Eutrophication is a process in which nutrients build up in a body of water, leading to the rapid growth of plants and algae. This growth can use up oxygen in the water, making it hard for other living things to survive. This often happens when chemicals used to help crops grow are carried by rain into rivers, lakes, or other water sources. Eutrophication can occur naturally over time or be caused by human activities. Manmade eutrophication happens when waste from sewage, industrial processes, fertilizers, and other sources enter water systems. This pollution often causes large amounts of algae to grow, which can reduce oxygen levels in the water and harm the environment. To address this issue, many policies have been created, such as the United Nations Development Program’s sustainability goals.

To prevent or fix eutrophication, it is important to reduce pollution from sources like sewage and farming. This includes limiting the flow of nutrients into water systems. Introducing organisms like shellfish and seaweed, which can control nitrogen levels, may also help. These organisms reduce the growth of cyanobacteria, which are the main cause of harmful algae blooms.

History and terminology

The word "eutrophication" comes from the Greek word "eutrophos," which means "well-fed." Waterways that receive too many nutrients often develop large amounts of algae. This overgrowth can make the water less fresh, as the plants and animals that once lived there die off in what is called a "mass extinction." This process leaves behind areas with no oxygen, known as "dead zones." These zones grow larger over time, destroying more life until they reach areas with enough oxygen to stop their spread.

Water bodies with very few nutrients are called "oligotrophic," and those with moderate nutrients are called "mesotrophic." When eutrophication becomes severe, it may be described as "dystrophic" or "hypertrophic" conditions. Eutrophication is defined as the process where water quality worsens because of too many nutrients, leading to too much plant growth—mainly algae—and the decay of these plants.

Eutrophication was first recognized as a water pollution problem in lakes and reservoirs in Europe and North America during the middle of the 20th century. Important research conducted in the 1970s at the Experimental Lakes Area (ELA) in Ontario, Canada, showed that freshwater lakes are limited by the amount of phosphorus they contain. The ELA uses a method that studies entire ecosystems and focuses on long-term research about how human activities cause eutrophication in freshwater environments.

Causes

Eutrophication occurs when too many nutrients, such as phosphates and nitrates, enter water systems. These nutrients can come from different sources depending on the location. Before the 1970s, detergents containing phosphates were a major cause of eutrophication. After these detergents were phased out, sewage and agriculture became the main sources of phosphates. Nitrogen pollution mainly comes from agricultural runoff with fertilizers and animal waste, sewage, and nitrogen from the air that falls into water, often from burning fuels or animal waste.

The amount of plant growth in water systems depends on how quickly nutrients are added to the water and how quickly they are removed. This means some areas have more of certain nutrients than others, and different ecosystems have different factors that limit growth.

In most freshwater systems, phosphorus is the main nutrient that limits plant growth. Phosphate sticks to soil particles and settles in places like wetlands and lakes. Because of this, more phosphorus is now building up in freshwater areas.

In marine systems, nitrogen is the main nutrient that limits growth. Nitrous oxide, a gas made when fossil fuels are burned, enters water from the air, increasing nitrogen levels and causing more eutrophication in oceans.

Cultural or human-caused eutrophication happens because of human activities. This problem became more noticeable after chemical fertilizers were widely used in agriculture during the mid-1900s. Phosphorus and nitrogen are the main nutrients that cause eutrophication because they make water rich in nutrients, allowing plants like algae to grow quickly. These algae can block sunlight for underwater plants, changing the plant community. When algae die, bacteria break them down, using oxygen and sometimes creating areas with no oxygen.

These oxygen-free areas can kill fish and other oxygen-dependent animals. They also affect animals on land by reducing access to clean water. Over time, plants and algae that thrive in nutrient-rich conditions can change the structure and function of ecosystems, leading to loss of habitats and biodiversity.

Human activities that add too many nutrients include runoff from fertilized fields, lawns, and golf courses, untreated sewage, and pollution from burning fuels. Cultural eutrophication can happen in both fresh and salt water, with shallow areas being most affected. In shallow lakes and along shorelines, wind and waves can stir up sediments, releasing nutrients into the water and worsening eutrophication. Poor water quality from eutrophication can harm human uses like drinking water, industry, and recreation.

Eutrophication can also happen naturally over time as nutrients from minerals and dead plant matter slowly build up in water. Natural eutrophication is common in lakes and is influenced by factors like climate change and geology. Some artificial lakes show the opposite process, becoming less rich in nutrients over time as nutrient-poor water mixes with nutrient-rich water. The main difference between natural and human-caused eutrophication is that natural eutrophication happens very slowly, over long periods of time.

