Eutrophication facts for kids
Eutrophication is a process where a body of water, or parts of it, gets too many minerals and nutrients. The main nutrients are nitrogen and phosphorus. It can also be described as a big increase in tiny water plants called phytoplankton.
Water bodies with very few nutrients are called oligotrophic. Those with a moderate amount are mesotrophic. If there are too many nutrients, it can be called dystrophic or hypertrophic. Eutrophication can happen in freshwater or salt water systems. In freshwater, it's usually caused by too much phosphorus. In coastal waters, nitrogen or both nitrogen and phosphorus are often the main cause. This depends on the area.
When eutrophication happens naturally, it's a very slow process. Nutrients build up in the water over a long time. These nutrients come from rocks breaking down and from tiny plants like lichens and mosses. But when humans cause it, it's called "cultural eutrophication." This happens much faster. Nutrients are added to water from things like untreated sewage, factory waste, and fertilizer from farms. Nutrient pollution is a big reason for eutrophication in surface waters. Too many nutrients make algal and aquatic plants grow too much.
A common sign of eutrophication is algal blooms. These can just be annoying, or they can become harmful algal blooms. Harmful blooms can really damage water environments. When these algae die, bacteria break them down. This uses up oxygen in the water, which can make it hard for other living things to breathe.
To stop or fix eutrophication, we need to reduce pollution from sewage and farms. Some new ideas, like using shellfish or seaweed, are also being tested. It's important to remember that many people use the word "eutrophication" in different ways.
What Causes Eutrophication?
More Plants and Animals Growing
Eutrophication means more living things are growing in a water body. This happens because there are more plant nutrients, usually phosphate and nitrate. More nutrients lead to more growth of aquatic plants, both large ones and tiny ones like phytoplankton. When there's more plant food, there are also more invertebrates and fish species.
As this process continues, the total amount of living things in the water increases. But the variety of different living things (biodiversity) goes down. If eutrophication gets very bad, bacteria break down all the extra plants. This uses up oxygen, which can create areas with very little oxygen. These low-oxygen areas are called hypoxic zones. They often appear in deep lakes during summer.
Lakes with strong eutrophication can run out of oxygen completely after big algal blooms. In the ocean, too many nutrients and water that doesn't mix well can also lead to low oxygen. This makes it hard for fish and other animals to live there.
Phosphorus is a key nutrient for plants. It's often the main thing that limits plant growth in most freshwater systems. Phosphorus sticks to soil, so it mostly moves into water through erosion and runoff. Once it's in lakes, it's hard to get it out of the water.
In marine ecosystems (saltwater), nitrogen and iron are usually the main nutrients that limit how much algae can grow. But in general, nitrogen, phosphorus, and iron can all be limiting. How much plants grow in any water system depends on how many nutrients come from outside sources. It also depends on how nutrients are recycled inside the water. Light is also important, so plants won't grow much in deep water or in winter when there's less light.
Where Do the Extra Nutrients Come From?
The extra phosphorus comes from things like phosphates in detergent, industrial waste, home waste, and fertilizers. Since the 1970s, many countries have stopped using phosphates in detergents. So now, industrial waste, sewage, and farming are the main causes of eutrophication.
The main sources of nitrogen, besides natural processes, are farm runoff (from fertilizers and animal waste), sewage, and nitrogen from burning fuels or animal waste in the air.
Human-Made Nutrient Pollution
Types of Eutrophication
Cultural Eutrophication
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Cultural, or human-caused, eutrophication speeds up the natural process. This happens because of human activities like clearing land and building cities. Water runoff flows faster, carrying more nutrients like phosphates and nitrates into lakes, rivers, and then into coastal estuaries and bays.
Cultural eutrophication happens when too many nutrients from human activities end up in water bodies. This creates nutrient pollution and makes eutrophication happen much faster. This problem became more obvious after chemical fertilizers were widely used in farming in the mid-1900s. Phosphorus and nitrogen are the two main nutrients that cause cultural eutrophication. They make the water rich, allowing some aquatic plants, especially algae, to grow very quickly and form dense blooms.
