Eutrophication: Hyper-Nutrition in Water

We explain the meaning, types, causes and consequences of eutrophication, a form of hyper-nourishment of water which can lead to algal blooms followed by hypoxic levels of dissolved oxygen.
Seashore green due to eutrophication
Eutrophication on Lake Bant, Germany. Photo: Ein Dahmer (CC BY-SA 4.0)

This article shows how human activities such as agriculture, industry, and sewage disposal are leading to eutrophication – the unsightly excess growth of algae in our streams, lakes and coastal areas. These algae blooms become so dense, that they can suffocate all marine life around them.

Eutrophication is a major problem for humans too, it degrades our beaches and costs the U.S. economy alone, around $2.2 billion annually. 1

What Exactly Is Eutrophication?

Eutrophication comes from the Greek, meaning well nourished.

Eutrophication occurs when a body of water – usually a lake or coastal area – has too many nutrients in it, especially nitrogen and phosphorus. The water is said to be, over-enriched. The presence of these nutrients causes tiny phytoplankton plants to grow out of control and form harmful algae blooms.

You might ask – isn’t this a good thing? After all, aren’t plants supposed to be good for the environment.

Well, not always.

Firstly, this layer of plant life or algae bloom, forms an impenetrable lid on the water. This lid prevents sunlight reaching other plants beneath so they can’t photosynthesize and produce oxygen. While most plants store enough energy to wait out these conditions, the real problem occurs when all the nutrients are used up and the water can no longer support all the excess life.

When this happens, the blooms die and sink. Then bacteria and other decomposers feed on their bodies in a chemical process of decay. During this process the decomposers consume oxygen.

In a healthy ecosystem, there is balance. The amount of living and dead plant matter is relatively constant, so the amount of oxygen in the water is constant too. However, where there is an imbalance, as in the case of eutrophication, this natural cycle is disrupted.

Sea creatures in the marine food web – like fish and aquatic insects – rely on a constant source of oxygen, which is dissolved in the water, to breathe. Without it, it they suffocate. And of course, when they die, the decomposers feed on them, using up even more oxygen, causing a positive feedback loop.

In this way, eutrophication is one of the main drivers of another serious problem – ocean deoxygenation. When oxygen levels fall so low, ‘dead zones’ are created. These are large areas of coastline and open sea which temporarily become uninhabitable for marine life.

Eutrophication also exacerbates ocean acidification. As those decomposers feed on dead animals and algae, they absorb oxygen but also expel carbon dioxide in the process. Excess carbon dioxide lowers the seawater pH – increasing acidity.

As you can see, it’s a triple whammy.

Types of Eutrophication

Natural Eutrophication

When we talk about eutrophication today, we are essentially referring to the man-made version, or cultural eutrophication.

However, eutrophication is also a natural process, particularly in lakes. When scientists discuss lakes, they might refer to a lake’s trophic state. The word trophic means nutrient, so they are talking about the lake’s nutrient levels. Essentially, how much phosphorus and nitrogen it contains.

When a lake first forms, it typically contains very few nutrients. It is said to be oligotrophicoligo meaning little. So there are few nutrients available to support life.

Overtime, perhaps thousands of years as the lake ages, sediments wash in and the lake becomes more fertile. When there is a medium amount of nutrients it is said to be mesotrophic and can support some algae blooms.

Eventually a lake may become eutrophic – meaning very fertile. This happens when enough nutrients are carried into the lake from the surrounding landscape.

The main difference between natural and anthropogenic eutrophication is that the natural process is very slow and happens on a geological time frame.

Much like climate change is natural – just not the rate at which it is occurring today.

Did You Know?

Paleolimnology is the study of lakes and lake sediments in order to reconstruct past climatic and environmental changes. Paleolimnologists recognize that climate change and geology are critical influences in regulating activities in lakes.

Cultural Eutrophication

Cultural eutrophication is essentially what everyone means these days when they talk about eutrophication. It is where eutrophication happens much faster because of human interference.

Human activities like farming and sewage disposal result in lots of extra phosphorus and nitrogen being flushed into streams and lakes, which eventually reach the ocean. 2 These nutrients drive the overgrowth of algae blooms.

Cultural eutrophication is yet another sign of the Anthropocene epoch.

How Widespread Is the Problem?

