Fertilizers are substances, either natural or artificial, that contain chemical elements designed to enhance plant growth and productivity. They replenish nutrients in the soil, providing essential components like nitrogen, phosphorus, and potassium for healthy development and high yields. While beneficial for agriculture, the widespread use of fertilizers introduces a challenge when these nutrients enter aquatic ecosystems.
The Journey of Nutrients into Waterways
Fertilizer nutrients, predominantly nitrogen and phosphorus, enter aquatic environments through several pathways. A significant route is agricultural runoff, where rain or melting snow washes excess fertilizers from farm fields into nearby streams, rivers, and eventually larger water bodies. These nutrients can also leach through the soil, moving downwards to contaminate groundwater sources.
Animal manure, also used as fertilizer, contributes significantly to nutrient loading, with runoff from livestock operations carrying high levels of nitrogen and phosphorus into waterways. Additionally, wastewater discharge, particularly from systems that do not fully remove nitrogen and phosphorus, can introduce these nutrients. Once in the water, these excess nutrients become problematic, disrupting the natural balance of aquatic life.
Eutrophication: The Over-Enrichment Process
The excessive influx of nutrients like nitrogen and phosphorus into water bodies initiates eutrophication. This process describes the over-enrichment of water, leading to an increase in the growth of algae and other aquatic plants. Initially, these nutrients stimulate rapid proliferation of microscopic algae, often resulting in dense, visible algal blooms on the water’s surface.
As these extensive algal blooms form, they create a thick layer that blocks sunlight from penetrating deeper into the water column. This deprivation of light causes submerged aquatic plants, which rely on photosynthesis, to die. Eventually, the vast quantities of algae within the bloom also begin to die off.
The decomposition of this dead plant matter is carried out by bacteria and other microorganisms. This process consumes substantial amounts of dissolved oxygen. As oxygen levels drop, the water body can become hypoxic (low oxygen) or even anoxic (no oxygen), creating conditions where most aquatic life cannot survive.
Ecological and Health Consequences
The progression of eutrophication to hypoxic or anoxic conditions has significant ecological consequences. Areas with low oxygen levels become “dead zones,” where fish and other marine organisms cannot survive, leading to mortality events. These dead zones disrupt aquatic food webs, as species unable to tolerate low oxygen levels are forced to leave or perish, impacting the entire ecosystem structure. The loss of biodiversity is a key outcome, with species requiring higher oxygen levels being replaced by those more tolerant of hypoxic conditions.
Beyond ecological damage, eutrophication also poses direct threats to human health and economic activities. Harmful algal blooms (HABs), often a result of nutrient pollution, can produce toxins that are dangerous to people and animals. Exposure can occur through direct contact with contaminated water, inhaling airborne toxins, or consuming seafood that has accumulated these poisons. Such toxins can cause a range of symptoms, including skin irritations, respiratory problems, gastrointestinal illness, and neurological effects.
Contamination of drinking water sources is another health concern, particularly from elevated nitrate levels originating from fertilizer runoff. High nitrate concentrations are linked to health risks such as methemoglobinemia, known as “blue baby syndrome,” a potentially fatal condition in infants that impairs oxygen transport in the blood. Emerging research also suggests links between nitrate in drinking water and an increased risk of certain cancers and premature births. Economically, dead zones and harmful algal blooms negatively impact commercial and recreational fisheries, leading to reduced catches and increased seafood prices. They can also harm tourism by closing beaches, creating unpleasant odors, and causing fish kills.