Background and History of Phosphorus
Lake Stevens is the largest natural lake in Snohomish County. The lake covers 1013 acres, and has an average depth of 62 feet (19 meters) and a maximum depth of 150 feet (46 meters). Lake Stevens is fed by Stevens, Lundeen, Kokanee, and Stitch creeks, which comprise the major sources of water feeding the lake. The Lake Stevens watershed area covers 4,536 acres including the lake’s surface. This 4:1 watershed to lake ratio indicates a relatively small drainage basin for a lake of this size. The outfall of the lake drains into Catherine Creek and then to the Pilchuck River.
From the 1950’s and into the 1980’s, Lake Stevens experienced frequent algal blooms, a decline in water clarity, and poor water quality due to increases in phosphorus loading. Initially, external loading was due to forestry and agricultural practices, and in later years, nutrients came from housing and commercial developments (Snohomish County 2008). Internal loading was occurring simultaneously from a natural chemical cycling process involving phosphorus and iron. In the presence of oxygen, phosphorus binds with iron and remains in the sediment. During the warmer summer months, the sediment in the lake doesn’t receive enough oxygen and the chemical reaction which originally immobilized phosphorus reverses, releasing phosphorus from its bond with iron. In 1994 an aerator system was installed to maintain the required dissolved oxygen levels in the bottom waters of the lake (the hypolimnion) to sustain iron and phosphorus bonding during months when oxygen levels at the lake bottom dropped.
Phosphorus is essential for plant and animal life in an aquatic ecosystem, however an excess of this nutrient acts as a fertilizer and stimulates the growth of algae. This increase dramatically accelerates the rapid growth and death of blue-green algae that clouds water, reduces dissolved oxygen, and can poison fish and wildlife – causing a threat to the health and overall quality of the lake and its surrounding environment (Ecology, 2011).