An established ecosystem in the pond includes full algal photosynthesis. During the day the algae produce oxygen by photosynthesis, while during the night they consume oxygen. Thus there will be a daily fluctuation in the oxygen level in the pond, and special care must be taken during the night when it may be necessary to supply additional oxygen. Similarly, the pH may fluctuate because photosynthesis fixes carbon dioxide and therefore the pH will increase during the daytime, while at night the algae release carbon dioxide and the pH will drop.
The ecosystem created will affect all nutrients since a nitrogen cycle will occur in the pond: nitrification will transform TAN to NO−2 and further to NO3−. If there are areas in the pond lacking oxygen, dentrification of NO−3 to N2 will also occur.
A major benefit of a pond is therefore that it is possible to utilize this biological production, which includes prey that is food for the fish. In addition, there will also be decomposition of waste. However, to achieve this state the water exchange rate must not be too high (see Section 14.3). The major disadvantages with production in ponds is the low production per unit volume, and the difficulties of maintaining control over the water quality and the actual fish biomass.
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