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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 6: Temperature stratification
The koi that we keep today have been bred from carp that evolved in natural lakes and, throughout their evolution, they rarely experienced temperatures below 4°C because of a peculiar effect called temperature stratification that takes place in deep, still water. What is temperature stratification and, more importantly, what does this have to do with koi ponds?
Temperature stratification
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Figure 1: In a lake in summer the
warmest water is at the top
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Hot air rises is an expression that everyone will have heard and will accept as being true. Hot air balloons use this effect to float above the ground. When air is heated it expands and, to use a very unscientific but descriptive expression, the warmer the air is, the lighter it will be. Exactly the same happens with water at any temperature higher than 4°C, the warmer it gets, the lighter it becomes. This means that, in warm weather, the warmest water tends to accumulate as a layer at the surface of a lake as shown in figure 1.
Looking at this effect from the other direction, we could say that as winter approaches, wind and cold air causes the surface water of a lake to cool and, as soon as it becomes colder than the water deeper down, the newly cooled surface water will sink to the bottom of the lake. Until the weather gets really cold, although the overall temperature of the lake will be much cooler, it will still be the case that the coldest water will be at the bottom because of the effect whereby cool water is slightly heavier than warmer water.
However due to an interesting property of the water molecule, all this changes as water is cooled below 4°C. Below this temperature, water molecules behave differently which means that a fixed volume of water stops getting slightly heavier as it cools and the reverse happens, it starts getting lighter.
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Figure 2: In a lake in winter the
coldest water is at the top
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When the overall temperature of a lake has fallen to 4°C, as the surface cools even more, the cold water stays at the top as shown in figure 2. Even though the surface becomes cold enough to freeze solid, the water near the bottom remains at 4°C for a very long time, cooling only very slowly. It is this phenomenon that fish have evolved to exploit.
During summer when the water was warm, the ancestors of the fish we now call koi would swim around throughout the whole volume of their lake looking for food and sunbathing near the surface. As winter approached and water temperatures fell, they would eat less and swim less but would still swim throughout the entire volume. As the surface water cooled below 4°C they would retreat to the deepest parts of the lake where the water would stay at around 4°C throughout most of the winter. At this temperature they would conserve their energy by not swimming very much and could spend the coldest periods of winter in the relatively warm water near the bottom of their lake without venturing into the colder water above. The ancestors of our koi evolved to cope with winter in unheated natural lakes but did not have to spend any significant amount of time in water colder than 4°C.
Stratification in koi ponds
When still water is below 4°C it will always be the case that the coldest water will be at the surface regardless of depth. Shallow puddles always freeze from the surface downwards showing that, even in a depth of a few centimetres, there is some small amount of thermal stratification, but the temperature difference in shallow water between the surface and the bottom will be very small. For there to be a significant difference between the top layer of water and the bottom, a lake or pond needs to be deep. Only in the deepest of koi ponds could the water at the bottom be prevented from falling below 4°C by the stratification effect. Also, any stratification that is trying to occur is negated if the filter pump is left running throughout the winter. This is because the warmer water near the bottom is being drawn out of the pond and, even if it isn’t chilled in the filtration system, it will be returned nearer to the surface where it will mix with cooler water which will cool it down a little. As the surface continues to lose heat and this mixing continues, the whole pond slowly becomes chilled leaving nowhere for the fish to spend the colder months of winter at the relatively mild water temperature of 4°C.
Why not turn off the pump?
In an average outdoor koi pond that is typically 4 ft deep, simply turning off the filter pump during winter would be one way to ensure that water at the bottom of the pond would stay a little warmer than water near the top. But although it would be warmer at the bottom, due to the action of wind on the surface, there would be sufficient stirring of the water to prevent it remaining at 4°C. Also, stopping the flow of water through the filter system would mean that the water in it would become stagnant. Bacterial activity slows right down as the temperature decreases but it doesn’t stop entirely. If the flow of water to and from the pond through the biofilter in figure 3 was stopped, the air stones would provide oxygen for the bacteria but their primary energy supply of ammonia would cease.
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Figure 3: The biological filter
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For bacteria to enter into a dormant state when their energy source stops they must shut down the chemical processes going on inside them in a way that will allow them to restart when conditions are more suitable. The ammonia bug (nitrosomonas) has evolved to be able to do this. It can enter a state of dormancy for long periods and then “wake up” and carry on as if nothing had happened but the nitrite bug (nitrobacter) has never evolved the ability to become dormant when times get bad. In the natural world, when its nitrite supply was interrupted, it simply metabolised different chemicals. This is not an option in a koi pond; its only source of energy is the waste product, nitrite, from its cousin the ammonia bug and if that bug becomes dormant, the nitrite bug very slowly starves to death.
