Dissolved oxygen - Aquatic Habitats
Oxygen in the terrestrial ecosystems occurs in the atmosphere along with other gases in a certain fixed concentration however, in aquatic ecosystems it is dissolved in water, where its concentration varies constantly depending on factors that influence the input and output of oxygen in water. In fresh water the average concentration of dissolved oxygen is 0.0010 per cent (also expressed as 10 parts per million or 10 ppm) by weight, which is 150 times lower than the concentration of oxygen in an equivalent volume of air.