Fertilizers are compounds rich in nitrogen (often nitrate) and phosphorus (usually phosphate), designed to maximize crop growth. When rain or irrigation washes these compounds off agricultural fields and lawns, this process is known as nutrient runoff. This excess of nutrients then flows into rivers, lakes, and coastal waters, fundamentally altering the chemical balance of these aquatic environments. Once these compounds enter a water body, they trigger a chain reaction of environmental consequences that severely impact water quality and ecosystem health.
Eutrophication: The Process of Nutrient Overload
Eutrophication describes the excessive richness of nutrients in a body of water, driven primarily by the influx of nitrogen and phosphorus from fertilizer runoff. In most freshwater systems, these two elements are naturally the limiting nutrients, meaning their scarcity controls the growth rate of primary producers like algae and aquatic plants. The sudden abundance of these compounds acts like a massive dose of fertilizer for the aquatic ecosystem, accelerating biological activity. Nitrates and phosphates become readily available to photosynthetic organisms, fueling rapid and uncontrolled growth. This initial phase transforms the water body into one dominated by an overwhelming amount of plant life, defining a eutrophic environment.
Algal Blooms and Surface Scum
The most visible consequence of this nutrient overload is the formation of an algal bloom, the rapid proliferation of microscopic algae and cyanobacteria. These blooms often manifest as green, murky water or thick layers of surface scum covering vast areas. The sheer density of the algae creates a biological blanket on the water’s surface, preventing light penetration to the depths below. This dense growth directly impacts submerged aquatic vegetation (SAV), such as seagrasses, which die off without sunlight, destabilizing the aquatic habitat. Furthermore, cyanobacteria, often called blue-green algae, thrive in these conditions and are the source of harmful algal blooms (HABs) that pose a threat due to the toxins they produce.
Creating Hypoxic Zones
When the massive biomass of algae and submerged vegetation dies, it sinks to the bottom. This dead organic matter becomes a feast for decomposers, primarily aerobic bacteria. The decomposition process is highly oxygen-intensive, meaning the bacteria rapidly consume the dissolved oxygen (DO) present in the water column. This biological oxygen demand quickly depletes the oxygen supply, especially in deeper layers where atmospheric replenishment is slow. The resulting condition of severely reduced dissolved oxygen is known as hypoxia, which stresses or kills most aquatic life, creating “Dead Zones” in the most severe cases of anoxia.
Contamination of Drinking Water and Ecosystems
Fertilizer runoff introduces specific chemical and biological threats to human health and the broader ecosystem. One serious concern is the direct contamination of drinking water sources by high levels of nitrate. Unlike phosphate, which tends to bind to sediment, nitrate is highly water-soluble and can easily leach into groundwater and surface reservoirs. Ingesting water with elevated nitrate concentrations is particularly dangerous for infants, as it can lead to methemoglobinemia, or “blue baby syndrome,” by interfering with the blood’s ability to carry oxygen. The harmful algal blooms (HABs) that flourish also produce powerful toxins, such as microcystins, which can be aerosolized, ingested directly, or accumulate in aquatic life, meaning bioaccumulation moves toxins up the food chain and presents a continuous health risk.