Algae Exists in All Water Bodies
Planktonic algae exist in most fresh-water bodies of water and serve multiple purposes as a food source for microorganisms and small fish and to oxygenate the water. Algae is typically kept in check by natural food chain activity. When nutrients suddenly increase by natural means or by outside influences on the water, algae can overproduce and create blooms which turn the water murky, slimy or even opaque.
Runoff and Fertilizer
Runoff from surrounding land areas that have been treated with chemical fertilizers is a common cause of algae blooms in ponds. Lawn and garden fertilizers have high nitrogen levels which can quickly boost algae production over what pond inhabitants can consume.
Sunlight and Temperature Changes
Warm temperatures and abundant sunlight increase the abundance of algae in ponds and exacerbate bloom conditions. This is due to the temperature of the water and the extra seasonal hours and intensity of sunlight. Microorganisms that eat algae are also fewer in warmer weather, which leads to an uptick in algae numbers. Absent fertilizer runoff problems, seasonal algae production is usually kept in check naturally by the onset of cooler temperatures and shorter days in fall and winter.