Algal blooms are growths of microscopic algae or cyanobacteria in a water body, often turning the water green, blue-green, or reddish-brown. This phenomenon presents a paradox: algae are photosynthetic organisms that produce oxygen, yet their explosive growth frequently leads to a severe depletion of dissolved oxygen in the water, a state known as hypoxia. The mechanism behind this oxygen loss is not the algae’s initial growth, but what happens when the bloom eventually dies and decays.
The Role of Nutrient Overload in Bloom Formation
The trigger for an algal bloom is an influx of nutrients into the aquatic system, a process called eutrophication. Nutrients like nitrogen and phosphorus are naturally present, but human activities have dramatically increased their concentration in waterways. These excess nutrients act like a fertilizer for the algae, allowing them to bypass natural growth limits.
Sources of this nutrient overload are diffuse and include agricultural runoff carrying fertilizer, sewage, and industrial waste discharge. In freshwater systems, phosphorus is often the limiting nutrient, fueling explosive growth. In marine environments, nitrogen is frequently the limiting factor, though both nutrients contribute to the problem.
How Decomposition Leads to Oxygen Depletion
The shift from oxygen production to oxygen consumption begins when the algal population dies and sinks, having exhausted its nutrient supply or reached the end of its lifespan. This accumulation of dead organic matter, called detritus, provides a food source for bacteria in the water column and on the bottom sediment. These bacteria consume the organic matter, breaking it down into simpler inorganic compounds.
Bacterial decomposition is a form of respiration, which requires and consumes dissolved oxygen from the surrounding water. Because the bloom generates a huge volume of decaying biomass, the bacteria multiply rapidly and consume oxygen at a high rate. This microbial respiration strips the water of the oxygen that aquatic organisms need to survive. While algae consume oxygen at night, the decomposition of the collapsed bloom is the main driver of severe oxygen depletion.
Impacts of Hypoxia on Aquatic Environments
When the concentration of dissolved oxygen falls below a certain threshold, less than two milligrams per liter, the water enters a state of hypoxia. Aquatic organisms cannot extract enough oxygen from the water to sustain their metabolic functions. Mobile organisms, such as fish and shrimp, will attempt to flee the affected area.
Those species that cannot escape, including shellfish, crabs, and benthic organisms living in the sediment, suffer mass mortality. This results in “fish kills” and the formation of “dead zones,” areas where the lack of oxygen has reduced biodiversity to near zero. The long-term consequence is the collapse of local fisheries and alteration of the ecosystem structure.
Reducing Nutrient Pollution to Prevent Blooms
Preventing algal blooms requires addressing the root cause: the excessive flow of nutrients into waterways. Management strategies must focus on controlling non-point source pollution, which is the diffuse runoff from large land areas. This involves better agricultural practices, such as implementing precision farming to reduce fertilizer applied and using cover crops to prevent nutrient runoff.
Upgrading wastewater treatment infrastructure is also necessary, as many older systems do not effectively remove nitrogen and phosphorus before discharging water. Additionally, restoring natural buffers, such as riparian zones of vegetation along streams, helps filter pollutants and absorb excess nutrients before they reach larger bodies of water.