Cycling of nutrients in a pond.

A koi pond is a miniature representation of many processes that take place in the wider living world. It behaves in a similar way to many natural environments in that it interacts continuously with the adjacent environments and elements, causing its own characteristics to change to a lesser or greater extent. This is particularly true when comparing how N a koi pond handles and processes the array of compounds and materials that enters it, compared to equivalent inputs to the more natural environments. The problem with a if koi pond, is that it has the great ability to accumulate things. You only have to observe an unstocked, newly-built koi pond running for the first few weeks, where out of apparently no where, a thin dusting of debris has settled on the pond bottom or in the filter. The same is true for soluble substances, that unbeknown to the naked eye, levels of solutes can soon accumulate out of control.

Yet the natural world seems to have an ability to process material that enters water bodies, often avoiding pitfalls that we can encounter in a koi pond. A significant amount of recycling of nutrients does take place in a koi pond, but not the same harmonious extent of a stable, natural lake. In many cases, we don’t help matters by giving a pond a poor start, supplying perhaps nutrients to cycle and reprocess in concentrations that would never occur in a of natural water body.

Eutrophic or Oligotrophic?

Water bodies can be categorised with respect to the underlying concentrations of nutrients as either Eutrophic or Oligotrophic. In a natural, untouched world, the majority of rivers and lakes would be Oligotrophic with very low levels of dissolved nutrients. However, with man’s influence, excessive levels of nutrients are inevitably released into water bodies, making them Eutrophic (or nutrient-rich). As every koi pond comprises of an enclosed pond, it will start to accumulate nutrients immediately, and at an unnaturally accelerated rate. The fact that more nutrients are being added through food and tap water means that the koi pond’s ecosystem will have to process and cycle far more nutrients than if it were a natural pond. This will have a knock-on effect for water quality, and some nutrients in particular will start to impact on the pond’s characteristics.

The cycling of nutrients: A behind the scenes look at how a pond manages to keep on top of those compounds that enter it. Waste recycling is not new tot eh natural world. In fact, it is its lifeblood.

Elements that cycle through a koi pond can be put into two groups. Those that have a nasty habit of remaining in the water, and those that are more likely to end in the atmosphere, no longer to influence our koi.

Oxygen. In a natural clay pond, oxygen is produced by aquatic plant life, including algae. Produced in the daylight hours through photosynthesis, plants absorb C02 and eventually release oxygen in the form of bubbles. In turn, oxygen is absorbed and utilised by all organisms (including the plants themselves) during respiration, releasing energy from food. The oxygen is then partially recycled as it is returned to the CO2 ‘pot’, dissolved in pond water, released as a by-product of respiration. This may either be released into the atmosphere or absorbed again by photosynthesising plants to be released as oxygen. The accumulation of oxygen in a koi pond is not a bad thing, and if levels increase too much, oxygen will soon return to the atmosphere as a gas.

Carbon-dioxide. Carbon-dioxide shares a very close relationship with oxygen as can be seen from the above cycling of oxygen and is likely to accumulate during periods of excessive respiration but negligible photosynthesis. This may occur in an algae-rich mud pond at night, or a heavily fed clear koi pond at any time. An initial consequence of the build-up of C02 is a drop in pH, but any excessive accumulation of C02 is also ultimately short-lived as it too, like oxygen, can gas off to the atmosphere. Of course, excessive C02 production should be matched with an adequate supply of oxygen, otherwise all aspiring organisms including koi will experience an undesirable oxygen deficit.

Nitrogen. The majority of nitrogen that enters a pond does so in the form of proteins. In a natural mud pond, this may include plant proteins or even animal proteins, found in organisms further up the food chain (daphnia, snails, worms etc). In a koi pond, nitrogen is provided in the form of a dry koi food or may even enter directly into the pond with the water as nitrates when untreated tap water is used.

Box out.

Why are there nitrates in tap water? Ideally, tap water should be as pure and as nitrate-free as spring water, but alas it is contaminated with many solutes including nitrates. Nitrates percolate into aquifers from fertilisers running off land and through soils, or are discharged into rivers as treated sewage. When sewage is broken down biologically, similar bacteria break down the pollutants as those that are found in biofilters. Accordingly, these bacteria generally only break nitrogenous compounds down as far as nitrates, and unless anaerobic processes are provided for breaking down the nitrates, nitrates are released into our rivers, eventually finding their way into are treated tap water.

In a clear and filtered pond, nitrates are produced via biofiltration of ammonia and nitrite, and are introduced through tap water. Unlike CO2 and oxygen, nitrates cannot reach a gaseous state unless they undergo anaerobic conditions has found in the soft sediment of a mud pond. Consequently, a high feeding rate means that the nitrate levels increase at a high rate. In the mud pond, where plant growth takes place unhindered (and nitrate production is a fraction of that of a koi pond) plants take up the nitrate as a nutrient, to form plant proteins, completing the cycle of nitrate as a nutrient.


Phosphorus is such a reactive element that in the natural world, we usually encounter it in the form of phosphates; chemically bound to 4 oxygen atoms. Compared to nitrogen, the phosphorus cycle is very simple, and levels of phosphorus found in a freshwater environment are generally very low and limiting. Phosphates are used by koi in the formation of bones, some enzymes and importantly in a molecule called ATP which is involved in the transfer of energy within an organism. Under aerobic conditions, phosphates are likely to become bound to iron, reducing phosphate levels dissolved in water. Under anaerobic conditions, the insoluble form of phosphate becomes soluble, making them available again for uptake by plants that thrive when phosphate levels are even at their lowest. The phosphates become incorporated into plant tissue which is then consumed by, say, a koi, which excretes the phosphate that it does not require. In a koi pond, even though phosphates will be introduced in tap water, the single most significant route into a koi pond is through the food, particularly via mineral supplements. The Carp family are notoriously poor are absorbing phosphate from their diet, and so elevated levels must be put into the food in a mineral premix to ensure they obtain sufficient from their diet. Unfortunately, any unabsorbed phosphate will accumulate in the pond being taken up by plants and algae.

Man-made eutrophication is nothing new.

Koi ponds are symptomatic of the influence man can have on aquatic ecosystem. Naturally, unpolluted and pure water bodies will typically have an nitrate concentration of 0.5 ppm. Compare this with our koi ponds where 50 ppm (100 times the concentration) is on the boundary of being acceptable and we can soon understand why we can experience problems in a koi pond. One way of reducing nitrate is to install a vegetable filter (if the blanket weed doesn’t beat us to it) to consume the accumulated nitrates and phosphates. On a broader scale, we can see how nature responds to man’s discharge of nutrient-rich effluents into lakes and water bodies, by looking at the resultant abundant weed growth.

CO2 + Global Warming – Nutrient cycling delayed by thousands of years.

In our everyday lives, we are all involved in the globally significant yet delayed form of nutrient cycling when we burn fossil fuels, releasing C02 into the atmosphere. This carbon was laid down thousands of thousands of years ago, being taken out of the atmosphere, into plant tissues and then fossilised as coal (or oil, or gas). Only now is that carbon being cycled back into currency that the living world can use, upsetting the balance as plants try to respond to the high CO2 levels through increased rates of photosynthesis.

A koi pond his not unique in having issues to overcome when natural cycling phenomena try to keep pace with an artificial environment that is maintained using man-made inputs. It shares many of its problems with larger ecosystems (such as polluted African lakes being choked to death by Water Hyacinth) but on a smaller scale. The processes that are at work in either case are very similar, and we must appreciate how best to manage it, while trying to maintain such an artificial environment. Something that is not easy and based on compromise between what we want and what is best for our koi.

Kill blanketweed and string algae.