Very. According to the Survey of the State of the World’s Lakes, a project promoted by the International Lake Environment Committee (ILEC), eutrophication affects 54 percent of Asian lakes, 53 percent of lakes in Europe, 48 percent in North America, 41 percent in South America and 28 percent in Africa. Some South African scientists believe that the figure is understated in their region, knowing 4 billion liters of untreated sewage is discharged everyday into their rivers and dams.

As for coastal waters – as much as 78 percent of U.S. and 65 percent of European coastal waters are affected.

Did you know?

The nitrogen cycle is one of the most important biogeochemical circular pathways for soil and plants. Other similar pathways include: the carbon cycle and the water cycle. Human activities including climate change, are stressing all of these systems to a greater or lesser extent, and just exactly how the planet is likely to adjust is not yet known.

What Causes Eutrophication?

The causes of eutrophication really depend on the location. For example, agriculture is the key driver in the northern Gulf of Mexico. However municipal wastewater is the main culprit in Narragansett Bay, Rhode Island. 3. There are broad scale regional differences, but generally agricultural sources are the primary contributors.


It was only in the 1960s and 1970s, that scientists made a connection between algal blooms and fertilizers. That fertilizers people were putting on their crops, gardens and golf courses, were seeping into the water table and causing unexpected problems. Many lakes in Minnesota, for example were severely damaged by eutrophication in the 1950s. At that time, farming in the area had become more intensive and agricultural practices changed to include increased fertilization and enhanced drainage.


Farmers spray their crops with treated fertilizers which contain nitrogen and phosphorus, to help the plants grow faster. While plants absorb some of the nutrients, unused nutrients runoff into nearby streams and eventually end up in coastal waters. Those nutrients stimulate the growth of algae in water – just as they stimulate the growth of plants on land.

Between 1960 and 1990, the use of treated fertilizers increased seven times. Farmers tend to apply more than is necessary, it can be hard to judge the right amount. As a result about 20 percent of those nutrients are lost to leaching and surface runoff. 4

As time moves forward, recent research shows that climate change induced precipitation (rain), will substantially increase nitrogen runoff. In a business-as-usual greenhouse gas emission scenario, this will result in a 14 percent increase in nitrogen runoff in the U.S. by the end of the century. This will be particularly evident in the mid-west, the corn belt of America.

Offsetting this increase would require a 24 percent reduction in nitrogen outputs, representing a massive change in farm management.

Note: The main source of nitrogen pollution is runoff from agricultural land. Whereas phosphorus pollution mostly comes from households and industry.

Land Clearing

Changes in land use, primarily land clearing and deforestation for the purposes of farming or building homes. We know clearing forests is bad for the environment because it releases carbon dioxide (CO2) – further fuelling the climate crisis and raising global temperatures. But now we know it also contributes to eutrophication, causing the runoff of phosphates into local water supplies. 5

Sewage Pollution

The main source of phosphorus pollution comes from human waste – which we flush down our toilets – and from using phosphorus-containing household detergents. Some domestic wastewater plants attempt to remove this phosphorous, but many do not make the necessary investment. Needless to say, phosphorus discharges are a bigger problem in larger urban areas, especially coastal cities.

There is good news however. Discharges of phosphorus from urban wastewater treatment plants in many European countries have fallen by 50 to 80 percent in the past 15 years. 6 The main reason for this success is the upgrading of wastewater treatment plants to include phosphorus removal, along with a shift to phosphate-free detergents.

The largest algal bloom ever recorded in China, turned the Yellow Sea green. Officials in the city of Qingdao had used bulldozers to remove 7,335 tonnes of the growth from beaches.

Hopefully, better urban planning – like sponge cities which are designed around proper water management – will help lessen the effects of global warming on humans living in high density areas. It’s all part of the new climate-proof approach to city planning, known as green infrastructure.

What Are The Consequences of Eutrophication?

