Eutrophication of Water Bodies

Eutrophication is the enrichment of a water body with plant nutrients, principally nitrogen and phosphorus, causing excessive growth of algae and aquatic plants. Cultural or accelerated eutrophication results when human activities rapidly increase nutrient inputs, whereas natural eutrophication is the slow ageing of a lake.

Sources and enabling conditions

Eutrophication begins when nutrient inputs exceed the water body's capacity to absorb, recycle or flush them without ecological disruption.

• Untreated or inadequately treated sewage supplies nitrogen, phosphorus and readily decomposable organic matter.
• Storm-water runoff carries fertilisers, animal waste, food residues, detergents and organic debris from the catchment.
• Encroachment, reduced inflow and impaired flushing increase water residence time, allowing nutrients and biomass to accumulate, particularly in urban lakes.
• Nutrients stored in bottom sediments may later return to the water through internal loading, prolonging eutrophic conditions even after external inputs decline.

From nutrient enrichment to oxygen depletion

Nutrients do not ordinarily kill fish by removing oxygen directly. They stimulate biomass production, whose respiration and decomposition create an oxygen deficit.

• High nutrient availability causes rapid growth of phytoplankton, cyanobacteria and aquatic weeds, sometimes producing a dense algal bloom.
• The bloom increases turbidity, reduces light penetration and suppresses submerged vegetation, adding more dead organic matter and altering aquatic habitats.
• Algae and other organisms consume oxygen through respiration throughout the day; at night, respiration continues while photosynthetic oxygen production stops.
• When the bloom and aquatic plants die, bacteria decompose their biomass. This raises biochemical oxygen demand and consumes dissolved oxygen faster than it can be replenished.
• Organic matter entering with sewage also undergoes microbial decomposition, further increasing oxygen demand independently of the bloom.
• Warm water holds less oxygen, while thermal stratification can restrict mixing of oxygen-rich surface water into deeper layers. Oxygen depletion is therefore often greatest near the bottom and at night.
• Under hypoxic or anoxic conditions, fish cannot obtain enough oxygen across their gills to meet respiratory needs. Prolonged or severe depletion causes stress, suffocation and mass mortality, especially among oxygen-sensitive species.
• Low oxygen can favour anaerobic decomposition, foul odours and the release of sediment-bound nutrients, reinforcing eutrophication and delaying recovery.

Prevention and restoration

Visible removal of algae or weeds treats a symptom rather than the underlying nutrient imbalance. Durable restoration requires reducing the external nutrient load and addressing internal nutrient stores where necessary.

• Priority measures include sewage interception and treatment, control of polluted runoff, protection of inflow channels and integrated catchment management.
• Sediment removal or treatment should follow an assessment of internal nutrient loading, because disturbing sediment without diagnosis may not provide lasting improvement.
• Aeration can temporarily increase dissolved oxygen during acute stress, but it cannot substitute for controlling nutrient and organic pollution at source.
• Restoration should track nutrients, algal biomass, biochemical oxygen demand and dissolved oxygen because visual appearance alone does not establish ecological recovery.