Turbidity is caused by undissolved substances (particles) in the water mass. Turbidity is defined as an expression of the optical attribute that causes light to spread and to be absorbed instead of being transmitted in straight lines through the sample. The turbidity varies with the size, shape, composition and refractive index of the suspended particles. These can be sludge, mud, phyto- and zooplankton, microbes, organic material and other small, insoluble particles of varying origin. In the open ocean, the dominating particles are usually phyto- and zooplankton or dead organic material. Particles of mud and similar material from rivers and erosive shores occur near the coasts. The turbidity is related to the concentration of suspended substances, but the relationship can not be quantified.