Algal blooms can block sunlight from reaching plants at the bottom. When algae die, bacteria break them down. This process uses up oxygen, which can lead to areas with no oxygen (anoxic conditions). This lack of oxygen kills animals that need oxygen, like fish and other water creatures. It also affects land animals, limiting their access to the affected water for drinking.
The growth of certain algae and water plants that like nutrient-rich conditions can harm entire water ecosystems. This can change food webs and lead to a loss of homes for animals and a decrease in the variety of species.
There are several sources of too many nutrients from human activity. These include runoff from fertilized fields, lawns, and golf courses. Untreated sewage and wastewater also contribute. Burning fuels can also create nitrogen pollution. Cultural eutrophication can happen in both fresh and salt water. Shallow waters are most easily affected. In shorelines and shallow lakes, wind and waves often stir up sediments. This can release nutrients from the sediments into the water, making eutrophication worse.
The poor water quality from cultural eutrophication can negatively affect how humans use water. This includes drinking water, industrial uses, and recreation.
Natural Eutrophication
Even though humans often cause eutrophication, it can also happen naturally, especially in lakes. Scientists who study old lakes now know that climate change, geology, and other natural things also affect how many nutrients are in lakes. Some lakes even show the opposite process, becoming less nutrient-rich over time. This can happen in artificial lakes that are very rich in nutrients when first filled. They might become less nutrient-rich over time. The main difference is that natural eutrophication is very slow, taking thousands of years.
Effects of Eutrophication
Harmful Effects on Nature
Eutrophication can have these effects on nature:
- More tiny water plants (phytoplankton) growing.
- Changes in the types and amounts of larger water plants (macrophytes).
- Less oxygen in the water.
- More fish kills.
- Loss of desired fish species.
Less Variety of Life
When an ecosystem gets more nutrients, the first ones to benefit are the primary producers. In water, this means things like algae grow a lot. This is called an algal bloom. Algal blooms block sunlight from reaching creatures living at the bottom. They also cause big changes in how much oxygen is dissolved in the water.
All plants and animals that breathe air need oxygen. Plants and algae make oxygen during the day through photosynthesis. But in eutrophic conditions, dissolved oxygen goes up a lot during the day. Then it drops sharply after dark. This happens because the algae and tiny organisms that eat dead algae use up oxygen when they breathe. When oxygen levels get too low (hypoxic), fish and other marine animals can't breathe and die. This means creatures like fish, shrimp, and especially animals that can't move much on the bottom, die off. In very bad cases, conditions become anaerobic (no oxygen). This allows certain bacteria to grow. Areas where this happens are called dead zones.
New Species Moving In
Eutrophication can allow new, competitive species to move in and take over from the original ones. This happens by making a nutrient that was once limited now very common. For example, more nitrogen might let new species out-compete the native ones. This has happened in New England salt marshes. In Europe and Asia, the common carp often lives in naturally eutrophic areas. It is well-suited to these conditions. Eutrophication in areas outside its natural home helps explain why this fish has spread so much after being introduced.
Toxins
Some harmful algal blooms caused by eutrophication are toxic to plants and animals. These toxic compounds can move up the food chain, causing animals to die. Freshwater algal blooms can be dangerous to farm animals. When the algae die or are eaten, they release neuro- and hepatotoxins. These can kill animals and might be a danger to humans.
An example of algal toxins affecting humans is shellfish poisoning. Toxins made during algal blooms are taken up by shellfish (mussels, oysters). Then, when humans eat these shellfish, they can get poisoned. Examples include paralytic, neurotoxic, and diarrhoetic shellfish poisoning. Other sea animals can also carry these toxins. For example, in ciguatera poisoning, a predator fish eats smaller fish with the toxin and then poisons humans.
Economic Effects
Eutrophication and harmful algal blooms can cost money. This happens because of:
- Higher water treatment costs.
- Losses for commercial fishing and shellfish industries.
- Losses for recreational fishing (fewer fish and shellfish to catch).
- Less money from tourism (water bodies look less appealing).
Water treatment costs can go up because the water becomes less clear (more cloudy). There can also be problems with the color and smell of drinking water.
