San Diego Phytoplankton Blooms: The Hidden Visibility Killer

You’ve checked the swell forecast. It’s flat. The wind is light. There’s been no rain for two weeks. The tide is perfect. You make the drive to Point Loma, kit up, and drop in — to find a wall of brown-green soup that limits your view to about 4 feet in any direction. No storm, no swell, no runoff. What happened? Phytoplankton blooms in San Diego are the visibility killer that no wave buoy can predict, the invisible threat that even experienced divers frequently fail to account for.

What Are Phytoplankton Blooms?

Phytoplankton are single-celled photosynthetic organisms — the base of the ocean food web. Under the right combination of conditions (sunlight, nutrients, calm water), they can multiply exponentially to form blooms that cover hundreds of square miles of ocean surface. In San Diego, the most common bloom-forming species include:

Lingulodinium polyedra: The dinoflagellate responsible for San Diego’s famous red tides and night bioluminescence. Blooms appear rust-red or brown in daylight; at night, mechanical disturbance (waves, a diver’s finning) causes brilliant blue bioluminescent flashes. Spectacular to witness but devastating to visibility.

Pseudo-nitzschia: A diatom that forms toxic blooms capable of producing domoic acid, which accumulates in filter feeders and can cause amnesic shellfish poisoning. When these blooms are declared, shellfish harvesting is prohibited — they also severely reduce visibility.

Noctiluca scintillans: “Sea sparkle” — a non-photosynthetic dinoflagellate that feeds on other phytoplankton and bacteria. Produces green bioluminescence. Blooms in warmer months.

Various diatom species: Non-toxic but bloom to high densities in spring upwelling conditions, turning water green and reducing visibility.

When Do Blooms Peak in San Diego?

Phytoplankton blooms in San Diego follow a seasonal pattern driven by the upwelling cycle:

Spring (March–May): Peak bloom season. Northerly winds drive strong upwelling along the San Diego coast, bringing cold, nutrient-rich water to the surface. The sudden injection of nutrients into well-lit surface water triggers explosive growth. This is when visibility is most unpredictable — a 20-foot visibility day can turn to 5 feet within 24 hours after an upwelling event fertilises a bloom.

Summer (June–August): Blooms continue but are more intermittent as upwelling weakens. Red tide events with Lingulodinium polyedra are most common during this period when warm, calm conditions prevail.

Fall (September–October): A second, smaller bloom peak often occurs in fall. Water clarity is often at its annual best in September but can be disrupted by storm-triggered blooms.

Winter (November–February): Lowest bloom activity. Shorter days, cooler temperatures, and storm-driven mixing inhibit bloom formation. Winter often produces the clearest deep-water conditions despite increased swell.

How Blooms Affect Spearfishing at San Diego Sites

The spatial distribution of blooms in the San Diego Bight is not uniform. Understanding these patterns helps you find clear water even during bloom periods:

Offshore vs. inshore: Blooms are typically denser in the nutrient-rich upwelled water close to shore. Moving offshore even 2–3 miles from Point Loma can sometimes find significantly cleaner water where bloom concentrations are lower.

Canyon upwelling at La Jolla: The La Jolla Submarine Canyon drives persistent localised upwelling, which means La Jolla reefs sometimes have denser bloom concentrations than the outer Point Loma kelp beds even on the same day.

North-south gradients: South La Jolla sites (Bird Rock area) can be in cleaner water than northern La Jolla sites when a bloom is tracking north to south along the coast.

Detecting Blooms Before You Dive

SCCOOS Harmful Algal Bloom Bulletins: The Southern California Coastal Ocean Observing System publishes weekly HAB bulletins at sccoos.org covering San Diego and Southern California coastal waters.

Scripps Institution of Oceanography water sampling: SIO’s Harmful Algae Research Program publishes real-time cell counts for San Diego near-shore waters. High counts of bloom species are published online.

NASA Chlorophyll Imagery: Daily MODIS satellite chlorophyll maps show bloom extent and intensity. Available on NASA Worldview and CoastWatch. Green and yellow patches over the San Diego Bight indicate active bloom conditions.

Visual observation: From a cliff or elevated vantage point (Sunset Cliffs, La Jolla bluffs), a bloom-affected ocean is visibly discoloured — green, brown, rust-red, or a peculiar murky turquoise rather than the clear blue-grey of clean offshore water.

The bioluminescence tip-off: If you’re at the beach at night and the breaking waves are glowing blue, Lingulodinium polyedra is blooming. Don’t expect great daytime visibility the next morning.

Using Bloom Data in Your Dive Planning

The Element app conditions score incorporates satellite chlorophyll data into its daily assessment for San Diego dive sites. When bloom conditions are active near your target site, the score is suppressed — a useful signal before you invest time and fuel in a session. Check the conditions score alongside a quick look at the current chlorophyll image, and let both data sources inform your go/no-go decision. On high-bloom days, even a flat swell and perfect tide can’t save the dive.