Algae are simple, photosynthetic organisms found in nearly every environment on Earth. The relationship between algae and vascular plants is complex, and whether algae pose a threat depends entirely on the environment and the rate of algal growth. While a small amount of algae is a natural and often beneficial component of an ecosystem, its ability to reproduce rapidly can fundamentally change the physical and chemical conditions necessary for plant life. Uncontrolled proliferation, known as an algal bloom, can indirectly cause plant death, turning a benign organism into a serious environmental problem.
The Direct Relationship: Is Algae Inherently Harmful?
Algae are not inherently harmful and in many ecosystems, they serve as the base of the food web while producing oxygen. The problem arises when environmental conditions, primarily the over-availability of nutrients, trigger a rapid, unchecked increase in the algal population, known as a bloom. This sudden growth alters the environment, creating conditions detrimental to surrounding plants.
The most severe threat often comes from cyanobacteria, commonly referred to as blue-green algae, which are technically bacteria rather than true algae. These organisms can produce potent toxins, such as microcystins, that are directly harmful to plants, animals, and humans. True algae, like green algae, are generally non-toxic, but their sheer physical presence during a bloom can have devastating indirect effects on plant survival. Excessive nutrient loading, particularly phosphorus and nitrogen, acts as fertilizer for the bloom, driving this harmful growth.
Mechanisms of Damage in Aquatic Environments
In aquatic settings, excessive algae growth damages submerged plants through three primary mechanisms. One immediate effect is light deprivation, where dense mats of algae create a barrier. This barrier prevents sunlight from penetrating the water column, inhibiting the ability of submerged vascular plants to photosynthesize.
Algae also outcompete plants for essential nutrients, a process known as nutrient competition. Fast-growing algae quickly sequester available macronutrients, such as nitrogen and phosphorus, from the water column. This rapid uptake leaves insufficient resources for slower-growing, rooted aquatic plants, which ultimately stunts their growth or causes them to starve.
The process of dissolved oxygen depletion, or hypoxia, is a major killer of aquatic plants and associated life. While algae produce oxygen during the day, they consume oxygen at night through respiration. More significantly, when a large algal bloom dies, bacteria decompose the dead organic material, a process that consumes vast quantities of dissolved oxygen. This massive oxygen consumption can create “dead zones” where oxygen levels are too low to support aquatic plant roots or animal life.
Algae’s Impact on Terrestrial and Soil-Based Plants
Outside of aquatic environments, algae primarily affect plants in containers, seed trays, and greenhouses by altering the soil surface and moisture conditions. In these damp environments, algae often form a dense, green crust or mat on the soil. This physical capping inhibits the emergence of delicate seedlings, preventing them from breaking through the surface.
The algal mat significantly interferes with water management by creating a barrier that impedes water penetration into the soil. Water tends to run off the surface, leading to dry soil layers beneath the crust despite frequent watering. This surface growth also leads to nutrient tie-up, as the algae rapidly absorb surface-level nutrients intended for cultivated plants.
The constant moisture maintained by the algal crust also creates an environment conducive to the development of fungal pathogens. This wet surface layer favors the growth of water molds, such as those that cause damping-off disease in young plants. The subsequent fungal infection kills the seedling at the soil line, demonstrating how algae indirectly promotes disease-based death.
Controlling Algae Growth to Protect Plants
Effective algae control focuses on reducing the environmental factors that enable rapid growth: excess light and nutrients. Reducing nutrient inputs is a primary preventive step, achieved through careful fertilization practices that prevent runoff of nitrogen and phosphorus. Using phosphorus-free fertilizers in susceptible areas can limit the frequency and severity of blooms.
In aquatic systems, light management is achieved by introducing shade to the water surface. Floating plants, such as water lilies or duckweed, can cover up to two-thirds of the water, limiting the light available to submerged algae. For soil-based problems, mechanical removal (lightly scratching or scraping the soil surface) breaks up the algal crust and restores proper water infiltration.
Biological control methods offer an alternative to chemical treatments by utilizing natural competitors. Introducing competitive plants absorbs excess nutrients, thereby starving the algae. Certain aquatic organisms, including snails and some fish species, can also be introduced to graze on the algae, helping to maintain a balanced ecosystem.