Effects

Eutrophication can cause several ecological effects: more phytoplankton, changes in types and amounts of macrophytes, less dissolved oxygen, more fish deaths, and loss of preferred fish species.

When an ecosystem gains more nutrients, primary producers benefit first. In water ecosystems, algae often grow more (called an algal bloom).

Algal blooms block sunlight for underwater plants and cause large changes in oxygen levels. Oxygen is needed by all plants and animals that breathe air, and it is made by plants and algae during the day through photosynthesis.

In eutrophic conditions, oxygen levels rise during the day but drop sharply at night because algae and microorganisms use oxygen to survive. When oxygen levels fall to very low levels, fish and other marine life can die. In extreme cases, no oxygen exists, allowing certain bacteria to grow. These areas are called dead zones.

Eutrophication can lead to competitive release by making a nutrient that was once scarce more available. This can change the types of species in an ecosystem. For example, more nitrogen might allow new species to grow and replace existing ones. This has been observed in New England salt marshes. In Europe and Asia, the common carp lives in naturally nutrient-rich areas and is adapted to these conditions. The spread of eutrophication in new areas helps explain why this fish successfully invades these places after being introduced.

Some algal blooms from eutrophication produce toxins harmful to plants and animals. Freshwater blooms can harm livestock. When algae die or are eaten, toxins like neurotoxins and hepatotoxins are released, which can kill animals and harm humans. For example, shellfish can absorb these toxins, making them dangerous for humans to eat. This causes illnesses like paralytic, neurotoxic, and diarrhetic shellfish poisoning. Other marine animals, such as predator fish, can also carry these toxins, as seen in ciguatera poisoning.

There are five toxins linked to harmful algal blooms (HABs): domoic acid, ciguatoxin, okadaic acid, brevetoxins, and saxitoxins. Except for ciguatoxin, these toxins cause different types of shellfish poisoning. Domoic acid causes amnesic poisoning; okadaic acid causes diarrhetic poisoning; brevetoxins cause neurotoxic poisoning; and saxitoxins cause paralytic poisoning. Different algae produce these toxins. For example, Alexandrium, Pyrodinium, and Gymnodinium species make saxitoxins. Saxitoxin is 50 times more deadly than strychnine and 10,000 times more deadly than cyanide.

Eutrophication and harmful algal blooms can harm economies by increasing water treatment costs, reducing commercial and recreational fishing, and lowering tourism income. Water treatment costs may rise due to cloudy water and issues with color or smell. However, controlled eutrophication might help increase fish production, boosting community income. Eutrophication can become harmful quickly, so it is not currently recommended due to high nutrient levels.

Human health risks from eutrophication include too much nitrate in drinking water and exposure to toxic algae. Nitrates can cause blue baby syndrome in infants and create harmful chemicals in treated water. Contact with toxic algae through swimming or drinking can lead to rashes, stomach or liver illness, and breathing or nervous system problems.

Causes and effects for different types of water bodies

When too many nutrients enter aquatic ecosystems, microscopic algae can grow quickly, forming an algal bloom. In freshwater environments, floating algal blooms are often caused by nitrogen-fixing cyanobacteria, also called blue-green algae. This happens when nitrogen becomes scarce but phosphorus levels stay high. Nutrient pollution is a major cause of algal blooms and the overgrowth of other aquatic plants. This can lead to competition for sunlight, space, and oxygen, which can harm ecosystems, food webs, and the diversity of species living in the water.

When large amounts of algae and plants die in nutrient-rich water, their decomposition uses up dissolved oxygen. This can cause fish to die and reduce biodiversity. Nutrients may collect in areas with little oxygen, often found in deeper water that is separated by layers of water. These nutrients may only return to the surface during seasonal changes in temperate regions or when water flows become turbulent. Dead algae and organic material from water entering lakes settle to the bottom and break down without oxygen, releasing gases like methane and carbon dioxide. Some methane may be broken down by bacteria, which can become food for zooplankton. This creates a cycle that supports the growth of phytoplankton and zooplankton, depending on oxygen levels in the water.

Excess growth of aquatic plants, phytoplankton, and algae can disrupt ecosystems by reducing oxygen levels, which are needed for fish and shellfish to survive. Dense algae on the water’s surface can block sunlight from reaching deeper water, harming plants that live near the bottom and affecting the entire ecosystem. Eutrophication also reduces the value of rivers, lakes, and the enjoyment of natural beauty. Health issues can occur when eutrophication makes it harder to treat drinking water.

