September 29 update: The Mt. Whitney pool is open again, and the boys have returned to their home pool for practice, according to the latest team update shared with this site. The temporary move to El Diamante has ended, and the earlier October 15 reopening estimate no longer applies. Follow the team’s current practice schedule for start and end times.
The pool had been closed because of black algae. The background below explains what it is and why it can be difficult to remove.
What is black algae?
The name is misleading. Pool “black algae” commonly consists of cyanobacteria: microscopic organisms that use light for energy. They can form dark, firmly attached colonies on pool surfaces. A 2025 study of recreational pools identified filamentous cyanobacteria as the principal organisms in these persistent biofilms. A biofilm is a community of microorganisms enclosed in a protective coating, which helps explain why an established patch can resist routine treatment. Research source ↗
What allows it to grow?
The Pool & Hot Tub Alliance’s water-quality committee explains that organisms can enter from the surrounding environment, including dust, water, swimmers and shared equipment. Warmth and light can support growth, while rough surfaces and tiny cracks provide protected places to attach. Low-flow areas can allow growth even on smooth surfaces. PHTA explanation ↗
LaMotte’s pool-care handbook also identifies insufficient or inactive sanitizer, inadequate circulation and elevated nutrients as contributors to algae problems. These are general risk factors; the specific cause at Mt. Whitney has not been confirmed to this site. The closure alone does not establish which condition was responsible. Pool-care reference ↗
Why reopening can take time
The protective coating makes established growth difficult to remove. The PHTA committee describes treatment as a combination of physical cleaning, sanitation and proper circulation, often requiring repeated attention. A date on the calendar does not establish that the work is complete. At Mt. Whitney, the team has now confirmed that the pool is back in use.