Turbidity is a measure of water clarity (cloudiness) and is affected by the concentration of suspended particles in the water column. Suspended matter may include clay, silt, organic matter, and plankton (microscopic plants and animals). The cloudier the water, the greater the turbidity. Turbidity is measured in NTUs (Nephelometric Turbidity Units).
The amount of suspended particles affects the dissolved oxygen (DO) concentration in water. As turbidity increases, the ability of sunlight to penetrate the water decreases. Sunlight is scattered and/or absorbed by the particles suspended in the water column. Reduction in the availability of light decreases the amount of phytoplankton, algae and other aquatic plants, thereby limiting photosynthetic activity (DO production is decreased). In addition to the reduction in DO levels, organisms (zooplankton and fish) dependent on plants for food are also affected.
Soil erosion from agricultural lands and construction sites, as well as urban runoff, can increase the turbidity in streams and rivers. Discharge from wastewater treatment plants may contain organic matter and plant nutrients (nitrogen and phosphorus) and may provide attachment places for other pollutants, such as metals and bacteria. Algae blooms can also cause an increase in turbidity.
Suspended matter remains in the water column as long as there is sufficient current to carry it. The deposition of suspended matter creates problems for aquatic organisms by covering up habitat and filling in lakes and slow moving areas of streams. By covering up habitat the amount of invertebrate food for fish is reduced and predators feed less efficiently in turbid water. If the sediment load is too high fish gills can become clogged. Turbidity also affects the human perception of water quality. Increased turbidity causes people to regard water as "dirty" or polluted.