The Ultimate Guide to Pool Algae: Identification, Biology, and Behavior

Green Algae (Chlorophyta)

The “Pea Soup” Bloom

This is the most ubiquitous form of biofouling, commonly caused by the species Chlorella vulgaris. If your pool looks like a bowl of vegetable soup, this is likely the culprit.

• Visual Identification: It typically manifests as a systemic infection of the water column. The water turns cloudy (turbid) first, then shifts to a deep, opaque green. While it can settle on walls, it lacks strong adhesion and brushes off easily.
• The Science (For the Pro): Green algae are “photosynthetic obligates,” meaning their survival is inextricably linked to direct light availability. They possess Chlorophyll a and b, which absorb light in the blue and red spectra, reflecting the characteristic green color.
• Behavior: Green algae is an opportunist, not a fighter. It generally lacks complex enzymatic defense systems. Its presence is almost always a symptom of operational failure—such as a lack of sanitizer or poor circulation—rather than the organism’s intrinsic toughness.

Yellow or “Mustard” Algae (Xanthophyta)

The Cryptic Survivor

Frequently misidentified as “sand,” “dirt,” or “pollen,” Mustard Algae is a distinct and evolutionarily divergent lineage. It is not a green algae; it belongs to a group known as Stramenopiles.

• Visual Identification: It appears as a fine, yellowish-brown dust. A key diagnostic trick is the “clouding effect”: when brushed, the algae disperses instantly into a yellowish cloud like smoke, only to settle back down hours later.
• The Science (For the Pro): Unlike green algae, Mustard Algae is photophobic (shade-loving). It hides on south-facing walls, behind ladders, and inside light niches.
• Why It Is Hard to Kill: This organism possesses a biochemical superpower. It produces high levels of antioxidant enzymes, specifically catalase and superoxide dismutase. These enzymes allow the algae to “detoxify” chlorine, effectively neutralizing the sanitizer before it can cause lethal damage to the cell.
• Re-Infection Risk: Its spores are resilient to desiccation (drying out). This means it can survive on dried swimsuits, pool toys, and vacuum hoses, leading to constant re-infection if equipment is not treated.

Black Algae (Cyanobacteria)

The Biofilm Fortress

The term “Black Algae” is a misnomer. This organism is not an algae at all, but a prokaryotic bacteria called Cyanobacteria. It is one of the toughest contaminants to remove from a swimming pool.

• Visual Identification: It manifests as dark black or blue-green spots, typically the size of a dime. It is almost exclusively found in gunite, plaster, or concrete pools because it requires a porous surface to anchor itself.
• The Science (For the Pro): Cyanobacteria do not just sit on the surface; they extend filaments (trichomes) deep into the microscopic voids of the plaster matrix. Aggressive brushing often only removes the visible “cap” of the colony, leaving the “roots” alive within the wall.
• Why Shocking Fails: The colony secretes a multi-layered slime known as the Extracellular Polymeric Substance (EPS). This biofilm acts as a hydrophobic barrier, repelling water and dissolved sanitizers. Standard chlorine reacts with the outer slime layer but cannot penetrate deep enough to kill the vegetative cells inside.

The “Vinyl Plague”: Stains Behind the Liner

The Teabag Effect

Owners of vinyl pools may encounter erratic, indelible stains known in the industry as “Vinyl Plague”. These appear as neon pink, charcoal, or dark grey splotches that look like they are inside the liner rather than on top of it.

• The Culprits:
  • Pink Stain: Caused by Streptoverticillium reticulum, a bacterium that produces a reddish pigment called prodigiosin.
  • Black/Grey Stain: Caused by Aureobasidium pullulans, a “black yeast” fungus that produces melanin.
• The Mechanism (The Teabag Effect): Think of your pool liner as a teabag. The “tea leaves” (the bacteria/fungus) are living in the soil behind the liner. As they feed, they release “tea” (dye/pigment). This dye is lipophilic (fat-loving), meaning it dissolves easily into the plasticizers of the vinyl and migrates all the way through to the pool side. Scrubbing the stain is like scrubbing the outside of a teabag—it doesn’t stop the tea from coming through.
• Why It Happens: Modern vinyl liners no longer use heavy metals like lead or cadmium (which used to kill these fungi) due to environmental regulations. Additionally, the plasticizers that make the liner soft are essentially a food source for these soil microorganisms.
• The Chlorine Diagnosis: How do you know if it is Vinyl Plague or just algae? Place a trichlor tablet in a white sock and hold it against the stain for 5 minutes.
  • If the stain bleaches away but returns in the same spot within 24–48 hours, it is likely Vinyl Plague. You bleached the surface dye, but the colony behind the wall just pumped more dye through.
  • If the stain does not change, try a Vitamin C tablet. If it vanishes instantly with Vitamin C, it is a metal stain (copper/iron), not biological.

The “Imposters”: Bacterial Slime and Water Mold

Sometimes, the “algae” in your pool is neither plant nor cyanobacteria. It is often a bacterial or fungal-like infestation.

Pink Slime (Methylobacterium & Serratia marcescens)

• What It Is:
  • Methylobacterium – A bacterium often found in soil and bathrooms. It is not photosynthetic (does not need light).
  • Serratia marcescens – Also a bacterium common in bathrooms and hospitals. Typically a darker redish color
• Behavior:
  • They have a high affinity for PVC plastics. You will typically find this heavy, pink/red gelatinous slime inside skimmer baskets, return jets, and cleaner hoses. Like Black Algae, it protects itself with a heavy biofilm.

White Water Mold (Oomycetes)

• What It Is: Often described as looking like “shredded tissue paper” floating in the water. Biologically, this is a water mold (Saprolegnia or Pythium).
• Behavior: It is a heterotroph, meaning it feeds on decaying organic matter. It thrives in dark areas with trapped debris, making your pool filter the perfect incubator. By the time you see the “tissue paper” in the pool, the infestation inside the filtration system is usually severe.

Summary: Know Your Enemy

Successful pool care requires accurate diagnosis. As the biology demonstrates, a treatment protocol that works on the planktonic Green Algae will fail catastrophically against the enzyme-rich Mustard Algae or the sub-surface Vinyl Plague.

• Green: Plant-based, floats, easy to kill.
• Yellow: Shade-loving, enzymatic resistance, hides in equipment.
• Black: Bacterial, rooted in plaster, protected by slime.
• Vinyl Plague: Soil microorganisms behind the liner, acting like a “teabag” to stain the vinyl.
• Pink: Bacterial, likes PVC, dislikes sunlight
• White Tissue Paper: water mold, feeds on decaying matter, grows in filter

By understanding the unique behaviors of these organisms, you can move beyond guessing and start solving the problem at its source.