In hyper-eutrophic ponds with abundant nutrients scums of filamentous green algae often become obvious near the water surface. These floating clouds of metaphyton originate on sediment, rock or macrophyte surfaces, from which they become disengaged and rise to the surface. Because of the high density of algal cells within the clouds of metaphyton, access to light and nutrients for many individuals may be poor, and the scums probably start to decompose soon after they appear. Constant replenishment from below, however, may result in the continued presence of metaphyton during much of the summer (Lembi, 1988).  (source)

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