Understanding the Water in Your Garden Pond…

Ponds & The Nitrogen Cycle

Ponds & The Nitrogen Cycle

It is crucial to have good water quality within any ecosystem, be it a garden pond, natural lake, stream or river as without it; problems can arise causing algal blooms and even fatalities with fish, invertebrates and amphibians.  Having the correct balance of flora and fauna within your pond will assist in having good water quality.  To have ‘good’ water quality can be dependent on what species you may have within the pond or where geographically the water body might be, but for purposes of this article; I will discuss garden ponds or wildlife ponds as a whole.

Water quality parameters will change throughout the season due to weather patterns such as rainfall, sunlight and wind and seasonal breeding patterns of fish, invertebrates and amphibians.  In order to understand the basics of water quality you need to have a basic knowledge of chemistry; however, please don’t worry; I will keep it simple and you won’t need to memorise the periodic table!

The basics of water chemistry and the Nitrogen Cycle, that I will come to later, is of benefit to anyone who has a garden pond, lake, water feature or aquarium and my aim is to help you understand what goes on beneath the water’s surface.

Water (H₂O) is made up of two hydrogen atoms and one of oxygen, physically this should be a gas, however, due to the oxygen atom being highly charged it attracts and sticks to other water molecules and forms a liquid.  This allows other molecules to dissolve in water which affects the suitability of water for aquatic life.  Examples can be how acidic (soft) or alkaline (hard) the water might be.  The lower the pH; the more acidic, the higher the pH, the more alkaline, a pH of 7 is neutral.  Topping up your pond with mains water can vary in pH depending on where you live so it is important to check this prior to filling your pond.  If you are lucky enough to have a natural spring, then this would normally be softer water.

The majority of aquatic invertebrates and fish prefer slightly harder water, but some aquatic plant species prefer more acidic, softer, water.  Rapid changes in any water quality parameter, particularly the pH, will cause stress to aquatic animals, so monitoring the waters temporary hardness; KH (carbonate hardness) is of benefit.  KH values of between 8°d and 12°d will indicate good buffering and a stable pH.  General Hardness or GH, is what makes water hard or soft and is related to the presence of calcium and magnesium salts in the water.  Harder water also plays a role in fishes colour and in harder water, alkaline, areas, the ‘Sumi’, or black in Koi carp will be more pronounced, whilst softer water will show ‘Hi’ colours like red.

Apart from the biological effects on water caused by livestock, the waters parameters can change from fluctuations in temperature.  Oxygen levels will decrease as the temperature increases which can cause risk to aquatic life, especially when plants use up oxygen levels at night during photosynthesis.  Low dissolved oxygen levels also cause an increase in CO², carbon dioxide, which will cause pH levels to drop.  Lakes do not suffer as much as smaller bodies of water, like garden ponds do, as larger water volumes are better at stabilising.

The Nitrogen Cycle…

I will keep this basic, but it is important for anybody that has an aquarium or pond to understand the principles as it explains the reasons as to why you shouldn’t overstock or over feed fish and why continued monitoring of the water is necessary.

To start; fish eat the food and discrete ammonium as a waste product into the water.  Ammonium is also produced from rotting detritus, dead fish and invertebrates and uneaten food on the bottom of the pond, although this is not directly toxic to the fish, it does depend on the other water quality parameters and often converts to toxic ammonia which will cause damage to the fish’s skin and gills.

Naturally occurring bacteria in the pond, called Nitrosomonas converts ammonia into nitrite which can be less toxic but is more toxic in harder water areas.  At high levels; ammonia prevents haemoglobin from being able to bind with oxygen, thus making it hard for fish to breath and signs of a high Nitrite or Ammonia level can be fish gasping at the water’s surface.

Nitrite is then converted into less harmful nitrate by naturally occurring Nitrobacter bacteria.  Pond plants and aquatic vegetation use the nitrate to grow and help keep the ponds inhabitants alive.  This is why having the correct fauna, flora balance is crucial.

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  • Nabila September 24, 2014 Reply

    very helpful information for my thesis

    • Andy November 1, 2014 Reply

      Hi, I am glad its of assistance to you 🙂

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