Algae Bloom Causes Carbon Dioxide Levels To Rise At Night

Carbon Dioxide (CO2) And Oxygen (O2) work hand in hand in a garden pond. As oxygen levels in the pond water increase, the level of carbon dioxide decreases and vice versa. This is a natural process. Oxygen levels are at their lowest at dawn, due to the processes taking place in your pond over night. During the day as sunlight appears the level of oxygen rises, peaking when the sun sets. After sunset the carbon dioxide which is at its lowest at this time of day starts to rise, peaking just before sunrise. It is important to not let carbon dioxide spikes, caused by green algae blooms from killing your pond fish. The best way to do this is to use a UV clarifier or a UV sterilizer.

How Does Carbon Dioxide Get Into My Pond?

Carbon dioxide is produced by aquatic pond life such as fish, snails, algae and aquatic plants through respiration (breathing). It is also produced as a result of rotting organic matter on the pond floor, including leaves, uneaten fish food and decaying algae blooms.

Carbon Dioxide Spikes Cause Fish Fatalities

Here is a question for those of you who have a fish pond! Have you ever woke up in the morning and found several of your pond fish dead? You probably stood there scratching your head, wondering why? There are two likely causes:

* A predatory animal such as a bird or a cat has physically killed the fish. The tell tale signs should be fairly obvious; damage to the body or head, for example.

* If your pond suffers from heavy algae blooms then the most likely cause is suffocation, through lack of oxygen. The pond water will need to have contained high levels of carbon dioxide and extremely low levels of oxygen for this to have happened.

If the cause of the deaths is due to the high level of green algae blooms then the best way to remedy this situation quickly is to install a UV clarifier or UV sterilizer, as these units will usually manage to get the algae bloom under control within about 5 days. With continuous use a UVC light will keep your pond water free from algae bloom. Please note however that a UV will not remove blanket weed or other filamentous string algae types.

What Is Photosynthesis?

Planktonic algae are a form of microscopic plant life that consumes carbon dioxide, during the day and reverts to using oxygen at night time. All plants carry out a process known as photosynthesis. Photosynthesis uses the energy of the sun to convert inorganic carbon (gained from CO2 dissolved in the water) into carbonic acid (H2CO3). During sunlight hours plants and algae absorb carbon dioxide from the pond water, causing the level of carbonic acid to fall. This causes the pond water to become more alkaline i.e. the pH level increases.

Carbon Dioxide Problems Are Lower In Winter

Oxygen levels in pond water are at their highest during the colder fall and winter months when there is significantly less biological activity taking place within your garden pond. There is an overall reduction in the level of nutrients (nitrates and phosphorous) present in the water. This is mainly due to the plant life and pond life producing less waste; the plants have stopped growing and pond fish feed less. There is also less sunlight resulting in algae populations diminishing. For this reason the overall fluctuations between oxygen and carbon dioxide levels during the day and at night will be significantly less. The chances of Koi and goldfish deaths are significantly reduced.

It is also much easier for atmospheric oxygen to dissolve in cold water than it is for it to dissolve in warmer water. In addition cold water is able to hold more oxygen than warm water. Have you ever noticed pond fish gulping for air at the pond surface on hot, sunny days? This is the reason why.

How Carbon Dioxide Affects Pond pH Levels

The ideal pH for garden pond water is between a pH of 7.5 to a pH of 9. Carbon Dioxide dissolves in water to produce carbonic acid (H2CO3), which has a pH of less than 7. As a result the pond pH will drop and become more acidic as the carbon dioxide levels increase in line with the increased respiration from aquatic life. This is exactly what happens during the night in your pond. During the day the opposite is true, where photosynthesis boosts the increase in oxygen, causing the pH level to rise and become more alkaline. It is when the pH level drops and becomes more acidic that the potential for carbon dioxide problems increase.

What is Pond pH?

The pH scale which ranges from 0 through to 14 signifies whether or not the pond water is acidic (below pH7) or alkaline (above ph7). A pH of 7 is neutral. The higher the number of free hydrogen ions (H+), the more acidic the water is. A neutral pH means that the number of positive hydrogen ions (H+) are the same as the number of hydroxyl ions (OH-).

Aeration Increases Oxygen Levels and Reduces Carbon Dioxide

Garden ponds can never have too much oxygen. A well aerated garden pond will suffer considerably less from the catastrophic effects of high carbon dioxide levels, such as fish fatalities. Using a waterfall, water feature or pond air pump will add valuable oxygen to the pond environment, which will push the pond pH up and remove carbon dioxide from the pond water.

Pond Buffers Increase Pond pH Levels

A buffer is any substance that reacts with an excess of hydrogen ions (H+). The most commonly used buffer is Calcium Carbonate (CaCO3) which is frequently used in garden ponds to prevent the pH from becoming acidic. It is a good idea to add a buffer to your pond filter to prevent fluctuations in pH levels.

CO2 + H2O ---> H2CO3 ---> H+ + HCO3-

In the above example, carbon dioxide + water form carbonic acid. This disassociates into two ions; including a free hydrogen (H+) ion.

CaCO3 + H+ + HCO3- ---> Ca (HCO3)2

In the above example, a buffer such as calcium carbonate (CaCO3) bonds with the free hydrogen ion, to form Calcium Bicarbonate Ca (HCO3)2.

As long as you have calcium carbonate in your pond filter, your pond water will be protected against swings in pH. A buffer will also work in reverse; releasing hydrogen ions back into the water should the water become excessively alkaline.

A pond should be a real enjoyment and will be if you are aware of what is actually happening below the surface. My advice to you would be to ensure that you use a branded biological pond filter, a reliable energy efficient pond pump and a UVC sterilizer or UVC clarifier. Ideally you should add additional oxygen into the pond water; water falls and pond air pumps are the best way to do this.

Remember to ensure that you regularly check your pond water for ammonia, nitrite, pH, total Alkalinity and possibly hardness using a suitable pond test kit to quickly identify any potential problems.

By: Sean Roocroft

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