Eutrophication is a serious global issue threatening our access to clean drinking water. As the population and economies grow all around the world, so does the need for water.
Eutrophication is a common problem in both freshwater and marine waters. It leads to toxic algae blooms in lakes, rivers, and streams, and green algae overgrowth in coastal waters.
Eutrophication causes harm to aquatic ecosystems. It may affect human health, disrupt local economies, and raise social concerns.
Introduction to the Concept
The term “eutrophic” means well-nourished or rich in nutrients. “Eutrophication” refers to the natural or artificial nourishment of water bodies in terms of nutrients.
The term “eutrophication” comes from the Greek eutrophos and it means “well-nourished”.
When the addition of nutrients results in adverse effects on living organisms and the environment, eutrophication becomes a form of pollution.
According to Britannica,
“Eutrophication, the gradual increase in the concentration of phosphorus, nitrogen, and other plant nutrients in an aging aquatic ecosystem such as a lake.“
A description of Eutrophication provided by (NOAA, 2007; UNEP, 2015) states,
“Eutrophication is the overabundance of nutrients in a body of water that results in harmful algal blooms, fish kills, and in some cases ecosystem collapse. It is a process driven by enrichment of water by nutrients, particularly compounds of nitrogen and/or phosphorus, leading to increased growth, primary production and biomass of algae; changes in the balance of nutrients causing changes to the balance of organisms; and water quality degradation.”
source: UNDRR
Eutrophication is when increased organic matter enriches an ecosystem, leading to harmful changes.
In coastal areas, it can cause toxic algal blooms, loss of seagrass, shifts in marine life, sediment buildup, and low oxygen levels. These effects can lead to fish kills and harm marine mammals, such as sea lions in California and manatees in Florida have been reported.
Natural vs. Human-Caused Eutrophication
Eutrophication is a natural, slow process in aquatic ecosystems. However, it become a faster, human-caused process due to excess phosphorus and nitrogen. While both increase organic production, the human-driven form happens quickly and has significant ecological and social impacts.
Nutrient Accumulation in Aquatic Systems and Algal Growth
Eutrophication is the process of water bodies acquiring high concentrations of nutrients, particularly phosphates and nitrates. These additions promote the excessive growth of algae. When the algae die and are decomposed by microorganisms, the available oxygen present in the water is depleted in the process. This oxygen-depleting process causes the death of other aquatic organisms, such as fish.
A freshwater body has a very low concentration of nutrients, which sees a gradual increase due to:
- Surface runoff
- Organic debris
- Wind-borne dust
- Excreta and exudates of animals
The nutrient status of such water bodies is improved by bacteria and blue-green algae that fix atmospheric nitrogen. This nutrient supply results in the moderate development of plants, animals, and microbes in the water.
When there is continued nutrient supply, the population of organisms increases.
Algae are a plant-like group of organisms that reside in water bodies. They make their own food through the process of photosynthesis. They use sunlight to make food from simple chemicals like oxygen and water.
Additional phosphates in water bodies lead to the explosive growth of algae, termed “algae blooms.” In this process, algae and plants consume greater amounts of oxygen in water due to their high population. This robs fish and other aquatic species of the necessary oxygen.
Oxygen Depletion and Ecological Consequences
When algae die eventually, bacteria require oxygen in order to decompose or break down the dead algae. This starts a cycle of oxygen consumption where more bacteria decompose more dead algae.
The bacteria then release more phosphate back into the water. More algae feed on this released nutrient. Oxygen is depleted in the lower levels, leading to the suffocation of species such as fish and mollusks.
The depleting oxygen levels also make water bodies lose their aesthetic, economic, and social value. Eutrophication also poses environmental effects. Anoxia, reduced water quality, and a reduction in the population of fish and aquatic organisms are some of the consequences. An increase in certain species’ populations may occur, resulting in an imbalance in marine ecosystems.
The bloom of phytoplankton in a water body has adverse effects on the health of organisms and the environment. This is mainly the result of anthropogenic activities that pollute water with nutrients in undesirable amounts.
