In the management of eutrophic deep-water lakes and reservoirs, the recurrent outbreaks of algae—especially cyanobacteria—have always been a persistent and challenging problem.

 

Traditionally, algae have been viewed as organisms that passively float on the water’s surface. However, research has shown that certain cyanobacteria have evolved an “intelligent survival strategy” involving active vertical migration—precisely moving between different water layers to evade control measures, thereby greatly increasing the complexity of eutrophication management.

I. Biological Mechanisms Underlying Vertical Migration of Algae

The vertical migration capability of cyanobacteria stems from their cells’ unique nanoscale structures—gas vesicles. These tiny gas-filled sacs, composed of more than 70 different proteins, can precisely regulate cell buoyancy by controlling their gas content, thereby generating a diurnal migration pattern:

·   Daytime Algae rise to the well-lit surface waters, where they rapidly synthesize carbohydrates through photosynthesis, thereby accumulating biomass and reproducing.

 

·   At night or under environmental stress It rapidly breaks down carbohydrates to produce gas, filling pseudovacuoles, reducing cell density, and sinking to a “safe zone” below the thermocline, thereby avoiding surface ultraviolet radiation, chemical agents, and disturbances from wind and waves.

 

Studies have confirmed that Microcystis ( Microcystis The migration speed of dominant species, such as those mentioned above, can reach 1–2 meters per hour, enabling them to complete vertical movement from the bottom layer to the surface within just a few hours. Some strains can even optimize their migration strategies by sensing ultraviolet (UV) intensity and synthesizing sunscreen compounds like mycosporine-like amino acids (MAAs), thereby further enhancing their survival adaptability.

II. The Ecological Vicious Cycle and Multiple Impacts Driven by Migration Behavior

Vertical migration is not only a survival strategy for algae; it also disrupts nutrient cycles and ecological relationships, creating a self-reinforcing feedback loop that is extremely difficult to break.

(1) Underlying “energy reserves” and natural enemy suppression

Algae that sink into deep-water layers can directly absorb nutrients such as nitrogen and phosphorus released from sediments, thereby building up “energy reserves.” Meanwhile, the oxygen-depleted conditions of deep water inhibit the survival of zooplankton species like copepods and cladocerans, protecting algae from predation and providing a safe space for population expansion.

(2) The closed loop of outbreak-deaths-re-outbreak

When environmental conditions—such as light and temperature—improve, algae with sufficient reserves rapidly rise to the surface and proliferate on a large scale. After their death, the algal biomass sinks to the lake bottom, where its decomposition releases additional nutrients, providing the material basis for the next outbreak and thus setting off a vicious cycle characterized by “accumulation of nutrients at the bottom—surface blooms—death and decomposition—re-release of nutrients.”

​

III. The Insights of Scientific Research into Migration Behavior

As early as the 1980s, British limnologist John G. Reynolds, through microscopic observations and tracer experiments, first systematically revealed the behavioral patterns of vertical migration in cyanobacteria. He discovered that cyanobacteria such as Microcystis can precisely locate themselves near thermoclines, adjusting their vertical position to maximize their utilization of light and nutrient resources, thereby confirming their “strategic” migratory behavior—a proactive adaptation to environmental conditions.

IV. Breakthroughs in Governance Technology: From “Superficial Combat” to “Deep-Rooted Control”

Traditional management methods—such as surface application of chemicals and manual removal—only target surface algae and are ineffective against the “algae seed bank” in deeper water layers, leading to rapid rebound after treatment.

 

In recent years, a super-nano aerosol reoxygenation technology targeting vertical migration has been gaining increasing application. This system can deliver dissolved oxygen to the bottom layer—where oxygen levels are most critically low—without disrupting stratification, thereby protecting cold-water aquatic habitats, inhibiting anaerobic release from sediments (especially the release of phosphorus), and achieving eutrophication control. As a result, it effectively improves the ecological environment in deep-water areas. The specific mechanism is as follows:

(1) Increasing the dissolved oxygen at the bottom transforms the anaerobic environment into an aerobic environment suitable for fish and benthic organisms, thereby reducing mass mortality of organisms caused by hypoxia.

(2) Inhibiting the Release of Harmful Substances: Sufficient oxygen can prevent the decomposition of iron-phosphorus compounds in sediments, thereby reducing the risk of internal pollution.

(3) Disrupting the vertical migration of algae: The oxygen-rich bottom environment is no longer suitable for harmful algae such as cyanobacteria, forcing them to rise toward the surface.

(4) Promoting Ecological Balance: After dissolved oxygen levels at the bottom of the water column rise, natural predators of algae—such as zooplankton—become active again, thereby suppressing excessive algal growth through biological regulation.

(5) Activating microbial degradation: Under oxygen-rich conditions, aerobic microorganisms efficiently decompose organic matter at the bottom layer, reducing sediment accumulation and slowing down the silting up of lakes and reservoirs.

(6) Promote material cycling: The addition of oxygen to the bottom layer accelerates the nitrification of nitrogen, helping to convert pollutants into harmless substances or gases that are released.

(7) Flexible seasonal adjustment: During periods of severe stratification in summer, targeted oxygenation is applied to avoid the high energy consumption associated with year-round operation.

​

The vertical migration behavior of algae is one of the core mechanisms underlying the persistent difficulty in effectively addressing eutrophication in deep-water lakes and reservoirs. In the future, effective management will require combining advanced oxidation technologies, migration-blocking techniques, and ecosystem-based regulation to fundamentally disrupt the survival strategies of algae and achieve long-term improvement of eutrophic water bodies.