Health Impacts
Eutrophication can affect human health. For example, too much nitrate in drinking water can cause blue baby syndrome. Also, chemicals formed during water disinfection can be harmful. Swimming in water with a harmful algal bloom can cause skin rashes and breathing problems.
Causes and Effects in Different Water Types
Freshwater Systems
When extra nutrients are added to aquatic ecosystems, tiny algae grow very fast. This creates an algal bloom. In freshwater ecosystems, these floating algal blooms are often made of nitrogen-fixing cyanobacteria (blue-green algae). This happens when nitrogen becomes limited, but there's still a lot of phosphorus. Nutrient pollution is a main cause of algal blooms and too much growth of other water plants. This leads to overcrowding and competition for sunlight, space, and oxygen. More competition for nutrients can disrupt entire ecosystems and food webs. It can also lead to a loss of animal homes and fewer types of species.
When large water plants (macrophytes) and algae die in lakes, rivers, and streams with too many nutrients, they break down. Microorganisms turn the nutrients in this dead material into a simple form. This breakdown process uses up oxygen, which lowers the amount of dissolved oxygen. Low oxygen levels can then cause fish kills and other problems that reduce biodiversity. The dead algae and other organic material that flows into a lake settle at the bottom. There, they break down without oxygen, releasing greenhouse gases like methane and CO2.
Too much growth of water plants, phytoplankton, and algal blooms messes up how the ecosystem normally works. This causes problems like a lack of oxygen for fish and shellfish. Dense algae on the surface can block light from reaching deeper water. This harms plants at the bottom and affects the whole ecosystem. Eutrophication also makes rivers and lakes less enjoyable and beautiful. Health problems can happen when eutrophic conditions make it hard to treat drinking water.
Phosphorus is often seen as the main problem in lakes polluted by sewage pipes. The amount of algae and the trophic state of lakes match well with phosphorus levels in the water. Studies in Canada showed a link between adding phosphorus and how fast eutrophication happened. Later stages of eutrophication lead to blooms of nitrogen-fixing cyanobacteria. These are limited only by the amount of phosphorus.
Coastal Waters
Eutrophication is common in coastal waters. In coastal waters, nitrogen is usually the main nutrient that limits growth in marine waters. This is different from freshwater systems, where phosphorus is often the limiting nutrient. So, nitrogen levels are more important than phosphorus levels for understanding and controlling eutrophication in salt water. Estuaries, which are where fresh and salt water meet, can be limited by both phosphorus and nitrogen. They often show signs of eutrophication. Eutrophication in estuaries often leads to low oxygen or no oxygen at the bottom. This causes fish kills and harms habitats.
Examples of human-made sources of nitrogen-rich pollution in coastal waters include sea cage fish farming and ammonia released from making coke from coal. Besides runoff from land, waste from fish farming, and industrial ammonia, nitrogen from the atmosphere can be an important nutrient source in the open ocean. This could provide about one-third of the ocean's new nitrogen supply.
Coastal waters include many different marine habitats, from enclosed estuaries to the open waters of the continental shelf. How much phytoplankton grows in coastal waters depends on both nutrient and light supply. Light is a big limiting factor near shore, where stirred-up sediment often blocks light.
Nutrients enter coastal waters from land through rivers and groundwater, and also from the air. There's also an important source from the open ocean, as nutrient-rich deep ocean waters mix in. Nutrient inputs from the ocean don't change much due to human activity. However, climate change might alter water flows. In contrast, human activity worldwide has increased inputs of nitrogen and phosphorus from land to coastal zones. How much these inputs have increased varies greatly depending on human activities in the surrounding areas. A third key nutrient, dissolved silicon, comes mainly from rocks breaking down into rivers and from offshore. So, it's much less affected by human activity.
Effects of Coastal Eutrophication
These increasing nitrogen and phosphorus inputs cause eutrophication in coastal zones. The effects vary depending on activities in the surrounding land and how much nutrient load there is. The location of the coastal zone is also important. It controls how much the nutrients are diluted and how much oxygen mixes with the air. The effects of eutrophication can be seen in several ways:
- Satellite monitoring shows that the amount of chlorophyll (a measure of overall phytoplankton activity) is increasing in many coastal areas worldwide. This is due to more nutrient inputs.
