Phosphorus is a naturally occurring element that is important to all life, serving as a building block for DNA and cell membranes. Despite its necessity, phosphorus becomes a powerful pollutant when introduced into aquatic systems in excess. In most freshwater environments, phosphorus is the limiting nutrient, meaning its scarcity controls the growth of algae and aquatic plants. When human activities increase the amount of phosphorus in the water, this natural growth control is lost, leading to severe ecological disruption. The consequences of this nutrient overload are far-reaching, altering the structure and function of lakes, rivers, and coastal areas.
The Mechanism of Eutrophication
Excess phosphorus acts as a fertilizer for aquatic ecosystems, stimulating a rapid and dense increase in primary production. This process is known as eutrophication. The abundant phosphorus allows phytoplankton, or microscopic algae, to proliferate unchecked, leading to massive growths called algal blooms.
This concentration of biomass disrupts the natural balance by reducing water clarity and blocking sunlight from reaching submerged aquatic vegetation. When light is blocked, these deeper plants die off, removing a food source and habitat for various aquatic organisms. The ecosystem shifts from a balanced state to one dominated by fast-growing, surface-dwelling algae.
Harmful Algal Blooms
A major negative effect of this over-enrichment is the formation of Harmful Algal Blooms (HABs), which are dense growths of specific algal species, primarily cyanobacteria. These cyanobacteria, often called blue-green algae, can produce potent substances known as cyanotoxins. These toxins include microcystins, which are liver toxins, and anatoxins, which are neurotoxins.
Exposure to these cyanotoxins poses serious risks to public health and wildlife. Ingestion of contaminated water can cause gastrointestinal issues, liver damage, and neurological symptoms in humans, and can be fatal to pets and livestock. These blooms can also contaminate drinking water supplies, requiring expensive treatment to remove the toxins and address taste and odor issues. The presence of HABs forces the closure of recreational waters, impacting tourism and local economies.
Depletion of Dissolved Oxygen
The most severe ecological consequence following an algal bloom is the depletion of dissolved oxygen in the water column, leading to hypoxia. While the living algae produce oxygen during the day, the problem arises when the massive bloom eventually collapses and dies. The volume of dead organic matter sinks to the bottom of the water body.
Bacteria and other decomposers consume this sinking organic material, a process that requires substantial amounts of dissolved oxygen. This decomposition activity rapidly consumes oxygen faster than it can be replenished from the atmosphere or surface waters. When dissolved oxygen levels fall below 2–3 milligrams per liter, the water enters a hypoxic state.
These areas are frequently called “dead zones” because they cannot sustain fish, shellfish, and other aerobic life. This lack of oxygen leads to mass mortality events. Less mobile organisms, such as mussels and bottom-dwelling crabs, are particularly vulnerable as they cannot escape the low-oxygen conditions.
Sources of Phosphorus Pollution
The excess phosphorus entering aquatic environments originates from various human activities, categorized as either point or non-point sources. Point sources are identifiable, single discharge locations, such as outfalls from municipal and industrial wastewater treatment plants. While these sources are regulated, they historically contributed significant soluble phosphorus to water bodies.
Non-point sources are now the major contributors of phosphorus to surface waters and are more difficult to manage due to their diffuse nature. This pollution largely comes from agricultural runoff, which carries phosphorus-rich soil particles and fertilizers from farm fields. Urban stormwater runoff is also a significant source, carrying phosphorus from lawn fertilizers, pet waste, and eroded soil into local waterways. Additionally, failing septic systems and atmospheric deposition contribute to the overall nutrient load.