Algae are microscopic organisms that form the base of many aquatic food webs. Like plants, algae generate their own food, a process that profoundly impacts water chemistry. Their presence is intricately linked to the concentration of dissolved oxygen (DO), which is the oxygen available for aquatic life to breathe. The question of whether algae increase dissolved oxygen does not have a simple answer because their influence is complex, dynamic, and changes significantly over a 24-hour cycle. Understanding this relationship requires looking at the biological processes of oxygen production and consumption that occur simultaneously.
Photosynthesis: The Daytime Oxygen Boost
The primary mechanism by which algae introduce oxygen into water is photosynthesis, a process entirely dependent on sunlight. During the day, algae absorb solar energy to convert water and carbon dioxide into sugars for growth. Oxygen is released as a byproduct of this conversion directly into the water column, significantly raising the concentration of dissolved oxygen. In aquatic systems with dense algal populations, oxygen production can be so high that the water becomes “supersaturated,” holding more oxygen than it could absorb from the atmosphere. This daytime surge causes DO levels to reach their daily peak in the late afternoon, with sunny days leading to higher oxygen production.
The Role of Respiration and Decomposition
While algae are net oxygen producers during the day, they are also continuous oxygen consumers through respiration, as is all other aquatic life. Respiration is the reverse of photosynthesis: organisms break down sugars for energy, consuming dissolved oxygen and releasing carbon dioxide. This consumption happens constantly, day and night, in every living algal cell.
At night, when photosynthesis halts, the respiration of the entire aquatic community becomes the dominant force controlling DO levels. The oxygen produced during the day is steadily consumed by the algae, fish, zooplankton, and bacteria. This leads to a predictable dip in dissolved oxygen that is lowest just before sunrise.
Decomposition and Biochemical Oxygen Demand
The most dramatic reduction in dissolved oxygen often follows the death of a large algal population, a process known as decomposition. When algae die, they sink and provide a massive influx of organic matter that must be broken down by aerobic bacteria and fungi. These decomposers are voracious oxygen consumers, using DO from the water to fuel the breakdown of the dead material. This biochemical oxygen demand (BOD) can be significantly more impactful than the nightly respiration of living algae. Rapid decomposition of a large volume of biomass can quickly strip the water of oxygen, leading to dangerously low levels that stress or suffocate other aquatic organisms.
Algal Blooms and Extreme Dissolved Oxygen Swings
An algal bloom is the rapid, excessive growth of algae, often triggered by an abundance of nutrients like nitrogen and phosphorus from runoff. Dense populations create an unstable environment marked by extreme diurnal dissolved oxygen swings. During the day, photosynthetic oxygen production can cause DO levels to become highly elevated, sometimes resulting in supersaturation that is stressful for some fish species.
The oxygen consumption at night or following the bloom’s collapse presents the greater ecological threat. When a dense bloom dies off, the ensuing bacterial decomposition consumes oxygen so rapidly that the water body experiences hypoxia, a condition of severely reduced dissolved oxygen. If oxygen is completely depleted, the water enters a state of anoxia, creating a “dead zone” where most aerobic aquatic life cannot survive. This severe depletion is the direct cause of mass fish kills. Therefore, while algae initially increase dissolved oxygen, their excessive growth sets the stage for a subsequent, devastating collapse in oxygen concentration.