Virtually every type of organism needs oxygen to breathe, in the same way as humans do. Terrestrial creatures obtain all the oxygen they require from a particularly rich source, namely, the air, 21% of which consists of pure oxygen. The organisms that live in garden ponds, on the other hand, are dependent on the oxygen dissolved in the water, which seldom amounts to more than 12 mg/l. In other words, the oxygen content accounts for no more than 0.0012% of the entire weight of the water. In comparison with air, one fifth of which consists of oxygen, this is an extremely small amount. It is therefore vitally important for all the organisms in the pond that the water be saturated to the highest possible degree with oxygen.
The amount of oxygen contained in the water depends on several different factors. Used oxygen can only be replaced by oxygen from the atmosphere or by the oxygen produced as a result of the photosynthesis of plants and algae. And photosynthesis will only create oxygen at those times during the day when there is sufficient sunlight.
Temperature, air pressure and, in particular, the area of the exchange surface relative to the volume of water will in large measure determine how much oxygen enters the water from the atmosphere.
The solubility of oxygen depends on a number of factors, and, in particular, on the ambient temperature (see figure 1). In the summer, for example, the solubility of oxygen will be lower than during the cooler seasons. And the warmer the environment, the greater the oxygen requirement of the organisms living in it. As a general rule it may be said that in a garden pond the metabolic rate - and thus the consumption of oxygen - will be doubled when the temperature rises by 10° degrees Celsius (see figure 2). At higher temperatures, therefore, there is a risk that the water will be insufficiently oxygenated.
The amount of oxygen in the garden pond fluctuates widely. Plants and algae which produce oxygen during the hours of sunlight switch their metabolism over from solar energy to combustion (respiration) at night, and then proceed to use up oxygen. During the night the consumption of oxygen can be so great that just before sunrise there is hardly any soluble oxygen left in the water. This means that the early morning hours are an especially critical time as regards the survival of the fish and all the other breathing organisms. These changes in oxygen levels are known as day/night fluctuations. The conclusion to be drawn is that it is not advisable to switch off aeration systems at night.
If, moreover, there are large quantities of nutrients in the water in the form of dead algae and plants, or superfluous fish food, there will be an enormous increase in the rate at which micro-organisms consume the oxygen, particularly if the water is warm. Indeed, the rate of consumption can be so high that there is not enough oxygen left for the fish and the other relatively highly developed organisms in the pond to survive on. If they die, the pond water will putrefy within a matter of hours. The damage to the biological balance is irreparable, and the pond will go off.
Introduction
pH Values