Blue-green algae, scientifically known as cyanobacteria, are microscopic organisms found in various aquatic environments. While naturally present at low levels, these photosynthetic bacteria can multiply rapidly under specific conditions, forming dense blooms. Some species produce cyanotoxins, which pose risks to water quality and the health of humans, pets, and wildlife.
Factors Influencing Bloom Persistence
The duration of blue-green algae blooms is influenced by environmental factors. Nutrient availability, particularly phosphorus and nitrogen, drives bloom formation and persistence. Excess nutrients often originate from agricultural runoff, wastewater discharges, and stormwater runoff, enriching the water body. Some cyanobacteria can fix atmospheric nitrogen, thriving even with limited nitrogen.
Water temperature plays a substantial role, as blue-green algae prefer warmer conditions. Optimal growth rates for many bloom-forming cyanobacteria occur above 20°C, with some thriving above 25°C. This explains why blooms are most common during warm summer and early fall months. Abundant sunlight also contributes to their growth, as these photosynthetic organisms require sufficient light.
The stability of the water column is another factor in bloom longevity. A lack of mixing allows buoyant cyanobacteria to concentrate near the surface where light is readily available. This stratification creates a warm, nutrient-rich upper layer that favors rapid proliferation. Lake morphology and depth also influence bloom persistence; shallower lakes are more susceptible. In these systems, nutrients accumulate and water warms quickly, creating ideal conditions for prolonged blooms.
Natural Bloom Decline and Disappearance
Blue-green algae blooms eventually dissipate through several natural processes. A primary mechanism is nutrient depletion, where the dense population of cyanobacteria exhausts available phosphorus and nitrogen. As their food source diminishes, their growth slows, and the bloom begins to decline.
Individual cyanobacteria cells age and die, a natural part of their life cycle. When these cells die, they sink to the bottom of the lake, contributing to the reduction of the surface bloom. Natural predators, such as zooplankton, can graze on cyanobacteria, though their impact on large, established blooms is often limited.
Weather changes can disrupt and dissipate blooms. Cooler temperatures inhibit cyanobacterial growth, as many species thrive in warmer water. Increased wind and heavy rainfall can physically break apart surface scums and mix the water column, dispersing concentrated algae and reducing light penetration. Natural viruses specific to cyanobacteria, known as cyanophages, can infect and lyse cells, contributing to bloom collapse.
Minimizing Bloom Duration Through Management
Human interventions can help reduce the duration and severity of blue-green algae blooms. A fundamental strategy involves nutrient reduction, focusing on decreasing phosphorus and nitrogen input into water bodies. This includes better agricultural practices to reduce fertilizer runoff, upgrading wastewater treatment facilities, and controlling stormwater runoff from urban and suburban areas, which often carries excess nutrients.
Aeration and circulation systems can prevent water column stratification, distributing oxygen more evenly and reducing surface algae concentration. These systems disrupt favorable conditions that allow blooms to persist. Algicides, chemical treatments, offer immediate control in specific situations. However, their use requires careful consideration as they are temporary solutions and may lead to toxin release from dying cells, potentially harming other aquatic life.
Emerging biological controls involve exploring methods like introducing specific bacteria or viruses that target cyanobacteria, though these are largely in research phases. Long-term watershed management is crucial for sustainable control, addressing the root causes of nutrient loading across the entire land area. This comprehensive approach aims to create healthier aquatic ecosystems less prone to prolonged blooms.
Consequences of Prolonged Blooms
The extended presence of blue-green algae blooms carries negative consequences for aquatic ecosystems and human interests. Many cyanobacteria produce harmful cyanotoxins. Prolonged blooms increase exposure risk, causing skin irritation, gastrointestinal issues, and liver damage in humans. Pets and wildlife are susceptible, with reported illnesses and fatalities from ingesting contaminated water.
As large blooms die and decompose, bacteria consume dissolved oxygen. This can lead to hypoxia (low oxygen) or anoxia (no oxygen) conditions, detrimental to fish and other aquatic organisms, potentially causing fish kills. Persistent blooms impact recreational activities, often leading to beach closures, fishing bans, and reduced tourism in affected areas.
Ecological disruption is a significant outcome, as prolonged blooms alter aquatic food webs and reduce biodiversity. Dense surface mats block sunlight, affecting submerged plants and other phytoplankton. Aesthetically, blooms make water appear like “pea soup” or spilled paint, with unpleasant musty or fishy odors, diminishing enjoyment and value of affected water bodies.