Discuss the causes and consequences of harmful algal blooms.

The Harmful Algal Bloom (HAB) or excessive algae growth refers to proliferation of algae in water bodies and oceans that causes negative impacts to other organisms. In some locations occurrence of HAB appears to be natural, while in most of the cases it is the result of human activities.


  • HABs are induced by eutrophication, i.e., presence of excessive nutrients in water body, most common of these nutrients are nitrates, ammonia, urea, phosphates, etc. Overabundance of these nutrients is caused due to:
    • Inflow of sewage water
    • Agricultural runoff containing pesticides and fertilizers
    • Discharge of Industrial effluents
  • Release of warm water in water bodies which causes thermal pollution.
  • Low water levels which reduce water flow & circulation.


  • They deplete the oxygen in the water and block sunlight from reaching to fish and plants.
  • With less light, plants beneath dies and fishes & other aquatic life starves to death.
  • When Algae eventually dies, the microbes which decompose the dead algae use up even more oxygen, which in turn causes more fishes to die.
  • It leads to creation of dead zones, where neither fishes nor plants can survive.
  • Emission of foul odour which contains harmful gases which affects human health.
  • Contamination of drinking water supply.

Hence, there is a need to reduce overuse of fertilizers in agriculture and regulate the release of untreated water from sewage and industries into water bodies.


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