A low level of phosphorus is usually present in natural waters as phosphate and it is an essential element of life. When too much phosphorus becomes available in water, algae in the water multiplies and grows much more rapidly. This may result in an algae bloom. Algae layers are thick layers of green slime that cover the surface of ponds or slow moving streams. Some algae in water are good but algae blooms are harmful to most other aquatic organisms. They cause a decrease in the dissolved oxygen level of the water. They prevent waves and the surface of the water from coming into contact with the air which provides the main source of oxygen for the water. Their dark color absorbs more heat energy from sunlight causing the water temperature to rise. Warm water holds less oxygen than cold water and this causes the metabolic rate of aquatic organisms to increase. In addition, the algae, which grow rapidly near the surface, block sunlight to plants that live on the bottom or down in the water column, causing them to die. Plants which grow fast, usually die fast, and sink to the bottom of the water source. Dead plant material is decomposed by bacteria that require oxygen and this causes oxygen levels in the water to decrease rapidly. A rapid decrease in dissolved oxygen can result in a fish kill and the death of many other organisms in the water.