Algal Bloom and Eutrophication

• Artificial fertiliser, which is especially rich in nitrogen, potassium and phosphorus, is constantly mixed into agricultural soil in order to supplement mineral deficiencies.
• Artificial fertiliser does not cling to soil particles and when exposed to rain, it is washed away by a process called leaching.
• Excess fertiliser on the surface of the soil is washed away by heavy rainfalls
• The nutrient-rich water that eventually ends up in rivers and dams is known as surface runoff
• Large amounts of minerals in the water cause a drastic increase in the growth of freshwater algae. This is known as algal bloom
• The dense mass of algae blocks sunlight from other photosynthetic organisms (aquatic plants) in the deeper layers of the water. This results in the death of many aquatic plants
• The dead plant material decomposes, leading to a drastic increase in decomposition bacteria.
• Decomposition bacteria use large amounts of oxygen during the process of decomposition.
• Less oxygen is available for other aquatic organisms which may cause their death. This process is known as eutrophication
• The release of raw sewage into rivers or dams has a similar effect with more serious and immediate consequences.
• The more organic material present in the water, the lower the oxygen levels.
• Eutrophication occurs when oxygen demand exceeds oxygen supply in the water, resulting in death for many aquatic organisms
• Possible actions to prevent algal bloom

  • Use of precise quantities of fertiliser to prevent excess runoff
  • Inject fertiliser below the soil surface to prevent runoff
  • Use specific combinations of fertiliser to suit each phase of growth
  • Install more water treatment plants in affected areas