Eutrophication of Water Bodies

Eutrophication is the enrichment of a water body with plant nutrients, principally nitrogen and phosphorus, which stimulates excessive growth of algae and aquatic plants. When human activities greatly increase nutrient inputs, the process is called cultural or accelerated eutrophication, as distinct from the slow natural ageing of a lake.

Sources of nutrient enrichment in urban lakes

Urbanisation increases both the quantity of nutrients entering lakes and the speed at which they are delivered.

• 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 lake catchment.
• Encroachment, reduced inflow and impaired flushing can increase water residence time, allowing nutrients and biomass to accumulate.
• Nutrients stored in bottom sediments may later return to the water through internal loading, prolonging eutrophic conditions even after external inputs decline.

The eutrophication sequence

Nutrient inputs exceeding the lake's assimilative capacity initiate a reinforcing chain of biological and chemical changes.

• High nutrient availability causes rapid growth of phytoplankton, cyanobacteria and aquatic weeds, sometimes producing dense blooms.
• Blooms increase turbidity and reduce light penetration, suppressing submerged vegetation and altering aquatic habitats.
• When excessive biomass dies, microbial decomposition raises biochemical oxygen demand and consumes dissolved oxygen.
• Oxygen depletion, particularly in deeper water and at night, can cause fish mortality and favour anaerobic decomposition, foul odours and further release of sediment-bound nutrients.

Implications for lake restoration

Visible removal of algae or weeds treats a symptom rather than the nutrient imbalance. Durable restoration requires reduction of the lake's external nutrient load through sewage interception and treatment, control of polluted runoff, protection of inflow channels and catchment management; sediment treatment should follow assessment of internal nutrient loading.