Upwelling Strength and Cold-Water Plankton Blooms: Predicting Blue-Green Algae in San Diego

San Diego’s coast is shaped by one of the ocean’s most powerful mechanisms: seasonal upwelling. Every spring and summer, northwesterly winds push warm surface water away from shore, drawing nutrient-rich cold water from the deep. This process fuels plankton blooms—and sometimes dangerous algal blooms—that directly affect your time in the water.

Understanding upwelling strength and its connection to blue-green algae (cyanobacteria) lets you make smarter decisions about when and where to paddle, dive, or swim.

How Upwelling Works Off San Diego

Upwelling intensifies from March through September, peaking in May and June. The Ekman effect—the deflection of moving water by Earth’s rotation—pushes surface water seaward while deeper water rises to fill the void. Off San Diego, this creates a dramatic temperature gradient: surface water can drop 10–15°F in weeks.

The strongest upwelling typically occurs along the north-facing coast near Point Loma and the La Jolla Kelp Forest, where wind stress and bathymetry align perfectly. Weaker upwelling occurs in Mission Bay and False Point, where geography provides more shelter.

Nutrients Drive Plankton, Not All Blooms Are Dangerous

Upwelling water is packed with nitrogen, phosphorus, and silica—exactly what phytoplankton crave. A surge of diatoms and other microalgae follows within days, creating the foundation of the coastal food web. Fish, kelp, and marine mammals depend on this productivity.

But not all plankton blooms are equal. Most are harmless. The problem arises when conditions favor blue-green algae (cyanobacteria)—a prokaryote that thrives in warmer, nutrient-rich, low-turbulence water. These blooms can produce toxins like microcystin and anatoxin-a, which cause rashes, respiratory irritation, and gastrointestinal illness in humans.

The Paradox: Strong Upwelling Can Suppress Harmful Blooms

Here’s where it gets counterintuitive. Intense upwelling—the kind that brings bone-cold water to places like Windansea Beach and Sunset Cliffs—actually reduces the risk of blue-green algae blooms. Why? Cyanobacteria prefer warmer water (above 65°F) and calm conditions. Vigorous upwelling keeps temperatures low and water mixing high, creating hostile conditions for these blooms.

Blue-green algae blooms are most likely during:

• Weak upwelling periods: April and early May, before upwelling fully kicks in
• Upwelling relaxation events: Brief wind reversals that allow water to warm and stratify
• Enclosed bays: Mission Bay, San Diego Bay, and Agua Hedionda Lagoon, where poor flushing allows warm water to linger

In these scenarios, water temperature climbs to 62–68°F, stratification increases, and nutrient-rich but stagnant conditions develop. Cyanobacteria flourish.

Predicting Blooms: Watch Wind and Water Temperature

The strongest predictor of blue-green algae risk is upwelling relaxation—a sudden shift from strong northwesterly winds to calms or southerly winds. When this happens, surface water warms rapidly, and stratification locks in. Blooms can develop within 5–10 days.

San Diego experiences predictable relaxation events in April, late May, and September. Monitoring local wind forecasts is your first line of defense. If you see a 5–7 day stretch of light winds or southerly flow, increase your vigilance for discolored water.

Water temperature is your second indicator. If you’re checking conditions at Torrey Pines State Natural Reserve or Mission Beach and the ocean feels unusually warm (above 65°F) after a calm period, that’s a red flag. Cold upwelled water (below 58°F) is a green light.

Visual and Olfactory Signs

Blue-green algae blooms often appear as a thick, greenish or brownish-green film or murky water. Some smell like cut grass or rotting plants—a telltale sign of cyanobacteria. If you see either, stay out. The bloom may be toxigenic or not, but the risk isn’t worth it.

Diatom blooms (harmless) appear as golden-brown or tan water and have no strong odor.

Where to Check Conditions

Satellite imagery and wind data reveal upwelling strength in real time. During strong upwelling, you’ll see a sharp temperature gradient on sea surface temperature (SST) maps—cold water hugging the coast from Point Loma to Del Mar.

During weak upwelling or relaxation, water warms evenly across the shelf, and stratification sets in. This is when cyanobacteria risk climbs.

Local beach water quality advisories from San Diego County Public Health are also essential; they monitor for harmful algae and close beaches when necessary.

The Bottom Line

San Diego’s upwelling is a blessing for ocean productivity and a safeguard against most harmful algal blooms. But weak upwelling periods and upwelling relaxation events—particularly in April, late May, and September—create windows of vulnerability. Pay attention to wind patterns, water temperature, and visual cues.

Use Element to track real-time upwelling strength, water temperature, and wind conditions before you head out. Plan your session around the ocean’s natural rhythms, and you’ll stay safe while enjoying San Diego’s incredible coastal playground.