Algal scum (green) on the shores of a reservoir. 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 one of the reservoirs on the Lee River, the Lee Valley, north-east London, England. Photographed during the 1991 drought.