Also known as - Algal bloom, Blue-green algae
These can be toxic
Algae are a form of microscopic plant often known as "phytoplankton". They develop in stagnant or slow flowing water bodies where there is a high concentration of nutrients (phosphorus and nitrogen) that are transported into the water supply, from land runoff, or as a result of high amounts of rotting organic matter, I.E. leaves falling into the water or the excessive feeding of waterfowl. It can occur as mats, blooms and scums in varying colours and density. Algal masses suspended in the water can shade the pond bottom, thereby reducing the growth potential of rooted aquatic weeds.
Algal mats are aggregations of filamentous algae growing in a submerged mass over the pond bottom, especially in shallow areas in sunlight. Photosynthesis produces oxygen and if enough gas accumulates in the mat a portion can break off and float to the surface creating an unsightly lumpy mess of green gooey matter.
Blooms are a suspension of Algal cells in the water giving it muddy appearance. Scums are a thin floating layer of algal growth in several colour variations, "blue-green algae" is commonly referred to in the press but strictly these forms are bacteria that contain plant pigments, the Cyanobacteria, a number of species of blue-green algae can produce toxins. Not all scums are a result of algae; pollen, airborne dust or accumulations of fine seed may also appear as a surface scum.
Other floating plants may at first glance look like an algal scum, Duckweed, Water meal and the Azolla (Water Fern) species can appear to be a scum but are in fact floating plant colonies; however the end result is the same - shading of the pond and a reduction of Oxygen levels. Duckweed is a tiny rapidly growing floating rooted perennial plant, Water meal is a genus of very small rootless plants that resemble specks of cornmeal. Azolla is another genus of floating rootless plants similar in size to duckweed vaguely resembling a fern. All of these plants are farmed in some countries for animal fodder and can appear on a water body at any time as they can be imported in by waterfowl.
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Duckweed on Brickfields pond in 2007
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