Turning off the filter pump during winter would allow the koi to stay a little warmer by not disturbing the bottom layer of warm water but it will also cause stagnant water in the pipe work and disrupt the bio-filter. Filter pumps should run throughout the winter.
Conserving heat during winter
If the average koi pond isn’t really deep enough to allow a natural temperature stratification to form and if the good water circulation techniques as described in part 4 will disturb any stratification anyway, the most obvious step to take to conserve heat during winter is to insulate everything possible and to cover the pond during the coldest months. Anecdotal evidence says that some pond keepers are managing to keep their ponds above 4°C by these methods, but what about those ponds where the water temperature falls below that point?
Animal Welfare Legislation
Keeping pets is currently a grey area. Some organisations regard it as cruel to keep animals confined in a cage and would like the practise stopped. How long will it be before confining fish in a pond is also targeted? Our best
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The Five Freedoms
1. Freedom from hunger and thirst
2. Freedom from discomfort
3. Freedom from pain, injury and disease
4. Freedom to express normal behaviour
5. Freedom from fear and distress
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defence as koi-keepers is to show that we provide our pets with as near a free-range lifestyle as is possible and we give our pets the conditions they would enjoy if they were free in a lake. It is generally thought by animal welfare organisations that if animals enjoy what are known as the five freedoms, they are being well cared for but it could be argued that if koi are subjected to water temperatures outside their natural range, this could breach the second freedom because they are being exposed to discomfort.
Whether or not you believe that is a valid argument, it is certainly true that when pond temperatures fall as low as they did and for as long as they did last winter, [UK winter 2010 / 2011 was prolonged and very harsh] many fish don’t survive. This is why I am now arguing that koi ponds should be prevented from falling below 4°C by a heater that, at the very least, is set to prevent the water falling below that temperature. In combination with a well insulated pond, this wouldn’t use a great deal of energy so it would be relatively inexpensive to run but would keep us on the right side of animal welfare legislation and prevent fish losses.
Aeromonas alley
Another reason to have a heater on a pond is to prevent an effect, sometimes called aeromonas alley, which is due to the falling efficiency of the koi immune system when compared with the activity of pathogenic bacteria such as aeromonas at low temperatures. Opinions vary slightly as to the precise temperatures, but in general, aeromonas alley can be described as follows: The koi immune system is designed to keep it healthy and combat infection with white blood cells playing a key part in the system by attacking or devouring invading bacteria. According to Luther Chien, the first proponent of the aeromonas alley theory that I recall reading, the koi immune system is at its most efficient at around 25°C and he also states that no white blood cells are found in the koi bloodstream below about 12.8°C. Aeromonas bacteria are not active below 4.4°C but their activity and ability to infect rises rapidly as temperatures increase above this point until they reach their peak activity at 15.6°C.
This means that at 4°C koi are almost inactive but, since the aeromonas bug is also inactive, they are safe from infection. If the water temperature rises slowly, the aeromonas bug becomes active and can infect koi because its immune system is weak until the temperature rises above 12.8°C so it is defenceless against this infection until then.
Although this temperature range is called aeromonas alley, aeromonas isn’t the only threat, there are other potentially harmful bacteria that could exploit the weakened koi immune system below about 13°C. Of course, if a pond is kept scrupulously clean there will be very little in the way of harmful bacteria in it but, even so, this temperature range will always be a potentially risky area where even small numbers of pathogenic bacteria can infect koi and cause infections they are unable to resist.
There are two strategies to avoid aeromonas alley. The first is to heat throughout the entire winter ensuring that the temperature stays above 13°C. This can be expensive and many koi-keepers prefer their koi to have a winter period to “reset their biological clocks”. A second, cheaper way is to set a heater to prevent the temperature falling below 13°C in autumn and maintain this temperature for as long as possible. Then, as the coldest part of winter approaches, readjust the heater to allow the temperature to fall reasonably quickly to 4°C. Maintain this temperature until spring when the heater can again be used to raise the temperature to 13°C until ambient temperatures are sufficient to keep the water above that temperature without the aid of the heater.
Insulating a pond against the cold is important but, where this is not sufficient to prevent the temperature falling below 4°C, a heater is essential. Also using a heater to avoid the risks of aeromonas alley is highly desirable. Next month I will discuss various methods of heating koi ponds and how some of them may not be as expensive as you think.
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