Red Tide and Harmful Algae Blooms: Freediving Safety in San Diego Summer
San Diego’s summer waters attract freedivers year-round, but June through September brings an annual challenge: harmful algae blooms (HABs) and red tide events. These natural phenomena can transform pristine dive sites like La Jolla Cove, Sunset Cliffs, and the kelp forests off Mission Beach into hazardous zones. Understanding what causes them, how to spot them, and when to stay out of the water is essential for every local freediver.
What’s Happening in the Water?
Red tide refers to the rapid multiplication of marine microorganisms—typically dinoflagellates like Lingulodinium polyedrum—that discolor the water red, brown, or green. Harmful algae blooms are similar events caused by toxic species that produce domoic acid, saxitoxin, or other neurotoxins. Both occur when water temperature, nutrient availability, and sunlight align perfectly, usually during our warmest months.
San Diego’s upwelling patterns—where cold, nutrient-rich water rises from the deep—can trigger blooms along our coast. The 2022 and 2023 seasons saw significant toxic blooms that closed beaches and warned divers away from popular spots for weeks at a time.
Why Freedivers Are at Risk
Unlike surfers or swimmers who spend minutes in the water, freedivers spend extended periods underwater and at depth, where toxin concentrations can be higher. Inhaling aerosolized toxins during breathing exercises on the surface is a real hazard. Consuming fish or shellfish contaminated by domoic acid can cause Amnestic Shellfish Poisoning (ASP)—symptoms include nausea, vomiting, memory loss, and in severe cases, neurological damage.
The risk isn’t just from direct ingestion. Freedivers who hunt or collect seafood during a bloom event face significant exposure. Even if the water looks clear at your favorite break—Windansea, Torrey Pines Shores, or Bird Rock—toxins may still be present at dangerous levels.
Spotting the Signs
Not all red tides are toxic, and not all toxic blooms are visible. However, watch for these red flags:
- Discolored water: Red, brown, orange, or murky green water near shore or at your dive site
- Dead fish or marine life: Unusual die-offs suggest toxin presence
- Foam or scum: Concentrated algae at the waterline
- Strong odor: A pungent, unpleasant smell coming from the water
Even clear-looking water can harbor toxins. This is why checking official sources is non-negotiable.
Check Before You Go
San Diego County Department of Environmental Health posts weekly HAB advisories and beach closure notices. The California Department of Public Health maintains a statewide HAB hotline and website with real-time data. Before heading to Coronado Cove, South Mission Beach, or any local site, spend two minutes checking these resources.
The Harmful Algae Bloom Hotline (1-844-HAB-NOAA) provides current conditions. The National Oceanic and Atmospheric Administration (NOAA) also offers satellite imagery tracking blooms along the California coast.
Download these resources and check them as part of your pre-dive routine, just like you check swell forecasts and wind conditions.
When to Stay Out
If there’s an active advisory for your intended dive location, don’t go. No single dive is worth the neurological risk. If the water looks obviously discolored or you smell something wrong, trust your instincts and walk away.
During peak bloom season (July–September), consider shifting your diving schedule to early morning or evening when conditions are often clearer and toxin concentrations lower. Alternatively, dive less accessible reefs and breaks that experience less human pressure—spots like the kelp forests off Point Loma or deeper offshore reefs may have better water clarity than heavily trafficked coves.
Practical Steps for Summer Freediving
- Check advisories weekly during June through September
- Avoid shellfish harvesting during any HAB advisory, regardless of visibility
- Bring a buddy who knows the signs of toxin exposure
- Rinse thoroughly after every dive—salt and algae residue can irritate skin and respiratory passages
- Report unusual conditions to San Diego County Environmental Health
- Stay informed about seasonal patterns; early summer 2023 saw blooms off Pacific Beach and Ocean Beach that lasted weeks
The Bigger Picture
Climate change is extending bloom seasons and increasing their intensity. San Diego’s coastal waters are warming, and upwelling patterns are shifting. What used to be a July–August concern now sometimes starts in June and lingers into October. Staying vigilant isn’t paranoia—it’s responsible diving.
Red tide and harmful algae blooms are part of San Diego’s summer diving reality. By checking conditions before every session and respecting beach closures, you’ll keep yourself safe while enjoying the best diving season our region has to offer. Use Element to track water conditions, wind, and swell—and always cross-reference with official HAB advisories before you enter the water.