The sight of dark, stubborn spots clinging to the walls of a swimming pool is a persistent pool nuisance, commonly known as black algae. It appears as dark, localized patches, typically on porous surfaces like concrete or plaster. While the organism itself poses biological risks, its true danger often lies in the environmental and chemical disruptions it causes. Understanding the nature of this organism is the first step toward effective management and ensuring a healthy swimming environment.
What Black Algae Really Is
The term “black algae” is a misnomer, as this organism is not a true alga but a type of bacteria called cyanobacteria. These prokaryotes perform photosynthesis like plants, which is why they are often referred to as blue-green algae. The colonies can range from deep blue-green to nearly black, appearing as small, dark spots on the pool surface.
This cyanobacterium is tough to eradicate because it forms a thick, protective, multi-layered biofilm. This outer shield, primarily composed of polysaccharides, makes it highly resistant to normal levels of chlorine and other common sanitizers. The organism embeds itself deeply into the pores and crevices of surfaces like gunite, plaster, and concrete, creating a root-like structure that allows it to survive even after the visible surface growth is scrubbed away.
Direct Health Concerns of Pool Cyanobacteria Exposure
The specific strains of cyanobacteria found in chlorinated swimming pools rarely present the severe toxicity associated with large, natural freshwater blooms. Cyanobacteria can produce cyanotoxins, which are harmful to humans and animals. However, chlorine in a properly maintained pool environment tends to keep the toxin-producing capacity suppressed or the toxins neutralized.
Direct risks to swimmers are generally mild. Skin contact may occasionally lead to irritation or a rash. If a swimmer accidentally swallows contaminated pool water, they may experience gastrointestinal upset.
The cyanobacteria colonies can serve as a physical harbor for other, more dangerous microorganisms. The protective biofilm offers shelter, potentially shielding pathogenic bacteria like E. coli from the pool’s sanitizing chemicals. This secondary bacterial proliferation poses a greater direct health risk than the cyanobacteria itself, potentially leading to infections or serious gastrointestinal illnesses, especially in children or individuals with compromised immune systems.
The Indirect Safety Hazards of Infestation
The presence of black algae introduces several indirect physical and chemical safety hazards. The rough, textured patches of growth create a slipping hazard for anyone walking on the steps or pool floor. This uneven growth increases the risk of falls and accidents, particularly when entering or exiting the water.
The cyanobacteria actively consume chlorine, the pool’s primary disinfectant. This consumption depletes the residual sanitizer level, compromising the pool’s overall sanitation. When chlorine levels drop below the recommended range, harmful pathogens, such as Giardia or various bacteria, can thrive.
A patch of black algae may conceal underlying structural problems in the pool shell, such as a crack, a leak, or erosion in the plaster or concrete. This prevents addressing minor damage before it develops into a costly repair issue.
Effective Eradication and Long-Term Prevention
Eliminating a black algae infestation requires mechanical removal combined with intensive chemical treatment. Aggressive manual scrubbing of all visible and affected areas is the first step. Due to the organism’s tough protective layer, a stainless steel brush must be used on plaster or concrete surfaces to break through the biofilm and expose the cells to the chemical treatment.
After the protective layer is compromised, a concentrated chlorine solution should be applied directly to the brushed spots. This is followed by a super-chlorination, or “shock,” of the entire pool water, often requiring a dosage of three to four times the normal amount of granular chlorine to kill the remaining bacteria. Running the pump and filtration system continuously for at least 24 hours after the shock circulates the high concentration of sanitizer throughout the water.
For long-term prevention, consistent water chemistry is necessary. The pool’s pH should be maintained between 7.4 and 7.6, and the alkalinity between 80 and 120 parts per million (ppm) to ensure the chlorine remains effective. Regular weekly maintenance should include brushing the pool surfaces, as this disrupts any potential new colonies before they can establish their protective layer. Preventive algaecides, particularly those containing silver or copper, can be used as a supplementary defense against cyanobacteria.