ABSTRACT
The aquatic environment contains relatively little oxygen. It is therefore essential for all forms of aquaculture to maintain an adequate level of oxygen in the rearing water. The decomposition of organic matter by the numerous microorganisms living in the water column or on substrates also consumes oxygen. This requirement for oxygen and the respiration constitutes the biological oxygen demand and may be important when the decomposing animals and plants are in closed systems for example in a pond. Using herbicides in water bodies is not always practical. The chemical oxygen demand is a measure of the chemical reactions which consume oxygen; these are low in the waters used in aquaculture. Carbon dioxide gas is scarce in the sea because it is part of a reversible chemical equilibrium with carbonate and bicarbonate ions and carbonic acid. The total level of carbon dioxide varies between 40 and 50 mg l-1.