The release of sewage effluents and agricultural runoff carries fertilizers to water systems.
Natural Sources of Nutrient Pollution
Nutrients can also enter natural water through natural processes, e.g., depositional environments. They can flow into systems on an ephemeral basis.
Excess nutrients are washed from land and soil into the water by streams, rivers, or lake floods. However, water bodies that don’t have fertilized land in surrounding areas are less likely to be impacted by the process of eutrophication.
Factors Involved in Eutrophication
Aquatic ecosystems rely on dynamic balances. Eutrophication disrupts this balance by altering the amount or form of nitrogen and phosphorus in the water.
The severity of its effects also depends on environmental factors like long water residence times, high temperatures, and sufficient light, which all promote eutrophication.
Classification of Aquatic Systems
- Oligotrophic – poor nutrient status and poor productivity
- Mesotrophic – moderate nutrient status and moderate productivity
- Eutrophic – rich nutrient status and rich productivity
Classification Process of Aquatic Systems
When there’s an addition of nutrients in the water system, the oligotrophic aquatic system turns into a mesotrophic aquatic system. With the continued addition of nutrients, the mesotrophic aquatic system turns into a eutrophic aquatic system.
This continued process of addition, population increase, and decay of organisms leads to water losing its aesthetic and economic value. The environment and the health of water bodies are severely impacted.
The process of eutrophication can be described as:
- Addition of nutrients to water systems:
- Excess nutrients, primarily nitrogen and phosphorus, enter water bodies through agricultural runoff, wastewater, and other human activities.
- Increase in aquatic organisms’ population:
- Nutrient-rich environments lead to the rapid growth of phytoplankton, algae, and other simple organisms.
- Death of algae, decay, and decomposition by bacteria:
- When algae die, they are decomposed and broken down by bacteria, a process that consumes oxygen from the water. The bacteria that decompose the dead algae require large amounts of oxygen, increasing the water’s biological oxygen demand.
- Excessive nutrients mean more growth, decay, and decomposition:
- With excessive nutrients, more algae grow and die, leading to a continuous cycle of growth, decay, and decomposition, further depleting oxygen.
- Algal bloom happens:
- A visible algal bloom occurs as the algae grow out of control due to the excess nutrients. When this happens at the water surface, it blocks sunlight access to the depth of water.
- Fish and other aquatic life forms die:
- Oxygen depletion (hypoxia or anoxia) occurs as bacteria consume more oxygen during decomposition, leaving insufficient oxygen for other aquatic life. With oxygen levels dropping, fish and other aerobic organisms die or migrate, leading to dead zones.
- Water loses its quality, aesthetic, and economic value:
- The water becomes turbid, discolored, and sometimes toxic, affecting its use for drinking, recreation, and economic activities like fishing.
- Populations dependent on such water are highly impacted:
- Local communities, industries, and ecosystems that rely on these water bodies for drinking, food, and livelihoods suffer severe consequences.
Causes of Eutrophication
Eutrophication is caused most often by human activities. Agricultural lands, farms, lawns, golf courses, etc., are heavily fertilized by humans for production. These fertilizers contain nutrients that are perfect for algae and plankton. When it rains or the wind blows, these fertilizers can run off from land into the water.
When nutrients increase in freshwater and marine ecosystems, plants grow more, blocking light. As the water fills with nutrients, new plant species take over, changing how the ecosystem works. These plant blooms use up oxygen, leading to low oxygen levels (hypoxia or anoxia) and can release harmful gases like CO2, H2S, and CH4.
Anthropogenic causes are more prominent. These include:
- Sewage effluents
- Agricultural runoffs
- Agricultural fertilizers and pesticides
- Excreta and exudates of living beings
- Organic wastes
- Direct discharge of municipal waste into water bodies
Aquatic life forms, just like other living beings, need specific nutrients for their survival. In general, nature provides the right balance of nutrients to ecosystems. Too many or too few nutrients cause an imbalance. When nutrients are abundant in an aquatic system, mainly due to human activities, we get the issue of eutrophication. Algae, plankton, and other
Microorganisms feed on these types of nutrients, such as nitrogen and phosphorus. When such nutrients are abundant, these organisms take over the water bodies.
