Pond algae utilize nitrogen and phosphorous to grow, which can be found in dead and decaying vegetation, fish and animal waste and fertilizer runoff. Residential areas that have a lot of pavement and not enough land cover to absorb rain will experience algae blooms in ponds and rivers because of the excess fertilizer and septic overflows that run into the water. This causes fish kills that in turn cause more algae to grow.
Too Much Sunlight
Constant, direct sunlight also helps scum to grow, which is why the late summer months feature algae problems. Ponds situated in direct sunlight, without any nearby vegetation to give it occasional shade, risk algae blooms. Too few aquatic plants in the pond can also cause algae problems because of the lack of sunlight interference.
Low Oxygen Levels
Algae uses the oxygen in a pond to grow. If there are few plants or fish living in the pond to produce oxygen, the algae will feed on its own growth cycle. Low oxygen levels are created often by too much algae, and it is the algae that will thrive on the environment it has created. Ponds are ideal conditions for the persistence of low oxygen levels because there is no natural turbulence such as waves or tides to re-oxygenate the water.
There are several types of pond scum, each with particular nutrients beneficial for particular animals and vegetation. Planktonic algae gives the pond the particular green tint associated with scum and is made up of beneficial plankton that feed the vegetation and animals in the pond. Pond scum is not a problem itself; it is a sign that there are other problems related to the correct balance of light and growth within and around the pond.
The correct balance of conditions will help keep the healthiest amount of algae. Locating a garden pond close to tall trees will help keep the sunlight down. Planting aquatic plants in your pond will keep the oxygen levels up. If the pond is a naturally forming pond, ensure the surrounding trees are healthy. Never use excess fertilizer, particularly if there is a lot of pavement around and little open ground. Both synthetic and organic algaecides can be used to kill off some of the algae and let the pond re-balance itself while you try to manage the elements that you can control.