Cyanobacteria are photosynthetic bacteria naturally present in most freshwater systems. A bloom is an excessive proliferation of these microorganisms that results in visible discoloration of the water, often appearing as a green, blue-green, or brownish-green scum. The duration of these blooms is highly variable, ranging from a few days to an entire season, depending on the lake’s specific environmental conditions. While cyanobacterial blooms are a natural ecological phenomenon, human activities that increase nutrient loading have made them more frequent, severe, and widespread.
Conditions That Trigger the Start of a Bloom
The onset of a visible bloom requires a specific combination of environmental factors that give cyanobacteria a competitive advantage over other aquatic organisms. One significant factor is elevated water temperature, as many bloom-forming species thrive in warmer temperatures than most eukaryotic algae. Warmer temperatures accelerate the growth rate of cyanobacteria, allowing their population to rapidly increase.
A second necessary condition is high nutrient availability, particularly phosphorus and nitrogen, which often enter the water body through runoff from agricultural lands, sewage effluent, and stormwater. High phosphorus concentrations are frequently associated with bloom formation. Cyanobacteria are highly efficient at absorbing and storing these nutrients, and some species can even use nitrogen gas from the atmosphere, giving them an advantage when dissolved nitrogen levels are low. The final trigger is low water flow or stagnant conditions, which allow the cyanobacteria to accumulate at the surface for better access to sunlight.
Factors Determining the Length of a Blue-Green Algae Bloom
Once a bloom is initiated, its duration is determined by how long these favorable conditions persist, which can keep the bloom going for weeks or months. A primary factor in sustaining a bloom is the thermal stratification of the water column. Stable, calm weather creates a warm, less dense surface layer over a cooler, denser bottom layer. Many cyanobacteria species can regulate their buoyancy, allowing them to migrate between the nutrient-rich bottom water and the sunlit surface, where they can outcompete other organisms.
Another mechanism that prolongs the bloom is internal nutrient recycling, especially the release of phosphorus from lakebed sediments. In stratified lakes, low oxygen levels that develop in the bottom water cause phosphorus stored in the sediment to be released back into the water column. This internal loading provides a continuous food source for the cyanobacteria, even if external nutrient runoff has ceased. Furthermore, the specific species of cyanobacteria present plays a role, as some are more resilient and form specialized resting stages that can persist in the sediment and re-emerge later.
How a Blue-Green Algae Bloom Naturally Ends
A bloom typically ends when the environmental conditions that favor its growth change, leading to a collapse or dissipation of the surface mass. The most effective natural terminators are strong wind events and a rapid drop in ambient temperature. Strong winds disrupt the stratification of the water column, causing the warm surface layer to mix with the cooler, deeper water.
This mixing process disperses the cyanobacteria cells throughout the water, moving them away from the light-rich surface and into the darker depths, where their growth becomes limited. A significant drop in water temperature below the optimal range for cyanobacteria will also slow down their reproductive rates, shifting the competitive advantage back to other types of algae. Eventually, the bloom may also dissipate due to the natural death of the massive cell population. This cellular breakdown can be caused by viral lysis or bacterial decomposition, or by the depletion of a limiting nutrient.
What Happens to Toxins After the Bloom Dissipates
A dissipating bloom does not immediately mean the water is safe, because many cyanobacteria species release cyanotoxins, such as microcystins, when their cells die and break apart. This release means that the highest concentrations of these compounds may occur just as the visible bloom is collapsing. The toxins transition from being contained within the cells to being dissolved in the water, where they can persist for an extended period.
The rate at which these dissolved cyanotoxins break down depends on several factors, including light exposure and microbial activity in the water. Microcystins can persist for days or weeks, with some studies showing them remaining above safe thresholds for up to 100 days. Therefore, clear water does not guarantee the absence of danger, and continued monitoring of toxin levels is necessary until tests confirm the concentrations have degraded below levels of concern.