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Aimed primarily at beginners to the hobby, this series of articles will take you step by step through the process of understanding how a good koi pond works.
Part 15: Nitrate and the nitrogen cycle
Testing the nitrate level is one test that hasn’t always been on the recommended list of pond parameters to monitor but attitudes toward this parameter are changing. It is the end product of what koi keepers call the nitrogen cycle in a conventional biofilter although in most koi ponds, unless they are heavily planted, what we called the nitrogen cycle isn’t really a complete cycle at all.
Nitrogen is essential for life
In part 1, I introduced the nitrogen cycle and gave a brief description of what was involved. The process by which ammonia is removed using biological action in a biofilter is usually referred to as the nitrogen cycle but that is just a convenient shorthand way to refer to it and it is only partially true. The full nitrogen cycle isn’t about how bacteria remove ammonia from an environment and turn it into nitrate. Nitrogen compounds take different forms and since nitrogen is essential for the metabolic processes that allow animals (including fish), bacteria and plants to grow and reproduce, it shouldn’t be surprising that different life-forms have evolved to find ways to use it in a self sustaining way.
Nitrogen is a gas that is common in nature, forming about 78% of the atmosphere, but if all life-forms could only take their nitrogen supply from the air, that supply would have been exhausted long ago and life on this planet may never have evolved or might have evolved in a very different way.
Fortunately, since nitrogen forms different compounds, different species are able to use those different compounds in different ways. As one species uses its particular preferred form of nitrogen, either its waste product or the whole organism itself becomes the food or the energy source for another.
To recap on something I referred to in part 1 of this series; the nitrogen cycle isn’t something that is specific to koi ponds or even ponds in general, different versions of the nitrogen cycle are common throughout nature, not just in water, but in soil and even in the atmosphere. The two main nitrifying bugs (nitrosomonas and nitrobacter) can successfully thrive in a wide range of environments, above and below water.
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Figure 1 The simplified nitrogen cycle
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Cycling nitrogen
Figure 1 shows a very simplified form of the nitrogen cycle as it applies to the aquatic environment. The full nitrogen cycle is much more complicated with many different pathways for nitrogen to take. This simplified version explains how pond owners don’t employ the full nitrogen cycle only part of it which is why nitrate becomes a problem in koi ponds.
Bear in mind that food contains protein.Protein is made from amino acids, which in turn are made from compounds called amines and these are molecules based on ammonia as is suggested by the similarity of the names; amino, amines and ammonia. Where there is an N in the chemical names below, that is the symbol for nitrogen. It isn’t necessary to understand chemistry to any great degree but by watching for the N, and the way it appears with different combinations of other chemicals, it can be seen that nitrogen changes from one form to another and eventually goes back to its original form.
In case you ever wondered why we refer to a nitrogen cycle taking place in our biofilters, the reason is that what a koi keeper would see as removing ammonia from the pond, a chemist would see as cycling nitrogen endlessly from one form to another. In other words, to a chemist it is a nitrogen cycle.
The simplified nitrogen cycle
In figure 1, fish eat food containing protein and the ammonia based compounds in it are converted into ammonia as it is metabolised. This becomes a waste product that must be excreted. Fish have a very efficient method of achieving this. They excrete up to around 90% of the waste ammonia through their gills at the same time as they excrete waste carbon dioxide from the respiration process as described in part 11. In effect, they are taking in nitrogen in one form, amine, and excreting it in another form, ammonia (NH3 or NH4)
Although the ammonia they excrete takes two forms; free ammonia (NH3) or ionised ammonia (NH4) according to the pH and temperature, for simplicity, we can just refer to it as ammonia for the purposes of this description.
Ammonia
After ammonia is excreted by fish as their waste product, the next species in the nitrogen cycle makes use of it. The ammonia bug (nitrosomonas) doesn’t actually eat ammonia as its food but it uses it in a complicated chemical process that provides a chemical energy that allows it to take carbon from the water. Bugs, in common with all species and animal groups, are carbon based life-forms so it is carbon they need in order to grow and to reproduce which is their sole purpose in life. They don’t actually have any need for ammonia other than to extract the chemical energy in it to power the biological processes. In this way, ammonia can’t actually be classed as a food but, for a basic understanding, they could be considered to take in ammonia from their surroundings and excrete nitrite as their waste product provided you remember that this description is a highly simplified one.
Nitrite
After the ammonia bug has taken in nitrogen in the form of ammonia (NH3), used it to fix the carbon it needs and then excreted its waste product which is another form of nitrogen, nitrite (NO2), this form becomes the energy source for the next species in the cycle, the nitrite bug. This bug takes in the waste nitrite from its cousin, the ammonia bug, and extracts energy from it to power its own needs to obtain the carbon that it needs to grow and reproduce. In a similar way to the ammonia bug, it doesn’t have any need for the nitrite as it stands but it breaks it apart and uses the chemical energy stored in it to allow it to extract carbon from its environment.
There is far less available energy stored in nitrite than is stored in ammonia. This reduces the rate at which the nitrite bug can extract the energy it requires to fix carbon in order to grow. The slow rate at which it can take in carbon means that it has a slow rate of growth which explains why, when a biofilter is maturing, the nitrite bug always is far slower to develop than the ammonia bug. Depending on individual environmental circumstances, the nitrite bug has a growth and reproduction rate that can be as slow as between a third and a half that of the ammonia bug. The slow rate of development has the often seen effect, as biofilters mature, of causing the nitrite spike to last longer than the ammonia spike, the ammonia bug colony grows faster and produces nitrite far more quickly than the nitrite bug colony can grow and use it.
