Most agricultural and lawn fertilizers cause algae to grow. Whether that is considered beneficial depends on whether algae is a desired product. Fertilizers can be intentionally applied to holding ponds to cause algae growth, while unintended fertilizer runoff can cause oxygen-depleting blooms that harm the environment.
Algae is the designation botanists give to a group of thousands of multicelled, water-dwelling plants that photosynthesize sunlight. There are three primary types of algae: red, brown and green. The more complex red and green algaes are commonly called seaweed. Microscopic types of algae form the base of the food chain in ocean and pond environments, feeding zooplankton that in turn feed larger fish and mammals. Some types of algae contain toxins harmful to aquatic life and humans. Clouds of blue-green algae are commonly called pond scum and are readily apparent in lakes and ponds in the summer.
Fossil evidence indicates that algae has existed for millions of years, and most botanists believe that land-based plants evolved from algae ancestors. According to environmental scientists at the Wisconsin Department of Natural Resources, records exist of algal blooms going back to the 12th century. An increase in nutrients reaching waterways from sewage discharges, agricultural chemicals, phosphorus-based lawn fertilizers and other human sources have made these blooms more prominent in the past several decades.
Fertilizers can be used to enhance algae growth for beneficial purposes. Raising fish in ponds for personal or commercial harvest requires cultivation of algae to create fish food and a proper habitat. Scientists and entrepreneurs are also exploring ways of combining excess fertilizer, such as sewage sludge or animal manure, with carbon dioxide emissions from power plants and factories to grow algae under stressed conditions to produce biodiesel and ethanol fuels.
An algae bloom may make a water body look like it is teeming with life--and it is, temporarily. Algae produce oxygen during daylight hours. The extra oxygen and plant matter can feed millions of micro-organisms and lead to increased fish populations. However, when the algal bloom dies off--either all at once if the nutrient source stops, or over time if the nutrient source keeps flowing--decomposition of the algae structures depletes the water body of oxygen, resulting in fish kills and other environmental damage. If the algae bloom is toxic, it can sterilize the pond.
Excess algae growing in a swimming pool may be caused by the inadvertent introduction of fertilizer into the water. Pool owners should consider lawn fertilizer running into the pool in water flowing along seams of a surrounding concrete walkway, or water added to the pool from a well that has been infiltrated with agricultural fertilizer.