ROBERT BROOK / SCIENCE PHOTO LIBRARY ROBERT BROOK / SCIENCE PHOTO LIBRARY
Algal bloom (green, bottom left) on a river, next to a storm drain outfall (across upper centre). An algal bloom is usually due to pollution of water with fertilisers, and is called eutrophication. The fertiliser in the water allows algae to grow much faster than usual. The blue-green colour of the algal bloom could be due to the presence of blue-green algae (cyanobacteria). These can form toxins that can kill other plants and animals. The fertilisers can be washed off surrounding farmland into storm drains by excessive rainfall. This is the River Lee, north-east London, England.
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