Pool Algae

Algae Classification and Growth Requirements

Pool algae are photosynthetic microorganisms that colonize pool water and surfaces when sanitizer levels drop below the threshold required to kill spores. Three species groups account for virtually all residential pool algae problems — each with distinct visual identification, surface attachment behavior, and chemical resistance profiles.

All algae species share four growth requirements: sunlight (or artificial UV), water temperature above 60°F, available nutrients (primarily phosphates and nitrates), and insufficient sanitizer residual. When free available chlorine drops below the minimum effective threshold — approximately 7.5% of the cyanuric acid level — dormant spores activate and reproduce through cell division, doubling their population every 6 to 8 hours under ideal conditions.

Chlorine kills algae spores at maintained residual levels, but active blooms require breakpoint chlorination — raising FAC to 10 to 30 ppm depending on species and severity. Algaecide provides a secondary prevention layer when chlorine levels temporarily dip between service visits. Phosphates accelerate algae reproduction by supplying the primary limiting nutrient — reducing phosphates below 125 ppb starves algae colonies of their growth fuel.

Green Algae (Chlorophyta)

Green algae (Chlorophyta) is the most common species in residential pools — accounting for an estimated 80 to 90% of all algae service calls. It presents as a green tint in the water progressing to full opacity, with slimy green film on walls, floors, and steps.

Green algae is free-floating — it suspends in the water column rather than rooting into surfaces. This characteristic makes it the easiest species to treat. Shock treatment with calcium hypochlorite at 10 to 15 ppm FAC kills active blooms within 24 to 48 hours. Dead algae cells turn gray-white and settle to the pool floor, requiring vacuuming — preferably to waste to prevent filter clogging.

Diagnosing green pool water starts with testing FAC and CYA to determine whether the chlorine residual fell below the effective sanitizing ratio.

Mustard Algae (Phaeophyta)

Mustard algae — also called yellow algae — appears as a yellowish-brown or tan powdery film on shaded pool walls, steps, and corners. Unlike green algae, mustard algae is chlorine-resistant at standard residual levels, requiring elevated treatment concentrations.

The defining characteristic of mustard algae is its shade preference and surface adhesion. It colonizes areas with reduced water circulation and minimal direct sunlight — behind ladders, under step overhangs, and on north-facing walls. Brushing removes the visible film temporarily, but spores embedded in the surface recolonize within 24 to 48 hours without chemical treatment.

Treatment protocol requires shocking to 20 to 25 ppm FAC — nearly double the green algae threshold — combined with aggressive brushing and a copper-based algaecide at 0.2 to 0.5 ppm elemental copper. All pool accessories that contacted the water — floats, vacuum hoses, brushes, robotic cleaners — must be sanitized or replaced, as mustard algae spores survive on equipment surfaces and reintroduce the colony after treatment.

Black Algae (Cyanobacteria)

Black algae is technically not algae — it is a cyanobacterium that forms dark blue-green or black spots on plaster, gunite, and concrete surfaces. It produces a waxy protective coating over its colony and extends root-like structures (rhizoids) into porous pool surfaces, making it the most difficult species to eradicate.

Standard chlorine levels — even at shock concentrations — cannot penetrate the protective biofilm without physical disruption. Treatment requires a stainless steel brush (never nylon, which slides over the waxy layer) to break open the colony heads, immediately followed by direct application of granular calcium hypochlorite or trichlor tablets held against each spot. The full-pool FAC level must reach 25 to 30 ppm and hold for 48 to 72 hours.

Black algae returns if any rhizoid structure survives within the plaster matrix. Recurrence within 2 to 4 weeks after treatment indicates incomplete root removal — often requiring professional acid washing or plaster resurfacing to eliminate embedded colonies permanently.

Charleston’s Extended Algae Growing Season

Charleston’s subtropical climate creates an 8 to 9 month window where water temperatures exceed the 60°F algae activation threshold — from mid-March through late November. Charleston’s humidity creates persistent algae risk because moisture in the air reduces chlorine off-gassing marginally but maintains the warm, nutrient-rich conditions algae require.

Summer water temperatures routinely exceed 88°F in July and August, accelerating algae reproduction rates while simultaneously increasing chlorine consumption. A pool that maintains 3 ppm FAC at 75°F may require 5+ ppm at 90°F to achieve the same sanitizing effect — a dynamic that creates vulnerability windows between weekly service visits.

Phosphate loading from live oak leaves, pine needles, palmetto debris, lawn fertilizer runoff, and coastal storm wash-in frequently pushes Charleston pool phosphate levels above 1,000 ppb — 8 times the recommended maximum of 125 ppb. Routine phosphate removal using lanthanum chloride compounds is standard practice on Charleston service routes. Algae prevention through weekly maintenance combines consistent chlorine management, phosphate control, and brushing to break the reproduction cycle before visible blooms develop.

Related Pool Care Concepts

Maintaining adequate chlorine residual at the proper ratio to cyanuric acid is the primary algae prevention strategy. Algaecide provides backup prevention when chlorine levels temporarily dip between service visits. Pool shock at species-specific concentrations clears active blooms through breakpoint chlorination. Phosphates supply the primary nutrient that enables algae reproduction — controlling phosphate levels reduces the growth rate even when chlorine momentarily drops.