The question of whether dead algae float is more complex than a simple yes or no, as the answer depends entirely on the organism’s state and the environment. Algae are microscopic, plant-like organisms found in nearly all water bodies, and they possess mechanisms to control their position in the water column while alive. When an algal bloom dies off, the immediate fate of the biomass differs from what happens days or weeks later as the material breaks down. The change in buoyancy is governed by cellular biology, density physics, and bacterial action.
How Living Algae Maintain Buoyancy
Living algae must constantly adjust their vertical position to access optimal levels of sunlight and nutrients for photosynthesis. Many species, particularly cyanobacteria (blue-green algae), manage this using specialized, protein-based structures called gas vesicles. These hollow nanocompartments are permeable to gases but exclude water, creating internal air-filled spaces within the cell. Algae actively regulate the number and strength of these vesicles to fine-tune their buoyancy, allowing them to rise toward the light or sink deeper to gather nutrients.
Another mechanism for buoyancy control involves storing energy as lipids or oils. Many types of algae produce these fats, which are significantly less dense than water. By increasing the proportion of these lighter molecules within their cells, the algae effectively decrease their overall cellular density. This lipid accumulation provides positive buoyancy, helping the organisms remain suspended in the upper, sunlit layers of the water column.
The Initial Sinking of Dead Algae Biomass
When algae die, the active biological processes maintaining buoyancy immediately cease, leading to a rapid increase in density. For cyanobacteria, the delicate protein walls of their gas vesicles collapse under the surrounding water pressure. This loss of internal air space instantly removes the primary flotation mechanism, causing the cells to become negatively buoyant. The dense cellular material, composed of proteins, carbohydrates, and other organic matter, initiates the sedimentation process.
This immediate physical consequence means the newly deceased algae biomass begins to sink toward the bottom of the water body, often referred to as the “rain” of organic matter. The cellular components are denser than the surrounding water, dictating this initial downward movement. This rapid sinking is the primary fate of an algae bloom immediately following a massive die-off event. The layer of dead cells accumulates on the bottom sediment, where the second phase of the buoyancy cycle begins.
Secondary Refloating Caused by Decomposition
Once the dense organic matter settles on the bottom, it becomes a concentrated food source for various microorganisms, especially anaerobic bacteria in the deeper, oxygen-depleted sediment layers. These bacteria begin the process of decomposition, breaking down the complex organic compounds in the absence of oxygen. This anaerobic breakdown produces various gases, including methane, carbon dioxide, and hydrogen sulfide.
As these decomposition gases are continuously generated, they become trapped within the dense, sunken mat of decaying algal biomass. The accumulating gas forms bubbles, which exert an upward force on the entire mass. Over a period of days or weeks, enough gas may be produced to overcome the negative buoyancy of the dead material. This secondary process causes large mats of dead algae to refloat to the surface, where they can be seen as foul-smelling slicks, completing the cycle of sinking and refloating driven by bacterial action.