Explain how eutrophication can lead to the death of aquatic organisms? (6 marks)

1 - Fertilisers (eg NPK) used in agriculture are washed into local water sources such as rivers and lakes by rainwater/surface run off

2 - These nutrients enable algae to grow, leading to the formation of algal blooms at the surface of the water, blocking sunlight from reaching the bottom. 

3 - Plants growing at the bottom of the pond are no longer able to photosynthesise to produce energy (and oxygen) and die 

4 - Bacteria decompose these plants aerobically, using up even more oxygen in the water for respiration

5 - Oxygen levels become so low (hypoxic) that the water source is no longer able to support aquatic life

6 - This causes a reduction in biodiversity