Eutrophication is a process where a body of water becomes overly enriched with nutrients. This enrichment fuels the excessive growth of plants and algae, which can severely degrade the quality of estuaries and coastal waters. In the United States, 65% of the estuaries and coastal waters analyzed are degraded by this nutrient influx. While some nutrient increase is a natural part of a lake’s aging process, human activities have significantly accelerated this phenomenon into a widespread environmental issue.
The Process of Eutrophication
Eutrophication begins when excessive nutrients, particularly nitrogen and phosphorus, are introduced into an aquatic ecosystem. These nutrients act as a fertilizer for microscopic aquatic plants known as phytoplankton, or algae. This abundance of food triggers a rapid population explosion of these organisms, an event commonly referred to as an “algal bloom.” These blooms can be dense enough to change the color of the water, sometimes resulting in phenomena like green tides.
The dense mat of algae that forms on the water’s surface blocks sunlight from penetrating deeper into the water column. Submerged aquatic plants, which depend on this light for photosynthesis, are starved of their energy source and begin to die. The loss of these plants represents a disruption to the ecosystem’s stability and food web.
The death of the massive algal bloom and decaying submerged vegetation sets the stage for the next phase. Large populations of bacteria move in to decompose the dead organic matter. Through their metabolic processes, these decomposers consume large amounts of dissolved oxygen from the water. This biological oxygen demand can outpace the rate at which oxygen is replenished from the atmosphere or through photosynthesis.
This depletion of oxygen leads to a condition known as hypoxia, where dissolved oxygen levels become very low. In extreme cases, it can lead to anoxia, a complete absence of dissolved oxygen. These low-oxygen zones, often called “dead zones,” are unable to support most aerobic aquatic life, such as fish and invertebrates. Organisms that cannot escape suffocate and die, leading to large-scale fish kills.
Sources of Nutrient Pollution
The excess nutrients that fuel eutrophication originate from several sources. A major contributor is agricultural runoff, a nonpoint source of pollution because it does not come from a single, identifiable outlet. Fertilizers applied to crops and fields contain high levels of nitrogen and phosphorus, and rain washes these chemicals into nearby waterways. Animal manure from livestock operations is another source of nutrient-rich runoff.
Urban and municipal areas also contribute to nutrient loading in waterways. Wastewater and sewage treatment plants may not fully remove all nitrogen and phosphorus from the water they discharge. Runoff from cities and suburbs carries nutrients from fertilized lawns, gardens, and golf courses into storm drains. Pet waste that is not properly disposed of is also a source of these compounds.
Industrial activities and atmospheric processes also add to the nutrient burden. Some industrial facilities discharge wastewater containing nutrients directly into waterways, which is known as point source pollution. The combustion of fossil fuels releases nitrogen compounds into the atmosphere. These compounds can travel long distances before being deposited on land and water through a process called atmospheric deposition.
Ecological and Human Impacts
The ecological consequences of eutrophication are far-reaching. The creation of “dead zones” reduces populations of commercially and recreationally important species and leads to a loss of biodiversity. As conditions favor a few tolerant species, more sensitive organisms are eliminated. Habitats like seagrass beds, which serve as nurseries for many species, are also destroyed due to light deprivation.
A dangerous consequence is the formation of Harmful Algal Blooms (HABs). While not all algal blooms are toxic, some species of phytoplankton produce potent neurotoxins and hepatotoxins. These toxins can kill fish, marine mammals, and birds. They can also accumulate in the tissues of shellfish, leading to a risk of poisoning for humans and other animals that consume the contaminated seafood, causing symptoms ranging from diarrhea to paralysis and memory loss.
The impacts affect human health and economies. The contamination of drinking water supplies can pose a health risk and require expensive treatment methods to remove toxins and nutrients. Industries that rely on healthy aquatic ecosystems suffer economic losses. Commercial fishing and aquaculture operations are harmed by fish kills and shellfish harvesting closures, while tourism declines as beaches are closed and recreational activities are deemed unsafe.