Nitrate
After the nitrite bug has extracted the energy it needs from the nitrite (NO2) it has taken in, it excretes its own waste product, nitrate (NO3). This is where most koi keepers break the true nitrogen cycle.
In nature, the waste product of the nitrite bug, nitrate, is used by the next species in the cycle - plants. Plants in natural lakes and rivers or marine plants in the sea all need nitrate in order to synthesise (manufacture from scratch) their proteins, the chlorophyll in them and even their DNA.
One of two things happens next. Either fish eat the plants directly and, in doing so, are eating the plant proteins directly. Or smaller fish and aquatic insects browse on the plants and take in the protein and then carnivorous fish eat these. Either way, the nitrate waste from the nitrite bug has been taken up by the plant, and then converted into protein which either directly becomes food for fish or indirectly becomes their food when they eat smaller fish or insects.
The cycle is complete
Nitrogen (N), which formed part of protein, was eaten by a fish and excreted as ammonia (NH3). The ammonia bug used the ammonia and in doing so converted it to nitrite (NO2), which was then used by the nitrite bug and converted into nitrate (NO3). Nitrate was used by plants which converted it into proteins again and became part of a fish’s diet when they ate it either directly or indirectly by eating smaller fish or insects which had already eaten the plant material.
The cycle is completed when fish metabolise the protein and excrete more ammonia to begin the cycle anew. In this way nitrogen doesn’t need to be depleted from the atmosphere but the same nitrogen atoms are used over and over again in an endless cycle where it simply changes from one compound to another and back again.
Very briefly, to avoid leaving a false impression before moving on; although nitrogen isn’t taken from the atmosphere during the aquatic part of the nitrogen cycle, the full nitrogen cycle is much more complex and some atmospheric nitrogen is involved. However, this is only “borrowed” and then “paid back” which means that even in the much more complex full cycle, nitrogen isn’t slowly being removed from the atmosphere. That’s another story and is only of passing interest to koi keepers in that, although we are putting carbon into the atmosphere that future generations will have to deal with, no living organisms are currently stealing their nitrogen.
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Figure 2 The full nitrogen cycle showing additional nitrogen pathways
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The full aquatic nitrogen cycle
The full cycle is shown in figure 2. The simplified cycle, as shown in figure 1, is still the main way in which nitrogen is endlessly recycled but there are additional factors which have to be taken into account if we wish to understand the full picture.
In nature fish aren’t fed pelleted food, their diet includes plants and insects or smaller fish. This means that they either eat plant protein directly or they eat insects and smaller fish that have browsed on plants and turned it into their own protein.
In either case, the nitrogen that the plants assimilated from the nitrate waste product from the nitrite bug (nitrobacter) will be eaten, either directly if fish eat the plants themselves, or indirectly after the plants have first been eaten by insects or small fish.
There are further paths that nitrogen can take in a natural waterway and these are also shown. Fish faeces also contains nitrogen waste and so do decaying dead plants and decaying dead insects. These are used by yet more bugs (heterotrophic bacteria) as their energy source. The waste product of these bugs is ammonia which is no different to the ammonia waste from fish and so it is also used as an additional energy source by the ammonia bugs and subsequently produces more nitrate as the end product.
This extra nitrate is also taken up by plants and so this little complication to the simplified nitrogen cycle that we often see drawn doesn’t really change the cycle in any way other than to give nitrogen a few extra paths it can take and doesn’t alter the fact that it’s still an endless cycle.
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Figure 3 The nitrogen cycle isn’t really a full cycle in the average koi pond
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Breaking the cycle
In a koi pond, unless it is heavily planted there are few, if any, plants that can take up the nitrate produced by conventional biofilters and turn it into plant protein that can then be eaten by fish resulting in them excreting more ammonia to complete the cycle.
As shown in figure 3, the process we usually refer to as a cycle in a koi pond starts with a fish, moves through the two bacterial stages which results in an abundant production of nitrate.
That is where it comes to a halt, the next stage that should take place is where the nitrate is used as a plant food to produce new growth which in turn becomes food for the fish again, but this stage is missing.
Since the nitrogen cycle is broken at the point where nitrate is being produced, unless we intervene in order to keep the rising nitrate level under control, it will just keep increasing because there is no process that normally takes place in a koi pond to remove it.
Plants are nature’s solution, but there must be a great deal of them to have any chance of reducing the level to near zero. Many koi keepers grow a few plants in the pond and it is true that rapidly growing plants will help, but to get an idea of just how many would be needed, think in these terms.
To make a self sustaining and self regulating system, there needs to be sufficient plant material in the pond to provide enough for the fish to eat without any additional food. That way, the total amount of nitrogen eaten will be recycled into just sufficient plant material to keep the cycle going, as happens in a natural eco-system.
Water changes are the usual way by which koi keepers reduce nitrate levels but are they the best answer?
Next month I will discuss the problems that can be caused if nitrate is allowed to build up in a koi pond. Nitrate isn’t just a plant food that can encourage planktonic algae (green water) or filamentous algae (blanket weed), it has more of a detrimental effect on koi than previously had been thought. What these effects are and what measures we can take to reduce these problems will be explained in part 16.
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