Phosphorus is often the main cause of eutrophication in lakes polluted by sewage. The amount of algae and the health of lakes are closely linked to phosphorus levels. Studies in Ontario showed that adding phosphorus increases eutrophication. Later stages of eutrophication involve blooms of nitrogen-fixing cyanobacteria, which are limited only by phosphorus levels. Solutions to phosphorus-based eutrophication in freshwater lakes have been tested in some cases.

Eutrophication is common in coastal waters, where nitrogen is the main cause. In coastal areas, nitrogen is often the key limiting nutrient, unlike in freshwater where phosphorus is more important. Estuaries, which are between freshwater and saltwater, can be limited by both phosphorus and nitrogen and often show signs of eutrophication. Eutrophication in estuaries can lead to low oxygen levels in deep water, harming fish and degrading habitats. Natural processes like upwelling bring nutrient-rich deep water to the surface, supporting algae growth.

Human activities contribute nitrogen pollution to coastal waters, such as fish farming and industrial ammonia releases. Nutrients also come from rivers, groundwater, and the atmosphere. Nutrients from the open ocean are less affected by human activity, but climate change may alter ocean currents. Human activity has increased nitrogen and phosphorus inputs to coastal areas, though the extent varies by location. A third nutrient, dissolved silicon, comes mainly from weathered sediments and is less affected by humans.

Increased nitrogen and phosphorus inputs stress coastal ecosystems. These effects vary by region and depend on human activities in nearby areas. The geography of coastal zones also influences how nutrients spread and how oxygen is exchanged with the atmosphere. The impacts of eutrophication include:

• Satellite data show more chlorophyll in coastal areas worldwide, indicating higher phytoplankton activity due to increased nutrients.
• Changes in nutrient levels can shift phytoplankton species, favoring some over others. For example, diatoms (silica-rich species) may decline compared to other species, leading to harmful algal blooms in areas like the North Sea and Black Sea.
• Low oxygen levels have existed in some coastal areas for thousands of years, but human activity has worsened this. For example, the Gulf of Mexico has a large area of seasonal low-oxygen water, which has grown since the 1950s. Climate change may make these issues worse by increasing water layering, which limits oxygen mixing.

Extent of the problem

Surveys found that 54% of lakes in Asia are eutrophic; in Europe, 53%; in North America, 48%; in South America, 41%; and in Africa, 28%. In South Africa, a study by the CSIR using remote sensing showed that more than 60% of the reservoirs checked were eutrophic.

The World Resources Institute has identified 375 hypoxic coastal zones worldwide. These zones are mostly found in coastal areas of Western Europe, the Eastern and Southern coasts of the United States, and East Asia, especially Japan.

Prevention

As a society, we can take steps to reduce eutrophication, which harms humans and other living things, and help maintain a healthy environment. Some ways to do this include:

Cultural eutrophication can be addressed by improving sewage treatment. For example, upgrading sewage treatment plants to remove more nitrogen and phosphorus can reduce the amount of these nutrients released into water. However, even with good secondary treatment, most treated sewage still contains high levels of nitrogen in forms like nitrate, nitrite, or ammonia. Removing these nutrients is costly and challenging.

Laws that control how sewage is treated and discharged have helped reduce nutrient levels in ecosystems. Untreated domestic sewage is a major source of nutrients in water, so providing sewage treatment facilities is important, especially in highly urbanized areas and developing countries where such facilities are limited. Technology to safely reuse wastewater from homes and industries should be a top priority in policies aimed at reducing eutrophication.

Agriculture also contributes to eutrophication, and the U.S. Department of Agriculture suggests the following solutions:

• Nutrient management – Fertilizers should be used correctly, in the right amounts, at the right time, and with proper methods. Fields fertilized with organic materials can reduce harmful nitrate leaching compared to conventionally fertilized fields. However, in some cases, organic farming may cause more eutrophication than conventional farming. In Japan, livestock produces enough nitrogen to meet agricultural fertilizer needs.

• Year-round ground cover – Planting cover crops prevents bare soil, reducing erosion and nutrient runoff even after the growing season ends.

• Field buffers – Trees, shrubs, and grasses planted along field edges can trap runoff and absorb nutrients before they reach water bodies. Riparian buffer zones, which are areas near waterways, help filter pollutants like sediment and nutrients. Creating similar zones near farms and roads can also prevent nutrient pollution.

• Conservation tillage – Reducing the frequency and intensity of tilling helps nutrients absorb into the soil.