- The types of phytoplankton can change because of more nutrients and changes in the amounts of key nutrients. Specifically, increases in nitrogen and phosphorus, with smaller changes in silicon, change the ratio of these nutrients. These changing ratios cause shifts in phytoplankton types. This particularly harms silica-rich phytoplankton like diatoms compared to other species. This leads to annoying algal blooms in areas like the North Sea and the Black Sea. Sometimes, too many nutrients can lead to harmful algal blooms (HABs). These blooms can happen naturally, but there's good evidence they are increasing because of nutrient enrichment.
- Oxygen depletion has happened in some coastal seas, like the Baltic Sea, for thousands of years. In these areas, the water layers don't mix much, which limits oxygen in deep water. However, more organic material that bacteria can break down flowing into these deep waters can make this oxygen depletion in oceans worse. These areas with lower dissolved oxygen have increased globally in recent decades. They are usually linked to too many nutrients and the resulting algal blooms. Climate change will generally make water layers separate more, which will worsen this oxygen problem. An example is in the Gulf of Mexico. Here, a seasonal area with no oxygen, over 5000 square miles, has grown since the 1950s. The increased plant growth causing this lack of oxygen is fueled by nutrients from the Mississippi River. A similar process has been seen in the Black Sea.
How Big is the Problem?
Surveys show that:
- 54% of lakes in Asia are eutrophic.
- 53% in Europe.
- 48% in North America.
- 41% in South America.
- 28% in Africa.
In South Africa, a study found that over 60% of the surveyed reservoirs were eutrophic.
The World Resources Institute has found 375 hypoxic (low oxygen) coastal zones worldwide. These are mostly in Western Europe, the Eastern and Southern coasts of the US, and East Asia, especially Japan.
Global Goals
The United Nations has goals for Sustainable Development. They recognize the harm eutrophication causes to ocean environments. They have set a goal to create an Index of Coastal Eutrophication and Floating Plastic Debris Density (ICEP) as part of Sustainable Development Goal 14 (life below water). SDG 14 specifically aims to: "by 2025, prevent and significantly reduce marine pollution of all kinds, in particular from land-based activities, including marine debris and nutrient pollution."
How to Prevent Eutrophication
Reducing Pollution from Sewage
Finland started measures to remove phosphorus in the mid-1970s. These efforts focused on rivers and lakes polluted by factory and city waste. They achieved 90% removal efficiency. However, some targeted pollution sources still showed runoff despite reduction efforts.
There are many ways to fix cultural eutrophication. Raw sewage is a direct source of pollution. For example, sewage treatment plants can be improved to remove nutrients. This means they release much less nitrogen and phosphorus into the water. But even with good treatment, most treated sewage still contains a lot of nitrogen. Removing these nutrients is expensive and often difficult.
Laws about sewage discharge and treatment have greatly reduced nutrients in ecosystems. Since untreated domestic sewage is a major cause of nutrient pollution, it's important to have treatment facilities in highly populated areas. This is especially true in developing countries, where treating home wastewater is rare. Using reused wastewater safely and efficiently, from both homes and factories, should be a main focus for policies about eutrophication.
Reducing Nutrient Pollution from Farms
There are many ways to help fix cultural eutrophication caused by farming. Safe farming practices are the best way to solve the problem. Some safety steps are:
- Nutrient Management Techniques: Farmers should apply fertilizer in the right amount, at the right time of year, with the right method and placement.
- Year-Round Ground Cover: A cover crop will prevent bare ground. This stops erosion and nutrient runoff even after the main growing season.
- Planting Field Buffers: Planting trees, shrubs, and grasses along the edges of fields helps catch runoff. They absorb some nutrients before the water reaches a nearby water body.
- Conservation Tillage: Reducing how often and how deeply land is tilled helps nutrients soak into the ground.
Reducing Nonpoint Pollution
Nonpoint pollution is the hardest source of nutrients to manage. However, studies suggest that when these sources are controlled, eutrophication decreases. The following steps are suggested to reduce pollution from widespread sources that can enter water ecosystems.
