Monitoring water pollution. A scientist fills a container with freshwater discoloured by an algal bloom. It occurs due to abnormal growth of algae, usually blue-green algae (as here). This growth may have natural causes such as strong sunlight, or more usually is due to surplus phosphorus or nitrogen in the water derived from sewage or other polluting human activities. Some algae liberate toxins harmful to fish, aquatic birds and humans. Algal blooms may also cause eutrophication (oxygen depletion) in bodies of water resulting in large scale death of animal and plant species. This sample was taken at Griffith in New South Wales, Australia.

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