Eutrophication – a destructive chain reaction in the ecosystem

Eutrophication - a destructive chain reaction in the ecosystem
Key learning

Eutrophication is a process that occurs when the water becomes overly enriched with nutrients. But how does eutrophication happen, and what is its impact? This article explains eutrophication and explores its impact on the Baltic Sea.

You know how it should be. The shallow waters are fresh and clear as the new day. The light salty scent of the Baltic Sea. Perhaps even a dog is straying into the coolness of the sea, seeking relief from the summer heat.

Suddenly, the summer wind brings the sour air from the blue-green algae clogging the seaside. Eutrophication, someone whispers when you look at the scene, sensing the dire conditions below the surface. You have heard the term before but do not know what it means. Yet, you experienced the negative effect of harmful algae blooming up close last year when your dog became sick after a cooling dip…

Keep reading as we explain eutrophication and explore its impact on the Baltic Sea.

What is eutrophication?

Again, you might be more familiar with the visual symptoms of eutrophication – the algae blooming, causing a green lid on the surface – than the term itself. However, eutrophication is a process that occurs when the water becomes overly enriched with nutrients.

Yet, many areas are naturally eutrophic, so it is not necessarily a harmful or undesirable condition. Let us consider the process in a lake.

Then, natural eutrophication is caused by a slow increase of organic matter like leaves or trees falling into the water. Sometimes it is referred to as “lake ageing.” And the natural ageing process occurs over a long period, for as many as hundreds of years.

Natural eutrophication (traditionally referred to as lake ageing) happens over decades or centuries.
Natural eutrophication (traditionally referred to as lake ageing) happens over decades or centuries.

But, when human activities speed up the processes, the consequences of eutrophication are more severe and damaging. Inputs of nutrients like nitrogen and phosphorus have been increasing for a long time in the Baltic Sea.

And modernisation is the primary force behind this.

Modernisation – the main driver of human-made eutrophication

The global developments during the 1950s and 1960s were remarkable, particularly within the Baltic Sea region. Modernisation led to more farming, industries, transport, growing cities, and other sea-based activities. Transforming the society (and lifestyle) from rural and agrarian to a secular, urban, and industrial community.

But decades of development brought unwanted impacts. And the modernisation that citizens longed for had consequences realised much later.

In the context of eutrophication, some of the critical activities were (and still are),

Agriculture
Urban and industrial wastewater
Transports
Modernisation is the main contributor to eutrophication.
Modernisation is the main contributor to eutrophication.

Agriculture intensified both due to mechanisation and the substantial use of fertilisers. Run-off from agricultural areas led to a considerable boost in nutrient input to the sea.

Urbanisation and, primarily, the large coastal cities are the main contributors of human-introduced nutrients via domestic and industrial sewage and wastewater.

The vessels transporting goods on the Baltic Sea increase nutrient input into the marine ecosystem via air emissions and sanitary wastewater discharge.

So, modernisation, unfortunately, contributes to eutrophication. But what are the consequences of human-induced eutrophication?

Key impacts due to eutrophication

The two most detrimental effects of eutrophication are,

Harmful algae blooming
Lack of oxygen
The excessive combination of nitrogen, phosphorus and sunlight makes the recipe for the blue-green algae.
The excessive combination of nitrogen, phosphorus and sunlight makes the recipe for the blue-green algae.

Harmful algae blooming – the visual effect of eutrophication

You might have experienced the high volumes of algae in the summertime. In itself, it is not a problem since most algae are an integral part of habitats.

But in the context of eutrophication, the villain is the blue-green algae. It is known as the cyanobacteria and has a specific skill as it can use nitrogen straight from the atmosphere.

And in combination with phosphorus-rich water from the deep and the summer sun, it makes the green mess you have seen on shores and beaches. The blue-green algae are causing harm and danger to both humans and animals.

The extreme effect of eutrophication: When the seabed lacks oxygen, exposed areas lose their function as a habitat and damage the food web. And, as a consequence, put biodiversity in the Baltic Sea at stake.
The extreme effect of eutrophication: When the seabed lacks oxygen, exposed areas lose their function as a habitat and damage the food web. And, as a consequence, put biodiversity in the Baltic Sea at stake.

Lack of oxygen – the choking effect of eutrophication

As you can learn in the infographic, eutrophication may lead to a lack of oxygen at the seabed. And unfortunately, the Baltic Sea is naturally prone to a lack of oxygen because of the limited water exchange with the North Sea and by being a shallow inland sea.

And an outcome of what happens when a seabed lacks oxygen is that exposed areas in the sea lose their function as habitat, damaging the food web. And, as a consequence, the biodiversity in the Baltic Sea.

Biodiversity is at stake – the extreme effect of eutrophication

Eutrophication is a significant threat to the Baltic Sea since it affects water quality and biodiversity. As we have heard, some species, such as blue-green algae, thrive from eutrophication. Yet, other species are in danger of disappearing from the sea.

Losing species such as the vital but fragile bladderwrack would be incredibly harmful to the ecosystem. Since the seaweed is an essential habitat for many invertebrates and young fishes and is a food source for several animals, it would lead to the decline of many other species.

So, if the bladderwrack declines as a species, it would affect biodiversity. And making the Baltic Sea even more vulnerable since a homogenous ecosystem is less durable when environments change. A diverse range of species is needed to protect the Baltic Sea from other impacts such as climate change.

Therefore, curbing eutrophication is essential for the protection of biodiversity.

Infographic of eutrophication

Eutrophication process infographic
1) Human activity enriches the water with nutrients, which leads to excessive algae growth. 2) Excessive algae growth blocks the sun. 3) Due to the lack of sunlight, other vegetation dies. 4) Dead vegetation decomposes and releases carbon dioxide. 5) Bacteria use a lot of oxygen when digesting dead plants. 6) Water becomes acid – known as acidification. 7) With less oxygen, fish and other species suffocate and die. 8) The extreme effect of eutrophication – areas with insufficient oxygen levels to sustain life.

Voice of the Ocean sustains ocean observatories

The Baltic states are working towards creating sustainable policies to reduce the input of fertilisers from land. And there are signs of hope and positive developments.

Nevertheless, we still see these harmful algae blooms and eutrophication events. Much work, research, funding, and fundamental societal changes are needed to help the Baltic Sea restore its durability.

To track how much of a difference mentioned policy changes are making, we need to measure how the nutrient and oxygen levels change on weekly, seasonal, and annual time scales.

As part of this effort, Voice of the Ocean sustains ocean observatories that collect dissolved oxygen measurements. We are also working to deploy nitrogen measuring devices on autonomous underwater robots to make gathering these crucial data easier.

Oxygen concentration measurements taken by one of our gliders in the Bornholm Basin. Here we see variability in oxygen concentrations during winter 2021. Alarmingly low oxygen levels prevail in the basin’s deepest regions (red colour).

Further reading

HELCOM
The European Commission
Stockholm University Baltic Sea Centre

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