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Turbidity is a measurement of how 'cloudy' the water is in a given water body. 'Cloudiness' in water can occur from suspended sediment, dead organic matter or algal blooms. Turbidity is measured in nephelometric turbidity units (NTU's)which is a comparison of the amount of light scattered by the suspended particles in the water.
Aquatic and marine plants do not grow as well if light levels in water are reduced. This is a major problem, as less plants photosynthesising in water means less oxygen in the water, which in turn affects fish and other organisms needing the oxygen. Furthermore, reduced plant life in water affects the local ecosystem as animal populations that feed off the aquatic plants are reduced.
An even worse problem is that increasing turbidity can indicate high nutrient levels which are a growth factor for blue-green algae (cyanobacteria). This algae is very toxic and competitive against other microorganisms, ruining river systems such as the Murray-Darling River System.
Find out more about blue-green algae