Buffer Zones Near Rivers
Studies show that stopping pollution between its source and the water is a good way to prevent it. Riparian buffer zones are areas of land next to a flowing body of water. They are created to filter pollutants. Sediment and nutrients are deposited here instead of in the water. Creating buffer zones near farms and roads is another way to stop nutrients from traveling too far. However, studies have shown that nitrogen pollution from the air can travel far past buffer zones. This suggests that the most effective prevention is to control the pollution at its main source.
Prevention Policies
Policies regulating how farms use fertilizer and animal waste are needed. In Japan, the amount of nitrogen produced by livestock is enough for the agriculture industry's fertilizer needs. So, it makes sense to require livestock owners to collect animal waste from fields. If left, it will leach into groundwater.
Policies for preventing and reducing eutrophication can be divided into four areas: technologies, public involvement, economic tools, and cooperation. "Technology" here means using existing methods more widely, not just new inventions. As mentioned, nonpoint pollution is the main cause of eutrophication. Its effects can be easily reduced through common farming practices. Less pollution reaching a watershed can be achieved by protecting its forest cover. This reduces erosion flowing into the water. Also, using land efficiently with sustainable farming practices reduces land degradation. This lowers the amount of soil runoff and nitrogen-based fertilizers reaching a watershed. Waste disposal technology is another factor in preventing eutrophication.
The public plays a big role in preventing eutrophication. For any policy to work, people must know how they contribute to the problem. They also need to know how to reduce their impact. Programs that encourage recycling and waste reduction are necessary. Education on smart water use is also important to protect water quality in cities and nearby water bodies.
Economic tools, such as property rights, water markets, and financial incentives, are becoming important for pollution control. Giving rewards to those who use clean, sustainable water management methods is a good way to encourage pollution prevention. By making governments pay for the negative environmental effects, they are encouraged to manage water more cleanly.
Because a body of water can affect many people far beyond its immediate area, cooperation between different organizations is needed. Agencies from state governments to water resource management groups and non-governmental organizations, down to local communities, are all responsible for preventing eutrophication. In the United States, the most famous effort between states to prevent eutrophication is for the Chesapeake Bay.
Nitrogen Testing and Modeling
Soil nitrogen testing (N-Testing) helps farmers use the right amount of fertilizer on crops. By testing fields this way, farmers have saved money on fertilizer, reduced nitrogen loss, or both. By testing the soil and figuring out the minimum fertilizer needed, farmers benefit financially while reducing pollution.
Organic Farming
Organically fertilized fields can "significantly reduce harmful nitrate leaching" compared to fields using traditional fertilizers. However, in some cases, organic farming can have a higher eutrophication impact than conventional farming.
How to Fix Eutrophication
Recovering from Eutrophication
Reducing nutrient inputs is key to fixing the problem. But there are two things to remember: First, it can take a long time, especially because nutrients are stored in sediments. Second, fixing it might need more than just reducing inputs. This is because there can be several stable but very different natural states. Lakes that have too many nutrients recover slowly, often taking many decades.
New ideas have been developed to deal with nutrient pollution in water. These ideas change or improve natural processes to shift nutrient effects away from harming nature. Nutrient remediation is a type of environmental cleanup. It focuses only on nutrients that are active in biology, like nitrogen and phosphorus. "Remediation" means removing pollution or harmful substances, usually to protect human health.
In environmental remediation, nutrient removal technologies include biofiltration. This uses living material to capture and break down pollutants. Examples include green belts, riparian areas, natural and built wetlands, and treatment ponds. These areas usually capture human-made discharges like wastewater, stormwater runoff, or sewage treatment. They are also used for land recovery after mining or development. Biofiltration uses biological assimilation to capture, absorb, and put pollutants (including nutrients) into living tissue. Another way to remove nutrients is bioremediation. This uses tiny organisms (microorganisms) to remove pollutants. Bioremediation can happen naturally, or it can be encouraged by adding fertilizers, which is called biostimulation.
