its proliferation: it is the intensive agriculture system taking place in the region, releasing a high quantity of nutrients in the water in an uncontrolled way. Brittany is one of the most important industrial farming regions of France characterised by intensive livestock farming and intensive use of fertilisers



In Spain, the salty Mar Menor lagoon, in the province of Murcia (South East), is one of the largest coastal lagoons in Europe. It is also one of the most endangered ones due to eutrophication.

The process is the same: an excess of nitrates coming from the agricultural industry and from poor sewage systems from the towns surrounding the lagoon have caused algae in the lagoon to bloom, resulting in the depletion of the oxygen from the water. Most of the seagrass was killed and thousands of fish were beached on the shore, unable to breathe. The damage done to the whole ecosystem is dramatic, not only from an ecological perspective but also for the local economy.

Make the ocean blue again

Now that we better understand the whole process of eutrophication and how anthropogenic land-based activities are causing coastal ecosystems to be unviable, let’s see what we can do about it.

Nature-based solutions

One of the ways we can help coastal ecosystems to recover from this source of pollution, or to prevent it from happening, is through restorative and mitigation actions. For example, the salt marshes in the lagoon of Mar Menor in Spain act as water filters, decreasing the nutrient concentrations of polluted waters flowing into the lagoon, and reducing the risk of eutrophication. Restoring and protecting some parts of the ecosystem which have remediation effects can be an effective nature-based solution to put in place. 

Transforming our agriculture system

Acting directly at the source is probably the most effective thing we can do. Because the agriculture industry is the main polluter, it is probably where we should try to implement change. Instead of promoting an intensive agricultural model, characterized by intensive use of fertilisers and a huge concentration of animals per unit area, we should encourage an agricultural model that takes into account the environment and puts in place practices that can reduce the nutrient inputs and outputs from farming activities. 

Today, the subsidies given to farmers do not allow them to transition towards eco-friendlier practices. Instead, they promote more intensity and more production, at the expense of the environment – terrestrial and marine. Changes need to be implemented at the national or in the case of Spain and France, at the European level, where the regulations follow the Common Agricultural Policy. As individuals, we have the opportunity to decide who represents us at these levels, and whom we want to see defending our ideas in these decision-making bodies. We also have the choice to choose more wisely which food we want to put on our plates and which type of agricultural system we want to support when we buy our food.

The environment as a person

Another initiative that has been proposed to effectively defend the coastal environment and prevent it from harm, is to give an ecosystem the legal status of a person which can facilitate regulations to be put in place or harmful actions to be punished. For example, in the case of the Mar Menor lagoon in Spain, the senate has recognised it as a legal « person » that can be protected, preserved and restored by the government and citizens. Now, the lagoon has the right to « exist as an ecosystem and to evolve naturally » and its whole surface area as well as the surrounding coastline is now legally represented by a group of caretakers made of local citizens, scientists and officials.


Water pollution is happening globally, and closer to us than we sometimes imagine. After reading this article, we invite you to consider how our food system is impacting the ocean and the life within it, and what solutions you could put in place, at your level, to help to remediate this global issue of eutrophication. 

If you are interested in learning more about the green tides issues happening in France and Spain, we invite you to look closer at the work being done by the local charities to protect their coastal environment. 

  • To learn more about the green tides issue in Brittany, you can read the comic Algues vertes, l’histoire interdite (Green algae, a prohibited story) by Inès Léraud and Pierre Van Hove (French only) and/or follow the work of the local NGO raising awareness about and fighting against it: Halte aux Marées Vertes. 
  • To learn more about the eutrophication issue in the Mar Menor lagoon, follow the local initiative aimed to protect it: Pacto For El Mar Menor.