Water pollution is a problem that matters to us all, regardless of where we live. It begins where people live and damages resources that we all enjoy, use and consume. The main impact and ecological effects of eutrophication include:

  1. Loss of wildlife
    In an ecosystem with increased nutrients, there are winners and losers. Some species such as zooplankton and phytoplankton (algae) reap immediate benefits and thrive. However, as oxygen levels decline, fish and other marine animals begin to suffocate. Those that can’t swim away, like shellfish and coral reefs, die off. There are so many other problems too. For example, higher rates of photosynthesis due to excess plant life, can raise the ocean’s pH level during the day. This in turn can blind some organisms which rely on a certain chemical balance by impairing their chemosensory abilities.
  2. New invasive species
    Increased nitrogen or phosphorus can attract non-natives species who have an ecological advantage over native species. For example, Daphnia galeata – a type of water flea – is an invasive species in Lake Constance that arrived only in the course of the eutrophication. Generally, the species that live and breed in shallow water fare better than deeper water species. 7 Several types of jellyfish also fare well in blooms and they tend to prey on juvenile fish. They also feed on marine microbes which drive the aquatic food web like copepod, krill, isopod and amphipod.
  3. Toxicity Dangers
    Some algal blooms, also called harmful algal blooms, are toxic to plants and animals. Toxic compounds can make their way up the food chain, posing a threat to animals and humans. For example, humans can develop paralytic, neurotoxic and diarrhetic shellfish poisoning by eating infected shellfish. Other marine animals can be vectors, accumulating the toxin and causing ciguatera – a type of fish poisoning that causes nausea, pain, cardiac and neurological symptoms in humans when ingested.
  4. Unattractive Environment
    Dense blooms can be foul smelling, reduce water clarity and ruin the beauty of a beach, river or lake.

How Can Eutrophication Be Prevented?

The technology currently exists to control excessive nutrient additions. However, more effective environmental regulations are needed to control agricultural nutrient pollution, and investment in more advanced wastewater treatment plants will be needed to reduce these inputs and improve water quality. The enhancement of the quality of freshwater and coastal systems will become essential as climate change and human population growth place increased demands for high quality water resources.

Farming Practices & Land Usage

Nitrogen Testing

Soil Nitrogen Testing (N-Testing) is a method which helps farmers optimize the amount of fertilizer they use on crops. If adopted, farmers will have to spend less on fertilizers and environmental pollution is reduced.


Encouraging farmers to cut down fewer trees, protecting forest cover – will help reduce the amount of erosion leaching into a watershed. Tree planting initiatives should also be encouraged.

Waste Disposal

Underdeveloped nations in particular need to build better wastewater and sewage facilities, particularly in highly urbanized areas.

Reversing Pollution

Research shows that restoring critical marine habitats, such as mangrove forests (mangals), seagrass meadows, corals reefs, oyster reefs and salt marshes, help to remove excess nutrients from water.

Attention has also been paid to clean-up measures, which have been mostly successful. Finnish phosphorus removal measures started in the mid-1970s and have targeted rivers and lakes polluted by industrial and municipal discharges. So far these efforts have achieved a 90 percent removal rate.

Seaweed farming

Seaweed aquaculture, usually kelp, offers an opportunity for climate change mitigation, as well as helping to absorb phosphorus and nitrogen. It is nature’s natural pollution remover.


This involves applying a phosphorus sorbent (something that soaks up phosphorus) to a lake or waterway. The manipulation of biochemical processes, usually the phosphorus cycle, to achieve a desired ecological response in the ecosystem is called geo-engineering. The sorbent is applied to the surface of the water and, as it sinks to the bottom, it gradually reduces phosphate levels. Sorbents have been used to manage eutrophication worldwide.

The Perfect Storm

Our hydrosphere is being battered on every front. From eutrophication, over-fishing and global warming to acidification, deoxygenation and rising sea levels. The effects of global warming on oceans are well documented and continue to cause concern among oceanographers everywhere.

The mangrove forests, seagrasses and coral reefs that provide homes and nursery grounds for marine animals and provide protection against coastal flooding are disappearing.

Toxic phytoplankton are damaging fisheries and public health, the economic cost in the EU is estimated at $1 billion per year. 8

Is it any wonder that we say, the oceans are facing a perfect storm.


  1. “Eutrophication of U.S. Freshwaters: Analysis of Potential Economic Damages.” Walter K. Dodds et al. []
  2. “The Globalization of Cultural Eutrophication in the Coastal Ocean: Causes and Consequences.” August 2020 by Thomas C. Malone and Alice Newton. []
  3. Agriculture’s Impact on Aquaculture: Hypoxia and Eutrophication in Marine Waters. []
  4. Sources of Eutrophication – World Resources Institute []
  5. Forestry Operations and Eutrophication – []
  6. European Environment Agency []
  7. Does eutrophication-driven evolution change aquatic ecosystems? 2017. Timothy J. Alexander et al. []
  8. (Hoagland and Scatasta, 2006 []
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