Oxygen Dissolved in water, often referred to as DO is a very important parameter of water quality and is an index of physical and biological processes going on in water.

There are two main sources of dissolved oxygen in water (i) Diffusion of Oxygen from air to water is a physical phenomenon and depends upon solubility of oxygen which, in turn, is influenced by factors like temperature, water movements and salinity etc. Photosynthetic activity is a biological phenomenon carried out by autotrophs (mainly phytoplankton) in water and depends upon autotroph population, light conditions and available gases etc.