What is phosphorus?
Webster’s Dictionary defines “nutrients” as something that nourishes or a nourishing ingredient in a food. Nutrients are the building materials that algae need from their environment in order to grow and reproduce. The more nutrients plants and algae have, the better they grow.
The primary food nutrients for algae are phosphorus and nitrogen, which they need in addition to the sugars they make in order to grow. Other nutrients necessary in smaller amounts include calcium, magnesium and iron.
In most King County lakes, phosphorus is less available than nitrogen and is called the “limiting nutrient” because as the algae are taking up nutrients from the water, the phosphorus supply runs low first. When the limiting nutrient runs out, the algae must stop growing. This means that when the phosphorus supply in the lake is increased, the algae in the lake can also grow more. This increase in growth, left unchecked in a lake or stream, can result in unsightly and nuisance algal blooms.
Where does phosphorus come from?
Phosphorus is a naturally occurring element in the environment that is essential to plant growth. It is vital in lakes to help algae and aquatic plants grow, which in turn support a healthy lake ecosystem by providing food and habitat for fish and other aquatic animals. Phosphorus is found naturally in plant and animal tissue, and we all need some phosphorus in order to live, grow and reproduce. When watersheds are developed, new or modified sources of phosphorus in the landscape can produce big increases in the amount of phosphorus getting into the water. 
There are a variety of phosphorus sources to consider. For example, when aquatic plants in shallow water die back and decay in the autumn, phosphorus is released into the water. Aquatic plants are essential as refuge for fish, snails and amphibians, but an overabundance of aquatic plants can mean big phosphorus releases to the water in the fall when they die back.
Phosphorus is also excreted by animals as part of their feces. This can create a problem if the waterfowl population becomes very large. Big-bodied animals such as horses and cattle can also release phosphorus through their feces into water bodies, particularly when they have direct access to a stream or lake, or if the runoff from their pastures enters nearby surface waters and their excreta are not picked up and collected into managed manure piles. Domesticated pets can also be contributors if their droppings remain in places where they can wash into the surface waters.
Failing septic systems can contribute phosphorus from human feces to nearby surface waters. Some synthetic detergents and other cleaning agents still contain soluble phosphorus. Soaps and detergents flushed down the drain into a septic system can
end up in nearby waters when the binding properties of the soil can’t hold any more phosphorus. Washing your car at home can
also contribute nutrients from the soap and the car surface into nearby surface waters through storm drains. Fertilizers for lawns and gardens also contain soluble phosphorus and when improperly used in our yards, they can wash into our lakes and streams.
What does it do?
All this extra phosphorus encourages nuisance algae blooms and excessive plant growth in lakes. When phosphorus increases greatly over the natural concentrations, the resulting algae can severely impact recreational uses. For example, activities such as swimming, boating and fishing all can be affected by algae blooms, both in terms of enjoyment and even health and safety.
Not only can excessive phosphorus hinder beneficial uses for humans, it also changes lake processes. Cloudiness caused by the algae shades the water and changes conditions for plants and animals that use vision to catch prey. Bacteria that decompose algal remains use up oxygen in the water that fish depend on, particularly in the cool deep habitats that trout love. If bluegreen algae blooms become dominant, there is also the possibility of toxin production that could affect the health of animals and humans.
What can you do?
To minimize the amount of phosphorus your own activities may be contributing to nearby water, there are a number of very
helpful things that people living in the watershed can do:
• Minimize use of phosphorus cleaners. Clothes-washing detergents have been phosphorus free since the 1970s and in July 2010 a ban on phosphorus in dishwashing detergents went into effect across Washington State. Read the labels to see which products contain phosphorus and try the ones without it. With our soft water, they should be very effective.
• Wash your car at a car wash that recycles its water, keeping soaps and road dirt out of the surface waters of the watershed. If you must wash your car at home, use water only.
• Use low or no phosphorus fertilizers on your lawn. Grass needs nitrogen and potassium in fertilizers to become lush and green. It doesn’t need the phosphorus for good color or leaf production and the excess phosphorus will leach out when you water, getting into streams and lakes.
• Don’t dispose of grass clippings in or near water.
• Don’t feed the geese and ducks. They will congregate on beaches near good food sources and their feces will wash into the surface water and eventually into the lake.
• Scoop your pet’s poop and dispose of it in the trash or into the septic system.
• Keep your septic system maintained and functioning properly. Have it checked every three to five years and don’t put anything down the toilet that the system can’t handle.