Explain the processes involved in eutrophication

Eutrophication is where a surplus of nutrients in a lake or pond leads to an overgrowth of plant life, causing a shift in the biodiversity. There are 5 steps to this process:

Step 1: The use of fertilisers on farmland is washed into lakes and rivers.

Step 2: The excessive amount of nutrients such as nitrates and phosphates causes water plants, including surface water algae, to grow quickly.

Step 3: This results in 'algae bloom' on the surface water which blocks sunlight from reaching the water plants below, preventing these plants from photosynthesising. The competition for nutrients rises and more water plants die.

Step 4: Decomposing bacteria break down the dead matter from the plants via aerobic respiration. As the number of decomposing bacteria increases the levels of dissolved oxygen lower further.

Step 5: The oxygen levels are too low to support other aquatic organisms like fish and these organisms begin to die. Eventually the lakes and rivers may be left lifeless.