How to Identify Toxic Algae Blooms Affecting San Diego Freediving Safety
Harmful algal blooms (HABs) are a real threat to San Diego freedivers, and knowing how to identify them before you hit the water can be the difference between a safe dive and a serious health incident. While San Diego’s year-round diving conditions are world-class, toxic algae blooms can appear unexpectedly, particularly during warmer months. Here’s what you need to know to stay safe.
What You’re Looking For: Visual Signs of Toxic Blooms
The most obvious indicator of a harmful algal bloom is discoloration in the water. In San Diego, this typically appears as:
Red, brown, or green water — The most common type is a reddish or brownish tint, often called a “red tide.” Some blooms appear murky green or even yellowish. If the water looks unusually colored compared to normal conditions, it’s worth investigating further before committing to a dive.
Foam or surface slicks — Toxic blooms often create thick, foamy patches on the surface or at the waterline. These accumulations can smell distinctly foul—like a chemical or musty odor that’s clearly abnormal. Trust your nose; if it smells wrong, it probably is.
Visible particles or cloudiness — Rather than the typical clarity you expect at Sunset Cliffs or Bird Rock, the water may appear murky with a visible haze, even in shallow areas. Bloom-affected water loses transparency rapidly.
Dead fish or marine life — While not always present, finding dead fish, sea urchins, or other marine animals near your intended dive site is a red flag. HABs can cause mass die-offs in localized areas.
Seasonal Patterns in San Diego
Toxic algal blooms in Southern California peak during late summer and early fall (August through October), though they can occur year-round. Warm water, nutrient runoff from coastal development, and calm conditions all contribute to bloom formation.
La Jolla Shores, Pacific Beach, and Mission Beach have experienced notable blooms in recent years, particularly in September and October. Ocean Beach and Sunset Cliffs are less frequently affected due to stronger water circulation, but that doesn’t mean they’re immune. Check local conditions before every dive.
Spring blooms (May–June) are less common but do happen. Winter diving at spots like Point Loma and Torrey Pines is generally safer, though occasional winter blooms have been documented.
Types of Toxic Blooms in San Diego Waters
Karenia brevis (Florida red tide) occasionally drifts into San Diego waters from the south, causing respiratory irritation and neurological symptoms in exposed freedivers.
Pseudo-nitzschia produces domoic acid, which accumulates in shellfish and can cause amnesic shellfish poisoning (ASP). While you won’t consume shellfish while freediving, this bloom type still indicates broader water quality issues.
Alexandrium species cause paralytic shellfish poisoning (PSP) and are less common in San Diego but have occurred during unusual warm-water years.
How to Check Before You Dive
Use Element — The Element app provides real-time water quality alerts and bloom reports for San Diego coastal areas, helping you plan safe dive sessions.
Check NOAA HAB Hotline — Call 1-800-HAB-NOAA or visit the National Oceanic and Atmospheric Administration website for official bloom advisories affecting Southern California.
Contact local dive shops — Operations like those in Ocean Beach and La Jolla maintain current water condition reports and will know if recent blooms have been reported.
Monitor local news — San Diego County Health and Human Services publishes beach advisories when blooms are detected. Check these before heading out.
What to Do If You Encounter a Bloom
If you arrive at your planned site and notice discolored water, foam, or unusual odors, don’t dive. No single dive is worth respiratory irritation, neurological effects, or gastrointestinal illness that can linger for weeks.
If you’re already in the water and notice symptoms—burning in your nose or throat, difficulty breathing, tingling lips, or nausea—exit the water immediately and rinse thoroughly with fresh water. Seek medical attention if symptoms persist.
Practical Precautions
Even during non-bloom season, maintain awareness of water conditions. Avoid diving in areas with visible runoff, murky water, or recent rain (which can trigger algae growth). Wear a wetsuit to minimize skin exposure. Rinse your entire body with fresh water after every dive, especially your face and sinuses.
San Diego’s freediving spots—from the kelp forests of Point Loma to the reefs off Bird Rock—are incredible year-round. Staying informed about algal bloom conditions keeps you diving safely and consistently.
Check Element before your next dive to get real-time water quality data and bloom alerts specific to your San Diego dive site.