Oxygen content of pond water

Without oxygen life on earth in its present form would not be possible. This also applies to for life in water and thus to your pond. Pond water is provided with oxygen through a certain number of sources. In sequence of importance they include:

  • Through the water surface by diffusion. Especially water which is moved by wind and rain absorbs much oxygen from the atmosphere.
  • By growing underwater plants. Especially so-called oxygen producing plants (waterweed, hornwort and pond weed) can produce huge amounts of oxygen under favourable circumstances.
  • By any algae present.

Colder water can contain more oxygen than warmer water. In general, upper water layers contain more oxygen than deeper water layers. Normally speaking no problem will occur with a view to provision of oxygen during the growing season.

However, there are a number of circumstances in which lack of oxygen may occur.

  • In case of unfavourable ratios between surface and depth. As referred to earlier, the water surface absorbs much oxygen from the atmosphere. A small water surface can of course absorb a limited amount of oxygen. Still deep water layers of 1 metre or more will not sufficiently get into touch with oxygen. Lack of oxygen may occur than especially for micro-organisms.
  • At relatively high water temperatures. Hot summer days can raise water temperature in a pond up to 25⁰C. At this temperature the saturation value is just 8 mg of oxygen per litre of water. Especially if a large number of fish is present then, this can easily result in lack of oxygen (compare: water of 10⁰C has a saturation value of 10.9 mg oxygen per litre).
  • In case of accumulation of CO2 in water. Beyond the growing season the amount of CO2 gas in the water may increase under unfavourable circumstances (too low GH value). In contradistinction to oxygen CO gas in water is heavier and will accumulate from the bottom. In the deeper water layers low oxygen or even oxygen less conditions will occur. This will result in a disaster for micro-organisms and in a later stage also for fish.
  • In case of an enormous growth of large stocks of oxygen producing plants (waterweed, hornwort and pond weed), in combination with higher water temperatures. During the day time oxygen producing plants will release oxygen under the influence of light and will absorb CO2. At night this process will be in reverse. Large stocks of oxygen producing plants, in combination with higher water temperatures can cause lack of oxygen especially in the early morning hours.

Oxygen problems manifest themselves especially in fish. They will be staying in the upper part of the water, gasp for breath and their movements are slow. Lack of oxygen in the lower layers of water can manifest itself by the fact that a little layer of oil will appear on the water surface, caused by withered micro-organisms. The solution for oxygen problems is always the installation of a strong air pump, which will duly set the lower water layers in motion.