Nitrogen Cycle

The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This transformation can be carried out via both biological and non-biological processes. Important processes in the nitrogen cycle include fixation, mineralization, nitrification, and denitrification. The majority of Earth's atmosphere (approximately 78%) is nitrogen, making it the largest pool of nitrogen. However, atmospheric nitrogen is unavailable for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems. The nitrogen cycle is of particular interest to ecologists because nitrogen availability can affect the rate of key ecosystem processes, including primary production and decomposition. Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramatically altered the global nitrogen cycle.

In a koi pond, ammonia is broken down by naturally occurring bacteria, firstly to nitrite, which is also poisonous to Koi, and ultimately to nitrate. In the quantities we can expect it, nitrate is not directly poisonous to Koi, although it can be a cause of blanket weed. Nitrate can be removed by other means such as a vegetable filter, or a denitrifying filter that employs different bacteria to convert it to gaseous nitrogen and release it to atmosphere. This biological process is referred to as "The Nitrogen Cycle". In reality, the Nitrogen Cycle works throughout the pond, not in the filter alone. All surfaces will support the necessary nitrifying bacteria. A filter, with the provision of large amounts of medium for bacteria to grow upon, ensures that enough biological action takes place to cope with the ammonia produced by the Koi. The 'biomass' (the bacterial colony) will establish itself to the load imposed upon it therefore it is keyed to the stocking level of fish living in the system.

Nitrosomonas bacteria convert ammonia (NH3) to nitrite (NO2)
Nitrobacter bacteria convert nitrite (NO2) to nitrate (NO3)








    When you first introduce fish to a new pond, the main problem is not the solid waste produced by the fish, it is the ammonia(NH3) released into the water. This is very toxic to the fish, especially in a small pond. The first of our friendly bacteria to spring into action are the Nitrosomonas bacteria. These bacteria derive all the energy they need for growth and reproduction from converting ammonia into nitrites. They live in several places such as soil, sewage, fresh water, etc. and they thrive in places where there are high levels of nitrogen compounds. These bacteria need large amounts of energy to divide and multiply and, because of this, it takes a while for them to develop in the pond in such numbers as to be of use. It is, therefore, very important that you do not stock a new pond to capacity immediately when it is initiated.

Once your first fish are put into the system and begin to feed, they will produce toxic ammonia and carbon dioxide (CO2) from their gills and solid waste matter. Ammonia is also introduced into the aquarium by decaying matter such as solid fish waste, uneaten food and dead plant matter. Nitrosomonas bacteria present in the water will begin to convert the ammonia into nitrites (NO2) and, in doing this, will begin to multiply. As the numbers of Nitrosomonas increase and the ammonia levels correspondingly decrease, nitrite levels in the water will rapidly start to increase.

Nitrite is almost as dangerous to fish as ammonia and this is where the second batch of 'friendly' bacteria come into action - the Nitrobacter. These microscopic rod-shaped bacteria begin to colonise the filter and feed on the nitrites (NO2) produced by the Nitrosomonas bacteria. They convert them to nitrates (NO3) which are far less harmful to fish and other animals. In doing this they, too, begin to multiply their numbers until a balance is achieved.
The byproducts, then, of this cycle are the carbon dioxide exhaled by the fish and the nitrates produced by the bacteria. Both of these are used up to some degree by any aquatic plants present. The carbon dioxide is used up by the plants in the action of photosynthesis which produces oxygen back into the water and the nitrates are consumed by the plants as fertilizer to aid their growth.

That is why proper proportions of fish and plants are essential to your pond environment. To give you an idea, a general rule of thumb is that you should have one inch of fish for every square foot of surface area in your pond. 40 – 60 percent of your pond surface should be covered with a mix of marginals as well as floaters. These will not only provide nutrients but will allow your fish some cover from possible predators.