The United Nations’ Sustainable Development Goals (SDGs) recognize the harm eutrophication causes to marine environments. SDG 14, which focuses on life below water, includes a plan to create an Index of Coastal Eutrophication and Floating Plastic Debris Density (ICEP) by 2025. A key goal is to prevent and reduce marine pollution, especially from land-based sources like nutrient runoff.

Policies and regulations are tools to reduce eutrophication. Nonpoint sources of pollution, such as runoff from farms, are major contributors. These effects can be lessened through practices like protecting forest cover to reduce erosion and using sustainable agriculture to minimize soil runoff and fertilizer use. Waste disposal technology also plays a role in preventing eutrophication.

Current policies often use "command-and-control" approaches, which set strict limits on pollution inputs, emissions, or technologies. While these policies are easier to enforce, they may not be the most cost-effective. Many countries use similar rules.

Because water pollution can affect people far beyond a specific area, cooperation among organizations is needed to prevent contaminants that cause eutrophication. Governments, water management agencies, and local communities all share responsibility for protecting water bodies. In the United States, the Chesapeake Bay Program is a well-known example of a multi-state effort to reduce eutrophication.

Reversal and remediation

Reducing the amount of nutrients entering water systems is an important step in restoring their health. However, two important points must be considered: First, the process can take many years because nutrients stored in sediments can remain in the environment for a long time. Second, restoring ecosystems may require more than simply reducing nutrient inputs, as some ecosystems can exist in different stable states that are hard to change. Lakes affected by excessive nutrients, such as those suffering from eutrophication, often take decades to recover fully.

In environmental cleanup efforts, methods to remove nutrients include biofiltration, which uses living materials like plants or microorganisms to trap and break down pollutants. Examples of biofiltration include green belts, riparian zones, natural or man-made wetlands, and treatment ponds.

The National Oceanic and Atmospheric Administration in the United States has developed a forecasting tool to help predict harmful algal blooms in areas like the Great Lakes, the Gulf of Maine, and the Gulf of Mexico. Short-term predictions can show the strength, location, and movement of blooms to warn communities at risk. Long-term studies in specific regions help predict larger factors, such as the size of future blooms and conditions that might worsen environmental problems.

Nutrient bioextraction is a type of bioremediation that involves growing and harvesting plants and animals to remove nutrients from water. This includes farming shellfish like oysters and seaweed to take nitrogen and other nutrients from natural water bodies.

Research suggests that oyster reefs can help reduce nitrogen levels, which may lower the costs for groups required to meet nitrogen emission limits. If oysters keep nitrogen levels in estuaries below certain limits, they can prevent enforcement actions that would otherwise require costly measures. Studies show that oysters and mussels significantly affect nitrogen levels in estuaries by filtering water, controlling phytoplankton, and removing nutrients through harvesting, burial in sediments, or chemical processes like denitrification. Early work on using shellfish to improve water quality was done by Odd Lindahl and others in Sweden. In the United States, shellfish restoration projects have been carried out along the East, West, and Gulf coasts.

Studies have shown that seaweed, such as kelp, can help reduce nitrogen levels in water. Seaweed farming can also help reduce the effects of climate change by absorbing nitrogen, phosphorus, and carbon dioxide while producing oxygen. Some seaweeds grow very quickly and can remove large amounts of nutrients from polluted areas. Large-scale seaweed farming is considered a promising solution to reduce eutrophication in coastal waters.

Another method to address low oxygen levels or eutrophication in specific areas is injecting compressed air directly into water. This technique was used to restore the Salford Docks in England. For smaller water systems, such as aquaculture ponds, pump aeration is commonly used.

Removing phosphorus from water can help reduce eutrophication. One common method uses alum, a type of aluminum sulfate, to absorb phosphate. Many materials have been tested for this purpose, but alum is widely used. These materials are often added to the surface of water bodies and sink to the bottom, reducing phosphate levels. This method has been used globally, including under the name Phoslock. A large study monitored 114 lakes and found that alum reduced phosphorus levels for up to 11 years. In deeper lakes, the effect lasted longer (up to 21 years), while in shallower lakes, it lasted about 5.7 years. Alum was less effective in deep lakes and those with high external phosphorus inputs.

In Finland, efforts to remove phosphorus from rivers and lakes began in the mid-1970s and targeted areas polluted by industrial and municipal waste. These efforts achieved a 90% reduction in phosphorus. However, some pollution sources did not show reduced runoff even after these measures were applied.