Oxygen content

Like man, virtually all life forms require oxygen to breathe. Land creatures can meet their oxygen requirements exclusively through air that always contains an abundance of oxygen (approximately 21%). On the other hand only the oxygen dissolved in water is available to creatures that live in the water in the garden pond. At a temperature of 13.9°C the saturation value (100% saturation) is exactly 10 mg of oxygen per litre. This round value is easy to remember and corresponds to 10g of oxygen per m³.

The oxygen content of the water is not stable, and depending on the gas exchange with air, it is subject to strong fluctuations. These fluctuations are caused by the processes of "oxygen consumption" (respiration) and "oxygen production" (photosynthesis), and they become even more extreme as water movement decreases, and thus the exchange of gas with the air decreases. Through the exchange of gas with the air oxygen content is always regulated in the direction of 100% saturation.

In practical terms this means that through intensive water circulation in conjunction with a good mixture of air and water, if there is under-saturation (measured values less than 100%), oxygen content is increased in the direction of 100%. In the case of over-saturation (values above 100%) oxygen is expelled until the balanced state of 100% is reached. Thus in sufficiently aerated garden ponds the measured values for oxygen always fluctuate around 100% ± 30%.

Consumed oxygen can only be replaced by input of atmospheric oxygen, or by oxygen production from the photosynthesis of plants and algae. Oxygen production from photosynthesis is limited to periods of sufficiently high solar radiation. Since oxygen production depends on light, dangerous day/night fluctuations occur; these fluctuations are most pronounced if there is high sunlight incidence, high water temperature and a low level of water circulation.

Oxygen input from the air is determined by the size of the moving air‑water‑ exchange surface relative to the volume of water. Gas exchange with the air is the only source of oxygen that is available 24 hours a day, and with adequate water circulation this gas exchange is capable of effectively compensating for disproportionate oxygen fluctuations.

In the FiltoMatic CWS, with the aid of the Venturi nozzle at the water inlet, 100% oxygen enrichment is achieved before entry into the filter system. After filtering in the FiltoMatic CWS the water should be routed back into the pond via a watercourse or a waterfall, in order to achieve 100% oxygen enrichment here as well.

Solubility of oxygen in water depends on temperature, among other things (Fig. 1). At higher temperatures water solubility is lower than it is at higher temperatures. This fact is generally known. However it has more of a subordinate influence on the life in the garden pond, particularly on goldfish and carp, since even at a water temperature of 30°C, for these fish species in the saturation area more than sufficient oxygen is available.

Fig 1: Oxygen saturation value (100%) depending on the temperature


Less well known but significantly more important is the fact that all life forms in the pond require more oxygen as their body temperature increases. Body temperature in turn is dependent on ambient temperature. The rule of thumb is that the speed of metabolic reaction, and thus oxygen consumption doubles with every 10 degrees of temperature increase (Fig. 2). Due to increased consumption at high temperatures it is easy for an undersupply of oxygen to occur, if this undersupply is not immediately compensated through water circulation!

Thus fluctuations in the oxygen content of the water are greater as the water becomes warmer and water circulation capacity decreases. The explanation for this is actually quite simple: In daylight hours plants and algae produce oxygen, and at night they switch their metabolism over from solar energy to combustion (respiration). Instead of producing oxygen they consume oxygen during the night hours. During the night oxygen consumption can become so great that virtually the entire volume of dissolved oxygen can be consumed before sunrise. Fish kill due to lack of oxygen (except for "accidents", such as power failure, introduction of liquid manure, etc.) always starts at night or in the early morning hours. This clearly indicates how important it is to ensure sufficient water circulation, particularly at night, and how absurd it is to switch off pumps, filters, or aeration systems at night.
In hot summers a high nutrient offering due to over-fertilisation, feeding fish and fish excreta regularly results in algal bloom with a lack of oxygen - particularly in the early morning. These factors regularly cause the entire pond to "overturn" and they cause fish kill, even in shallow natural ponds. However fish kill in this case can certainly be prevented with adequate aeration and water circulation.

Over time the oxygen that is lacking cannot be added to the water through water treatment agents, but rather can only be supplied by water circulation and the exchange of gas with the air.

The new OASE Oxytex CWS offers you an optimal possibility to sustainably supply your pond with oxygen and simultaneously increase the circulation capacity.

Fig 2: Oxygen content and pH value of an over-fertilised garden pond without water circulation in summer (no fish stock, no feeding, fluctuations only due to photosynthesis (=oxygen production) and respiration (=oxygen consumption))