The Nitrogen Cycle

The nitrogen cycle is the basis for how an aquaponics system works.

Fish excrete ammonia through urine, faeces and their gills. Based on the nitrogen content, 82.8% of the ammonia is excreted through the gills, with 17% through defaecation and urine .

The process called Nitrification is the process where the ammonia breaks down through the action of bacteria to initially nitrite and ultimately nitrates .

There are two bacteria that are associated with the breakdown of ammonia to form nitrite (NO2) and nitrate (NO3), they are :
  • Nitrosomonas and
  • Nitrobacter

In traditional aquaculture and aquarium systems these bacteria are utilised in the biological filters that are used to decrease the level of ammonia and nitrite in the water and are also present in the substrate of ponds and dams.

The Nitrosomonas bacteria converts the ammonia into nitrite and then the Nitrobacter bacteria converts the nitrite into nitrate.
This process allows the valuable nitrate nutrients to be accessed by the plants in the system

When you start your system, the ammonia levels will begin to rise and colonisation of Nitrosomonas bacteria (which are naturally present in the water) will start to occur in the system.Bacteria numbers start to rise as the ammonia levels increase due to feeding the fish to the point where the ammonia begins to convert to nitrite. The presence of nitrite in your system will encourage the colonisation of a second group of bacteria - Nitrobacter and then they start the conversion of nitrites to nitrates. As you recycle the nitrate laden water through your grow beds the nitrate is taken up by the plants and the return water nitrate levels will be depleted. Over time the system should come into balance provided here is sufficient grow beds in the system to reduce the nitrate levels. In the short term however, it may be necessary to carry out water exchange to ensure that nitrite and nitrate levels are maintained at optimum.