Fertilizers and Eutrophication

Fertilizers

Fertilizers used include:

• organic - manure
• inorganic - nitrates and phosphates

Both types of fertilizer can cause problems if they get into waterways. This is called leaching and can occur by:

• the addition of too much fertilizer 
• the addition of fertilizer when the plants are unable to uses it because they are not actively growing (e.g. in winter)
• the addition of fertilizer immediately before a heavy rainfall 

The nutrients contained within the fertilizer (particularly the phosphates and nitrates) are used by organisms (bacteria and algae) in the waterway. 

The increasing of nutrient levels in waterways is called eutrophication

Eutrophication

The addition of high levels of nitrates and phosphates (and of organic nutrients) to waterways (lakes and rivers) causes eutrophication.

• Algae use the nutrients and increase in number to form an algal bloom.
• Algal blooms harvest most of the light entering the water depriving other plants of this essential energy. So the non-algal plants die. Eventually the algae themselves die. 
• The remains of these two groups of plants provide nutrients for aerobic bacteria
• The aerobic bacteria can also use the nutrients leached into the water

So the result of increasing the level of nutrients in soil is an increase in aerobic bacteria

• Large numbers of aerobic bacteria reduce the oxygen in the water (the water is said to have a high BOD [biological oxygen demand])

The reduction in water oxygen means that there is insufficient to support fish (which either die or migrate away)

Organisms that can tolerate low oxygen levels (e.g. Chironomus and Tubifex) proliferate

The number of species able to tolerate low oxygen is fewer so the biodiversity (a measure of how many species there are present) of the waterways is reduced

Go to Biology Index