Fertilizer runoff poses a significant threat to aquatic ecosystems. Fertilizers are substances applied to soil to provide plants with essential nutrients, primarily nitrogen and phosphorus. These compounds boost agricultural yields or enhance growth. However, when applied in excess or before heavy rainfall, these nutrients can wash off the land and enter nearby water bodies.
Aquatic ecosystems include freshwater systems like lakes, rivers, and wetlands, and marine environments such as estuaries and coastal waters. These complex communities interact with their surrounding water environment. The delicate balance within these systems can be disrupted by external factors, including pollutants like fertilizer runoff.
Nutrient Overload and Algal Blooms
Excess nitrogen and phosphorus from fertilizer runoff act as potent nutrients for aquatic plants and algae, leading to a rapid and excessive growth phenomenon known as eutrophication. The visible result of this overgrowth is often an algal bloom.
These dense algal blooms block sunlight from reaching submerged aquatic vegetation, hindering their ability to photosynthesize and grow. Some algal blooms are particularly concerning because they are classified as harmful algal blooms (HABs). HABs involve the excessive growth of toxin-producing algae or cyanobacteria. These toxins can pose severe risks to aquatic life and potentially to human health.
Oxygen Depletion and Aquatic Dead Zones
When the large masses of algae from a bloom eventually die, they sink to the bottom of the water body. Decomposer bacteria then break down this dead organic matter, a process that consumes significant amounts of dissolved oxygen from the water. This rapid consumption of oxygen can lead to conditions of low oxygen, known as hypoxia, or even a complete absence of oxygen, termed anoxia.
These severe oxygen-deficient conditions create what are commonly referred to as “dead zones.” In these dead zones, most aquatic life cannot survive, as they require oxygen for respiration. Such areas become biological deserts, forcing mobile organisms to flee and causing stress or death for those unable to escape.
Harm to Aquatic Organisms
The oxygen depletion resulting from the decomposition of algal blooms directly impacts aquatic organisms. Fish, crabs, shellfish, and other invertebrates that cannot escape low-oxygen zones often experience stress or die in large numbers. The destruction of submerged aquatic vegetation due to blocked sunlight also removes vital habitats and food sources for many species, disrupting the base of the aquatic food web.
Harmful algal blooms (HABs) further compound this problem by producing toxins that are directly poisonous to fish and other wildlife. These toxins can cause illness or death in aquatic animals that ingest them or are exposed to them. The cumulative effect of oxygen depletion, habitat loss, and direct toxicity from HABs leads to a significant loss of biodiversity within affected aquatic ecosystems.
Broader Impacts on Humans and Ecosystem Services
Fertilizer runoff and its consequences extend beyond ecological damage, affecting human health and various ecosystem services. Harmful algal blooms can contaminate drinking water sources, posing risks to human health through the toxins they produce. Exposure to these toxins, whether through ingestion, skin contact, or inhalation, can lead to gastrointestinal illness, skin irritation, respiratory problems, and even liver or neurological damage.
Economically, the impacts are substantial. Fisheries suffer from fish kills and habitat destruction, leading to reduced catches and economic losses for the industry. Tourism is also negatively affected, as fouled beaches, unpleasant odors, and swimming bans deter recreational activities and diminish the aesthetic appeal of water bodies. Furthermore, these environmental degradations impair essential ecosystem services, such as natural water purification and the provision of healthy habitats, which are crucial for the well-being of both the environment and human society.