Nutrient bioextraction is bioremediation that involves growing plants and animals. Nutrient bioextraction, or bioharvesting, is the practice of farming and harvesting shellfish and seaweed. The goal is to remove nitrogen and other nutrients from natural water bodies. It has been suggested that removing nitrogen using oyster reefs could benefit those who face nitrogen emission limits. This is similar to other nutrient trading ideas. If oysters keep nitrogen levels in estuaries below certain limits, they effectively save polluters the costs they would otherwise have to pay. Several studies have shown that oysters and mussels can greatly affect nitrogen levels in estuaries. Also, studies have shown that seaweed can improve nitrogen levels.
Shellfish in Estuaries
One idea to stop and reverse eutrophication in estuaries is to bring back shellfish populations, like oysters and mussels. Oyster reefs remove nitrogen from the water and filter out tiny particles. This reduces the chance of harmful algal blooms or low-oxygen conditions. Filtering activity is good for water quality. It controls the amount of tiny water plants (phytoplankton) and stores nutrients. These nutrients can then be removed from the system by harvesting the shellfish, buried in the sediments, or lost through denitrification. Early work on improving marine water quality with shellfish was done by Odd Lindahl et al., using mussels in Sweden. In the United States, shellfish restoration projects have been done on the East, West, and Gulf coasts.
Seaweed Farming
Seaweed aquaculture can help with and adapt to climate change. Seaweed, like kelp, also absorbs phosphorus and nitrogen. So, it's useful for removing too many nutrients from polluted parts of the sea. Some cultivated seaweeds grow very fast. They can absorb large amounts of nitrogen, phosphorus, and carbon dioxide. This produces a lot of oxygen, which is great for reducing eutrophication. It is believed that growing seaweed on a large scale could be a good solution to the eutrophication problem in coastal waters.
Geo-engineering in Lakes (Chemical Phosphorus Removal)
Geo-engineering means changing natural chemical processes, usually the phosphorus cycle. The goal is to get a desired natural response in the ecosystem. Geo-engineering methods typically use materials that can chemically make phosphorus unavailable for organisms (like phosphate) in the water. They also block phosphate from being released from the sediment (internal loading). Phosphate is a main cause of algal growth, especially cyanobacteria. So, once phosphate is reduced, algae can't grow too much. Thus, geo-engineering materials are used to speed up the recovery of water bodies with too many nutrients and to manage algal blooms.
There are several materials that absorb phosphate. These include metal salts (like alum, aluminium sulfate), minerals, natural clays, industrial waste products, modified clays (like lanthanum modified bentonite), and others. The phosphate absorber is usually put on the surface of the water. It then sinks to the bottom of the lake, reducing phosphate. Such absorbers have been used worldwide to manage eutrophication and algal blooms (for example, under the commercial name Phoslock).
One way to fix eutrophication uses chemical phosphorus removal with aluminum sulfate. This salt is commonly used to clean drinking water. Aluminum sulfate, or "alum," is used to reduce the amount of phosphorus. In a large study, 114 lakes were watched to see how well alum reduced phosphorus. Across all lakes, alum effectively reduced phosphorus for 11 years. While the effect lasted different amounts of time (21 years in deep lakes and 5.7 years in shallow lakes), the results show how well alum controls phosphorus in lakes. Alum treatment works less well in deep lakes and lakes with a lot of outside phosphorus coming in.
History of Eutrophication
Eutrophication was first recognized as a water pollution problem in European and North American lakes in the mid-20th century. Important research done at the Experimental Lakes Area (ELA) in Ontario, Canada, in the 1970s, showed that freshwater bodies are limited by phosphorus. ELA uses a whole ecosystem approach and long-term studies of entire lakes. They focus on human-caused eutrophication.
Words to Know
What "Eutrophication" Means
The word "eutrophication" comes from the Greek word eutrophos, which means "well-nourished."
Eutrophication on Land
While eutrophication usually refers to water systems, some scientists use the term "terrestrial eutrophication" for land ecosystems. This means "enrichment of an ecosystem with a limiting nutrient." It can be caused by nitrogen settling on land ecosystems. For example, more CO2 in the air can make the eutrophication of the boreal forest worse.
See also
Images for kids
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Sodium triphosphate, once a common ingredient in many detergents, was a major cause of eutrophication.
