Algal scum (green) on a river. Algae are microscopic aquatic plants. An increase in their numbers can be caused by the presence of fertilisers or an i

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Algal scum (green) on a river. Algae are microscopic aquatic plants. An increase in their numbers can be caused by the presence of fertilisers or an increase in sunlight. They may then contribute to the water becoming stagnant and losing its oxygen content. The density of algae may also increase as water levels fall during a drought. This is a tributary of the River Lee, London, England.

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Photo credit: © ROBERT BROOK/SCIENCE PHOTO LIBRARY / Alamy / Afripics
License: Licensed
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