74°F and Climbing: What a Record-Warm Week Means for San Diego Diving and the Kelp Rot

We've been talking about 72°F water all week like it was the ceiling. It isn't. The way this is trending, we could see the surface touch 74°F somewhere along the San Diego coast in the next few days. For our kelp and our diving, that's not a rounding error — every degree above the low 60s piles more stress onto a system that's already straining. Here's what I expect to see locally, and how to keep diving through it.

74°F is a big deal for a cold-water coast

It's easy to hear "74" and think it sounds pleasant. For a swim, sure. For a Southern California kelp forest, it's the wrong kind of warm. Our best diving runs on cold, nutrient-rich water — usually high 50s to mid-60s at the surface. Giant kelp is a cold-water plant, and its whole engine depends on nutrients that warm water simply doesn't carry.

At 74°, that surface layer is nutrient-starved. The kelp can't feed, growth stalls, and the plants start spending down their reserves. Warm water also holds less dissolved oxygen and tends to sit still — it stratifies, capping cooler water below with a warm lid. Everything about that combination favors decay and blooms over healthy, growing kelp.

The kelp rot accelerates

Kelp rot creating the "sea snot" effect. The "sea snot" also allows less sunlight into the water creating a darker diving experience.

This is the one you flagged earlier, and 74° makes it worse, not better. Here's the loop I expect to intensify this week: heat starves and bleaches the kelp — older blades lose pigment, go pale and whitish, then break down. That degrading tissue dumps nutrients and organic matter into the water. Those nutrients feed plankton blooms and encrusting foulers like the lace bryozoan (Membranipora), which pile onto the already-weak blades and weigh them down. Brittle, fouled, rotting blades then shed and break loose. More decaying material in the water, more food for the bloom — round and round it goes.

What that looks like in the water:

• A pale, tattered canopy with more white crust and bleached blades, especially in the top few feet where the heat is worst.

• Loose kelp everywhere — blades and chunks breaking off in your hands and drifting through the column.

• A funky, organic smell and a lot of particulate near the surface as tissue breaks down.

• Thinning canopy over the next couple of weeks if the heat holds — the same canopy-loss pattern we saw in the 2014–2016 "Blob" and the 2018 warm spells.

The kelp rot isn't a side effect here — at 74° it's close to the main event.

Blooms and surface debris get worse

Warm, fertilized, sunlit, and calm is a recipe for a plankton bloom, and the decaying kelp is throwing fuel on it. Expect the brown-green tint, the white marine-snow wisps, and the "sea snot" mucilage to hang around or intensify. There may be a clamworm spawn mixed in too — stringy biological slime that won't show up on any chlorophyll or turbidity chart, so the water can read "clean" on paper and still haze you out.

Cloudy and darker conditions due to blooms and "sea snot"

The one upside: if it's the dinoflagellate red tide, the same bloom that slimes you by day can glow electric blue at night. Worth a sunset lap if the bioluminescence fires up.

What the fish do

Warm water reshuffles the deck. On the plus side, warm-water and pelagic species push in closer — this is the kind of window where yellowtail, dorado even the occasional billfish or wahoo show up nearshore, and the offshore bluefin stay out in the cooler water by Cortez Bank. On the minus side, the resident kelp-forest fish get stressed and often drop deeper, chasing the cooler, oxygen-richer water below the warm lid. If your target lives in the kelp, look for it under the thermocline, not up in the warm soup.

How it impacts your actual dive

A few practical things to plan around this week:

• Murky, warm surface, cleaner cold water below. Blooms and rot are mostly a top-layer problem. Expect to punch through the soup up high into better visibility at depth — divers have been finding 15 to 25 feet down low while the surface is coated. If your spot keeps you shallow, you'll be in the muck.

• A real thermocline. With a 74° lid, the drop into cold water at depth can be sharp. Dress for the bottom, not the surface — you'll want the exposure protection when you drop through it. I've had this happen too many times, where I put on a 3-2 wetsuit only to get in the water and realize it's 60° just 5 ft below the surface.

• Entanglement and navigation. All that loose, drifting kelp is a snag hazard. Keep your knife accessible, watch your trail line, and don't push into rotting canopy you can't see through.

• Timing and spot choice. Mornings before the wind and boat traffic stir the surface tend to fish cleaner. Deeper reefs, more exposed points, and anywhere upwelling still touches will beat the warm, stagnant coves.

• Watch for a bounce-back. The fix for all of this is cold water. The moment upwelling kicks back in or a swell mixes the column, the surface temp drops, the bloom fades, and the canopy starts to recover. Keep an eye on the buoys — a 3 to 4 degree drop changes everything.

The honest read

This is a marine-heatwave week, and 74° would be near the top of what we see on this coast. Diving doesn't shut down, but it changes: shallow and warm means soup and rotting kelp, deep and cold means your window. Pick spots and depths that let you get under the warm lid, treat the loose kelp with respect, and know that this is a seasonal stress event — not the new normal. It clears when the water turns over.

I'll keep logging what we actually see against the forecast this week, because a run this warm is exactly the kind of thing worth having a dated record of.

Trying to figure out which day and which spot will dive clean through this? That's exactly what I built the SpearFactor Trip Optimizer for — it weighs water temp, swell, and bloom conditions so you're not gambling on the warm days. And if you're going to be dropping through sharp thermoclines into cold water this week, brush up first with the free freediving safety course at freedivingsafety.com and the full SpearFactor Master Class at SpearFactor.com.