Light has a key role for aquatic ecosystems, both in marine and freshwater. It penetrates underwater and interacts with dissolved and particulate water constituents, the optically active constituents (OACs). They absorb and scatter the light, giving water its characteristic colour and affect the light availability underwater. The three main OACs are phytoplankton, coloured dissolved organic matter (CDOM) and suspended particulate matter (SPM) and vary in time and space. Absorption and scattering represent the inherent optical properties (IOPs) of water and depend solely on the OACs present in the water. In addition, water bodies have apparent optical properties (AOPs) that depend both on OACs and the incident light field.
The chlorophyll in the phytoplankton absorbs blue and red wavelengths and reflects green. Therefore, the oceans appear blue-green depending on the concentration of phytoplankton. CDOM is primarily tannin-stained water released from decaying detritus. High CDOM concentrations appear yellow-green to brown. CDOM absorbs ultraviolet (UV) light in the surface waters which is harmful for phytoplankton but competes with phytoplankton for light. Inorganic suspended matter (ISM) is the suspended sediment in the water. It is a component of SPM and strongly scatters longer (red) wavelengths. High ISM concentrations give water a reddish-brown colour. Pure water, however, absorbs longer wavelength red light. As natural waters vary in their composition, oceanographers introduced ocean classification schemes based on the optical properties of water. The main differentiation is between Case 1 open ocean waters and Case 2 coastal waters. In open ocean waters, the optical properties are dominated by phytoplankton and covarying material. In coastal waters, optical properties are dominated by suspended sediments and CDOM that vary independently of phytoplankton.