Introduction. Freshwater bodies such as ponds, lakes and reservoirs are sensitive ecosystems. Their productivity depends upon the amount of nutrients available in the water. When the supply of nutrients, particularly nitrogen and phosphorus, increases beyond the natural requirement of the system, the water body becomes abnormally rich in plant growth, especially algae and aquatic weeds. This process of nutrient enrichment and ecological deterioration is called eutrophication. It is a major water pollution problem discussed in Environmental Studies.
Meaning and definition
The word eutrophication comes from the Greek word “eutrophos” meaning “well nourished”. In scientific terms it may be defined as:
“The process by which a lake or other water body becomes excessively enriched with nutrients, leading to dense growth of algae and other aquatic plants, deterioration of water quality and disruption of the normal ecological balance.”
In simple language, eutrophication means that a water body becomes over-fertilised and gradually turns into a weed-choked, dirty and oxygen-poor system.
Types of eutrophication
- (i) Natural eutrophication: This is a very slow, age-long process. Over hundreds or thousands of years, nutrients are carried into lakes from surrounding soils and rocks by rainwater and surface runoff. Gradually, the lake becomes more productive, sediment accumulates at the bottom, the depth decreases and the water body may finally turn into a marsh and then dry land. This is part of the natural ageing of lakes.
- (ii) Cultural (accelerated) eutrophication: Human activities have greatly speeded up this natural process. Discharge of sewage, industrial effluents, fertiliser-rich agricultural runoff and detergent-containing wastewater introduce large amounts of nitrogen and phosphorus in a very short time. This rapid, man-made nutrient enrichment is called cultural eutrophication and is responsible for most of the present-day problems.
Main causes (sources of nutrients)
Important sources which supply extra nutrients to water bodies are:
- Domestic sewage: Untreated or partially treated sewage contains organic matter, phosphates and nitrates.
- Agricultural runoff: Excess application of chemical fertilisers (NPK) and use of animal manure on fields; rainwater washes these nutrients into nearby ponds and canals.
- Industrial effluents: Wastes from food-processing units, dairies, sugar mills, distilleries, etc., carry high organic load.
- Detergents: Phosphate-based detergents used in households and industries add phosphates to water.
- Solid waste and feed from fish farms: Organic waste dumps and leftover feed also contribute nutrients.
Step-wise process of eutrophication
In a typical eutrophic lake, the following sequence is usually observed:
- Increase in nutrient levels (nitrogen and phosphorus) due to external inputs.
- Rapid growth of phytoplankton (algae) and free-floating plants; algal blooms form a thick green scum on the surface.
- Shading of deeper layers: Sunlight cannot penetrate, so submerged plants die due to lack of light.
- Accumulation of organic matter: Dead algae and plants settle at the bottom as organic sludge.
- Decomposition by bacteria: Microorganisms decompose this organic matter, consuming large amounts of dissolved oxygen.
- Oxygen depletion: Dissolved oxygen level falls drastically; in severe cases, the bottom layers become anoxic (without oxygen).
- Death of aquatic animals: Fish and other organisms die or migrate; bad odour and floating dead fish make the water aesthetically unpleasant.
- Conversion towards marshy land: With continued nutrient input and siltation, the water body becomes shallower, weed-infested and may ultimately turn into a marsh.
Effects and impacts of eutrophication
- Loss of biodiversity: Sensitive species of fish and invertebrates disappear; only a few tolerant species and algae dominate.
- Deterioration of water quality: Water becomes turbid, smelly and unfit for drinking, bathing, recreation or fisheries without expensive treatment.
- Fish kills and economic loss: Sudden oxygen depletion can cause massive fish deaths, affecting fishermen and local economy.
- Health hazards: Some algal blooms (blue-green algae) produce toxins which may cause skin irritation, stomach disorders or liver damage in humans and animals.
- Increased cost of water treatment: More chemicals, filtration and treatment steps are needed to remove colour, odour and organic matter.
Control and prevention (in brief)
Effective control of eutrophication focuses on reducing nutrient input to water bodies:
- Treating municipal sewage properly before discharge, including removal of phosphates and nitrates.
- Promoting judicious use of fertilisers, soil testing and integrated nutrient management in agriculture.
- Creating vegetative buffer zones and wetlands along fields and lakes to trap nutrients from runoff.
- Regulating industrial effluents and encouraging low-phosphate or phosphate-free detergents.
- For already eutrophic lakes: desilting, mechanical removal of weeds, aeration of water and diversion of sewage.
Exam-oriented recap
- Definition: nutrient enrichment of water leading to excessive growth of algae and weeds and decline in water quality.
- Two types: natural eutrophication (very slow) and cultural/accelerated eutrophication (due to human activities).
- Main causes: sewage, fertiliser runoff, detergents, organic industrial wastes.
- Main effects: algal blooms, oxygen depletion, fish kills, bad odour, loss of biodiversity and high water treatment cost.
- Control: reduce nutrient input, treat sewage, regulate fertilisers and effluents, restore affected water bodies.
Conclusion: Eutrophication represents the nutrient overloading and ecological degradation of lakes and other water bodies. While it is a natural ageing process, human activities have accelerated it to dangerous levels. Controlling the entry of nitrogen and phosphorus at their source is essential to protect freshwater ecosystems, maintain aquatic biodiversity and secure clean water for domestic, agricultural and industrial use.