Turbidity is a measure of the ‘cloudiness’ or ‘opacity’ in appearance of fresh, estuarine or seawater caused by suspended particles in the water column. Its measurement provides an indication of the ‘clarity’ of water and thus water quality. Suspended particles cause light to be scattered and absorbed rather than transmitted in straight lines through the water. Particles may include clay, silt, fine inorganic and organic matter, soluble colored organic compounds, and small microscopic organisms.

One approach going back as far as the 1900s measured turbidity by diluting a reference suspension sample which could then be used to derive a standard scale to calibrate instruments such as ‘turbidimeters’ used to quantify turbidity often using candle light. Later other methods were developed to estimate turbidity using various light sources but still all dependent on human estimates.

Photoelectric detectors became popular as they were sensitive to very small changes in light intensity while providing better precision, but were still limited in their ability to measure extremely high or low turbidities. Finally, measurements changed in the 1970s when the nephelometric turbidimeter (or nephelometer) was developed which determines turbidity by the light scattered at an angle of 90° from the incident beam. This latter method has been adopted by the EPA as one of their Standard Methods  

High turbidity also impacts biological organisms. Human-induced resuspension-turbidity effects often result from large-scale dredging operations to remove or redistribute sediments, ship and boat traffic, and land runoff. Observed effects are typically site specific as a consequence of sediment grain size and type, hydrological conditions, faunal influences, currents, water mass size and configuration, etc. Potential effects of turbidity and sediment resuspension have received a great deal of attention due to runoff and sediment dredging concerns. High turbidity can for example affect the eggs, larval, juvenile and adult fishes and shellfish in aquatic ecosystems.

Turbidity reduces:
(1) light levels, which in turn causes reduced submerged aquatic grasses through shading and decreased primary production,
(2) affects feeding behaviors of organisms,
(3) gill efficiency, which causes clogging, and
(4) reproduction and larval growth and survival. High turbidity can affect clam, mussel and oyster feeding causing these filter-feeders to shut down and ‘clam-up’. In addition to the above effects, turbidity can affect an organism’s immunological, physiological and histopathological systems, increasing disease.