In an eutrophic situation, fish, birds, and humans are negatively affected.
Effects of Eutrophication
Eutrophication causes several effects. The main effects are harmful algal blooms, loss of biodiversity, and low-oxygen environments (hypoxia or anoxia) that can kill aquatic life.
In river bays and lakes, plants like water chestnuts and water ferns can overgrow and deplete oxygen. Toxic cyanobacteria, such as Microcystis and Planktothrix, often thrive in lakes.
In coastal areas, the decay of green algae like Ulva leads to hypoxia and mass mortality of marine life, along with health risks from hydrogen sulfide.
Furthermore, marine eutrophication can trigger harmful algal blooms. This has been reported in many places in northern Europe along the coasts of the Baltic Sea, Kattegat-Skagerrak, eastern North Sea, Norwegian Sea, and the Barents Sea.
Eutrophication threatens the environment, economy, and human health. It affects shellfish production, fishing, and tourism.
Aesthetic and Economic Consequences
In general, simple plant growth and decay are promoted by the eutrophication process, favoring simple algae and plankton. This results in rapid growth and decay mainly because simpler organisms have a shorter life span compared to complex plants.
This growth and decay cycle disrupts the natural functions of ecosystems. It reduces the quality of water, affecting dependent living organisms.
The color of the water changes with eutrophication pollution; it becomes cloudy, with shades of green, yellow, brown, or red.
It affects the human population that uses the water source for consumption and agriculture purposes.
The overall value of water sources, such as rivers, lakes, and estuaries, is reduced due to eutrophication. Human society faces the consequences in terms of reduced fishing, hunting, and reduced recreational value and aesthetic pleasures.
Solution to Combat Eutrophication
Eutrophication in aquatic ecosystems can be addressed using three types of levers: physical, chemical, and ecological levers.
- Physical Levers:
- These aim to decrease water residence time (how long water stays in a system) or mix the water column. Mixing can prevent stratification, improve oxygen distribution, and reduce conditions that favor algal blooms.
- Chemical Levers:
- These involve re-oxygenating the water or precipitating phosphorus using substances like lime or aluminum. However, these methods have limits, such as potential impacts on water chemistry and costs.
- Ecological Levers:
- These include using algaecides to remove algae or introducing species to adjust the food web. While these methods can address symptoms in specific areas, these may cause other environmental issues like toxin release or harm to non-target organisms.
- Control Measures:
- It’s crucial to manage phosphorus and nitrogen flows from watersheds over the long term. This is needed considering how nutrients transfer and are retained. Watershed management strategies, such as controlling agricultural runoff, better sewage treatment, and buffer zones, are some of the long-term solutions.
- Consensus on Reducing Inputs:
- Scientists agree we need to limit nitrogen and phosphorus from urban, industrial, and agricultural sources. Nutrient cycles are linked, so controlling one affects others. This helps maintain the ecological balance.
- Phosphorus plays a key role in controlling algae growth in lakes and rivers. It’s often the limiting nutrient, meaning controlling phosphorus levels helps prevent excessive algae. Aquatic plants and microorganisms can absorb phosphorus in eutrophic waters. Harvesting plants, using microorganisms, and reducing phosphorus in detergents also help lower nutrient levels and fight eutrophication.
- Restore Wetlands:
- Wetlands naturally filter out excess nutrients before they reach larger water bodies. This helps reduce nutrient pollution.
- Create Buffer Zones:
- Planting vegetation near rivers or lakes helps trap and absorb nutrients. Thus, preventing them from entering the water.
- Use Sustainable Farming:
- Reducing fertilizer use and planting cover crops can minimize nutrient runoff from farmlands. Targeted, adaptable efforts are more effective and affordable.
- Maintain Septic Systems:
- Regular maintenance of septic systems prevents nutrient-rich wastewater from leaking into waterways.
- Public Education:
- Raise awareness about reducing nutrient pollution to help protect water quality.
