Humans against the planet: Eutrophication

Today we want you to familiarize with a phenomenon that you probably have witnessed while going for a walk in the nature. It goes by the name of eutrophication (from the Greek “well fed”) and you might not be aware of its implications. As always, lets go through it step by step:

How does this phenomenon manifest itself?

Eutrophication occurs in aquatic ecosystems such as lakes or ponds and we appreciate it visually when we see excessive growth of algae on the surface of the water (algal bloom). Sadly, the apparent simplicity of this phenomenon hides a tragedy, which is the death of most part of the biodiversity that inhabits the ecosystem (fish are the most shocking evidence). In the following photo you can see an eutrophic pond where the surface area is invaded by algae, which can be indistinctly macroalgae or prokaryotic and eukaryotic microalgae. Remember, you can refresh the information on algae by reading our article The colonies of the future.


© Copyright N Chadwick.
The most dramatic evidence of eutrophication is the death of fish. Photo: Colourbox

What causes it?

Pretty simple. As its name suggests, eutrophication occurs when there is an excess of nutrients in the water. Although many other parameters may have to do with it, it is mainly linked to the increase of phosphorus (P) in the form of phosphates, and nitrogen (N) in the form of nitrates and ammonium. The availability of these two elements is limited in aquatic ecosystems and they are very necessary for algae to grow. In fact, P is the most limiting nutrient in freshwater ecosystems while N is in marine ecosystems. One could easily think that this is good for the ecosystem, as “more nutrients” can be associated with biosphere growth and a benefit to the environment. But that is not the case.

What happens is that the increase in nutrients triggers a very rapid growth of algae (bloom), which in turn provide oxygen to other organisms that will also grow exponentially. Eventually all this biomass will no longer have resources to sustain itself and will gradually decompose. When we talk about decomposition, we mean that other bacteria will make use of this biomass, consuming oxygen and releasing CO2 as they grow. Therefore, the water ends up in a situation of hypoxia (lack of oxygen) and high acidity (due to CO2). The algae themselves are the ones that can usually re-introduce dissolved oxygen into the water when they do photosynthesis, but because of the dead biomass layer on top of the pond, light will not penetrate sufficiently into the water and the situation will worsen.

The result? A loss of animal and microbial diversity due to the impossibility of most species to survive without oxygen and light. However, nothing in nature is as simple as it seems. Many other parameters can stimulate eutrophication and were set aside for our explainaiton. Some of them are: temperature increases, salinity decreases, intervention of certain microbes, light intensity increases or water perturbations (resolubilization of nutrients retained in sediments).

Where do the nutrients come from?

Here is where humans are to blame. The arrival of new nutrients in aquatic systems usually has to do with human influence. The most common cases are:

  1. Fertilizers used in agriculture. They precisely aim to provide nitrogen and phosphorous to the crops. Ideally, fertilizers are to be absorbed by roots or to be retained in soil layers close to the surface, but reality is that a fraction of these fertilizers ends up in water reservoirs.
  2. Urban and industrial wastewater. Wastewater contains numerous nutrients in a variety of forms, toxic or not, depending for who. When untreated wastewater is dumped or leaked into natural reservoirs, not only its contaminants can be a threat to the ecosystem but also its available nutrients.
  3. Animal wastes. Animal excrements and other farm waste are filled with N and P in the form of nitrates, ammonium and phosphates, as well as other cations such as magnesium and potassium.
  4. Other sources. Water can also be polluted with nutrients that come from sources not mentioned above. For example, forest pruning spoils that are not removed from site can contribut to eutrophisation. Another example are salts that accumulate in aquifers and rivers and originate when water enters in contact with pollutants in the air (such as nitrogen and sulfur oxides).

Is there anything to do about it?

Eutrophication was recognised as a worldwide problem in the middle of the twentieth century and research currently indicates that approximately 50% of lakes in Asia, Europe and America are eutrophic. One of the most shocking examples is the case of the Caspian Sea, which has very high levels of eutrophication (especially in the northern side). As you can see in the following satellite photo, the green twirls indicate the presence of algae.

NASA images by Norman Kuring, NASA’s Ocean Color web. Caption by Adam Voiland.

The best solution is to prevent the entry of pollutants derived from human activity, either with firm legislation regarding fertilizer doses or strict control of waste dumping. However, there are other initiatives that try to address the problem after it occurs. Among them we find the establishment of mollusk reefs (oysters and mussels) in the estuaries of rivers as they filter suspended solids and remove nitrogen from the water column. Establishing seaweed algae has also been shown to help mitigate pollutants because they absorb N, P, and CO2. Precisely, it is believed that these large-scale algae may also be one of the best options for mitigating and adapting to climate change.



  • Catalan wikipedia for eutrophication.
  • ILEC/Lake Biwa Research Institute [Eds]. 1988-1993 Survey of the State of the World’s Lakes. Volumes I-IV. International Lake Environment Committee, Otsu and United Nations Environment Programme, Nairobi.



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