Algae are aquatic, plant-like organisms that use sunlight for photosynthesis but lack the roots, leaves, and stems of true plants. They are a natural part of all aquatic ecosystems, ranging from microscopic, single-celled forms to larger structures. Concern arises when their population grows rapidly and excessively, a phenomenon called an algal bloom. These blooms occur in freshwater and marine environments, often causing the water to turn noticeably green or other colors. Excessive algal growth is not caused by a single factor, but by the convergence of specific conditions that allow for uncontrolled proliferation.
The Essential Ingredients: Nitrogen and Phosphorus
The primary drivers for excessive algal growth are the macronutrients nitrogen (N) and phosphorus (P), which algae require to build cells and reproduce. In many freshwater systems, phosphorus is the limiting nutrient; even a small addition can trigger rapid growth until the supply is exhausted.
The accumulation of excessive nutrients is known as eutrophication, a process that accelerates the water body’s aging. When both nitrogen and phosphorus are supplied in excess, algal growth far outpaces the ecosystem’s ability to maintain balance.
The relative amounts of these two nutrients, expressed as the Nitrogen-to-Phosphorus (N:P) ratio, influences which specific species will dominate a bloom. A molar ratio of approximately 16:1 (the Redfield ratio) is cited for balanced growth. When the ratio deviates, certain species gain a competitive advantage. For example, a low N:P ratio often favors cyanobacteria (blue-green algae). These organisms can fix atmospheric nitrogen to meet their needs, allowing them to outcompete other species and proliferate into potentially harmful blooms.
Environmental Factors Accelerating Growth
While nutrients provide the fuel, physical and meteorological conditions accelerate the speed and intensity of a bloom. Water temperature is a significant factor, as warmer water increases the metabolic rates of algae, allowing them to grow and reproduce faster. Most harmful bloom species, particularly cyanobacteria, thrive in warmer temperatures and dominate in hot conditions.
Increased temperatures also favor algae by reducing the solubility of gases like oxygen and carbon dioxide in the water. This stresses aquatic plants, which compete with algae for nutrients, shifting the ecological balance. Furthermore, warmer surface water can lead to thermal stratification, where a layer of hot water sits atop cooler, denser water, preventing the mixing of the water column.
Calm, slow-moving, or stagnant water provides an ideal environment for algae to remain concentrated near the surface. Staying close to the surface maximizes their exposure to sunlight for photosynthesis. In stratified water bodies, some cyanobacteria can regulate their buoyancy, migrating down to nutrient-rich bottom layers at night and floating back up to the surface during the day, giving them a competitive advantage.
How Algal Spores and Nutrients Enter Water Systems
The continuous supply of nutrients that sustain blooms occurs through several distinct pathways. Nutrient pollution from human activities is the major contributor of excess nitrogen and phosphorus into aquatic environments. A significant source is agricultural runoff, where rain washes chemical fertilizers and animal manure from farm fields into waterways. In urban areas, stormwater runoff carries pollutants like pet waste and lawn fertilizers into streams and lakes. Wastewater effluent also contributes, as sewer and septic systems can discharge human waste containing high levels of nitrogen and phosphorus.
Atmospheric deposition from the burning of fossil fuels, which releases nitrogen into the air that falls onto water bodies, is another non-point source. Algae cells and spores enter new water sources through both natural and human-mediated transport. Spores can be carried long distances by the wind or transported by animals, such as on the feathers of birds. Human activities, including the movement of boats (via ballast water) or fishing gear between bodies of water, can introduce new algal species to an environment.