General Remarks
All organisms require nutritional components to support their constructive and destructive metabolic processes (anabolism and catabolism). The constructive metabolic processes promote the various forms of growth, whilst the destructive processes serve to release energy.
The growth of all the organisms living in a garden pond depends on a variety of factors. If one of these factors is less than fully available, growth will be limited. In such a case there is said to be a limiting factor. Fish, water plants, algae and micro-organisms are all subject to different limiting factors; these are outlined below.
- Fish and animals need nutritious, high-quality, high-energy forms of nutrition (insects, fish food etc.) which also set limits on growth.
- Water plants require nutrition, carbon dioxide and light. They can obtain their nutrition from the water or from soil. For water plants the factor limiting growth is light.
- Algae have the same basic requirements as water plants. However, with the exception of green algae, they can only feed off nutrients dissolved in the water. As a rule, the factor limiting the growth of algae is the quantity of nutrients in the pond.
- Micro-organisms contribute to the process of mineralization by degrading dead organic biomass. The nutrients required by the micro-organisms may come direct from the biomass, or they may be contained in the water. The factors limiting the growth of micro-organisms are therefore the availability and the energy content of dead biomass.
Some of the intermediate and final products created by the metabolic processes are released into the water as nutrients, the most important of which are nitrogen (ammonium or ammonia, nitrate, nitrite ) and phosphor (phosphate).
Algae can grow only when sufficient nutrients are available to them in the water. If further nutrients in the form of, say, fish food are added, the natural limitation on the growth of algae imposed by the exhaustion of the supplies of normally available nutrients will have been removed. The disadvantages already outlined will then manifest themselves.
Ammonium and Ammonia
Ammonium is the first inorganic nitrogen compound to be produced when albumen decomposes. The amount of ammonium/ammonia in the pond water is crucial, because ammonia is highly toxic for fish.
The balance between ammonium and ammonia shifts constantly.
This shifting balance is dependent on the pH value of the water. If the pH value rises, the toxic ammonia will tend to prevail. At a pH value of 7 the ratio of ammonium to ammonia is 99:1. If the pH value rises to 9, the ratio changes to 70:30. In other words, the higher the pH value of the water, the more the increased levels of ammonium/ammonia will endanger the garden-pond fauna.
Ammonium/ammonia in the water is degraded by micro-organisms , which are the agents of the process known as nitrification (see figure 4, page 8). Degradation takes place in two stages and involves different sets of micro-organisms.
In the first stage, the ammonia/ammonium is oxidized by bacteria and converted into nitrite. The bacteria involved in this process are referred to as first-order nitrificants. Then the nitrite is broken down by other micro-organisms - so-called second-order nitrificants - and converted into nitrate. In both oxidation processes the bacteria obtain the oxygen they require from the water. The first stage takes longer than the second, because the first-order nitrificants grow only very slowly.
Nitrite (NO2-)
During nitrification, which only takes place when the water temperature is at least 10°C, the ammonium is degraded by certain types of micro-organism, and thus converted, first into nitrite, and then into nitrate (see figure 4, page 8). Nitrite, the intermediate product in this process, is a fish poison at concentrations of greater than 0.2 mg/l. However, if the pond contains micro-organisms which can break the nitrite down directly into nitrate, there will be no danger to fish.
Nitrate (NO3-)
Nitrate is the temporary end product of the decomposition of albumen; it is created when the nitrite into which ammonium has been degraded in the first stage of nitrification is further degraded by the micro-organisms known as second-order nitrificants (see figure 4, page 8). Unlike ammonia and nitrite, nitrate is not a fish poison and therefore does not pose a direct threat to the fish in a garden pond.
Nitrate is, in fact, a fertilizer, and will accelerate the growth of the pond plants. Water in which concentrations of nitrate are increased will automatically encourage vigorous plant growth and the spread of cloudlike algae. This will upset the biological balance. When the algae die they sink to the bottom of the pond, where they are degraded by micro-organisms which consume large quantities of oxygen. This degradation process will set free the nitrate which until then had been bound in the plant cells of the algae, and this in turn will promote algae growth. This cycle can only be broken if the micro-organisms convert the nutrients into biomass, or into atmospheric nitrogen, which is not available to plants.
The further processing of the nitrate and its conversion into atmospheric nitrogen is performed by another group of bacteria, namely, the denitrificants. Denitrification may be defined as the reduction of nitrate into a bound nitrite (i.e., nitrite which is not set free) and into gaseous nitrogen (see figure 3). Gaseous nitrogen is chemically stable and no longer bio-available. Denitrification thus effectively interrupts the cycle of nitrate production and consumption. However, denitrification can only take place in an environment where there is little oxygen.
Carbon Dioxid
Carbon Cycle