Phosphorus (P) is an element found in all living things and is fundamental to life. The form most readily available to aquatic organisms is phosphate. This nutrient is a component of adenosine triphosphate (ATP), the primary energy currency of cells, and is a structural part of DNA and RNA. In freshwater ecosystems, phosphorus often acts as the limiting nutrient, controlling the overall amount of plant and algae growth. While necessary for a healthy ecosystem, even a small increase in phosphate concentrations above natural background levels can accelerate aquatic plant growth, leading to significant water quality degradation.
Runoff from Agricultural Land
The agricultural sector is a primary source of elevated phosphate levels in waterways, mainly due to commercial fertilizers and livestock waste management. Fertilizers contain phosphorus compounds that remain on the soil surface until absorbed by crops or bound to soil particles. If heavy rainfall occurs shortly after application, a concentrated pulse of soluble phosphate can wash directly into nearby streams and rivers.
Applying manure and other organic wastes also contributes significant phosphorus. Manure is often applied to meet nitrogen requirements, frequently resulting in the over-application and accumulation of phosphorus in the soil. This saturation reduces the soil’s capacity to bind phosphate, making the excess nutrient more mobile and prone to runoff. Soils with high phosphorus levels, resulting from decades of nutrient surpluses, continually release elevated amounts of phosphate into surface runoff.
Phosphorus loss is also linked to soil erosion from tilled fields. Erosion carries particulate phosphorus, bound to sediment particles, into the water column. This bound phosphorus settles in streambeds and lake bottoms, where it can be released into the water as the sediment decomposes. Conservation practices, such as reduced tillage, minimize soil erosion and reduce the transport of this particulate phosphorus to aquatic environments.
Discharge from Wastewater Systems
Urban and domestic wastewater systems contribute phosphate primarily through human waste and household cleaning products. Human waste accounts for a substantial portion of the phosphate load entering treatment facilities. Historically, phosphate-containing detergents added heavily to this load, though their contribution has been reduced in many regions due to regulatory action.
Municipal wastewater treatment plants (WWTPs) remove solids and organic matter, but conventional processes are not effective at removing dissolved phosphate. Conventional biological treatment may only remove about 20% of the phosphate present. Modern facilities often employ chemical precipitation, using metal salts like aluminum or iron to bind with soluble phosphate, transforming it into a solid precipitate removed with the sludge.
Enhanced Biological Phosphorus Removal (EBPR) uses specialized bacteria that absorb and store excess phosphate. Outside of centralized treatment, decentralized systems like septic tanks also pose a risk. Aging or failing septic systems, especially those with compromised leaching fields, allow wastewater to seep into groundwater or surface water, introducing phosphate into the environment.
Natural Geological Contributions
Phosphate occurs naturally in the environment, providing a background level. This baseline phosphate comes primarily from the slow, natural weathering of phosphate-rich rocks and mineral deposits in the earth’s crust. As water flows over these materials, it slowly dissolves and releases phosphate ions into the soil and water.
The amount of naturally occurring phosphate is strongly influenced by local geology and soil type. In unpolluted surface waters, the natural background level of total phosphorus is very low. This natural process establishes the baseline productivity of the aquatic ecosystem, which is significantly altered by human-driven sources.
The Consequence: Eutrophication and Algae Blooms
The influx of excessive phosphate initiates eutrophication, the over-enrichment of a water body by nutrients. Since phosphate is the limiting nutrient in most freshwater systems, its abundance triggers a rapid, unchecked growth of aquatic plants and algae. This proliferation often results in dense algal mats and scums known as algal blooms, which can include potentially toxic types like cyanobacteria.
These blooms cloud the water, reducing the depth to which sunlight can penetrate, which kills submerged aquatic vegetation by blocking photosynthesis. The massive amount of algae eventually dies and sinks, where bacteria consume the dead organic matter. This decomposition consumes large quantities of dissolved oxygen (DO) from the water, leading to hypoxia. Hypoxic conditions create “dead zones” where fish and other organisms cannot survive, leading to widespread die-offs and a collapse of the aquatic food web.