Sea Snot Season: What That Slimy Stuff at La Jolla Actually Is

If you've slipped into the water at La Jolla lately and come up coated in goo — brown-green water, white wisps drifting like smoke, squishy multicolored clumps riding the surface — you're not imagining it, and you're not alone. Divers up and down the coast are asking the same question: what is this stuff?

Short answer: it's a plankton bloom — a red tide — and the slime is the bloom's own mucus mixed with the debris of dying plankton.

Here's the longer version, and why it's happening right now.

A red-tide bloom — reddish-brown dinoflagellates and their scum collecting along the shoreline.

What you're actually swimming through

That brown-green band stretching from outside the cove all the way to shore isn't floating junk — it's a water mass. It's a dense bloom of microscopic phytoplankton, most likely a dinoflagellate. In our waters the usual summer culprit is Lingulodinium polyedra.

When these blooms get thick, the algae ooze sticky sugar compounds (polysaccharides), and that's the source of everything gross you're seeing:

Marine snow — the white wisps like smoke and the crud coating everything. These are tiny clumps of dead and dying plankton, mucus, and detritus drifting down through the water column. In a heavy bloom, it's a blizzard down there.

Sea snot (marine mucilage) — the squishy multicolored clumps on the surface. Those same exudates aggregate into gelatinous globs, then get herded into slicks and bands by wind and current.

So the discolored band and the slimed feeling? That's the bloom itself.

Could it just be kelp rotting in the warm water?

Great instinct — and you're partly right, but it's mostly the other way around.

Kelp decay looks like brown tatters and chunks of actual kelp tissue: recognizable blade fragments, coarse, concentrated around the kelp beds. A plankton bloom, by contrast, discolors the water itself — microscopic algae coating everything, spiking chlorophyll, and (as we'll get to) glowing at night.

But the heat hunch is real, and the two are linked. This marine heatwave — surface temps running 70 to 72°F — is genuinely stressing the giant kelp. Warm water slows its growth and makes older blades deteriorate and slough off. That degrading kelp then dumps nutrients and organic matter into the water, which feeds the bloom. It's a loop: heat degrades the kelp, the decaying kelp fertilizes the plankton, and the plankton bloom is what you actually see and feel. The kelp rot is a supporting actor, not the star.

Why it's happening now

It's a perfect storm for a bloom. The marine heatwave has the surface sitting at 70 to 72°F, upwelling is still feeding nutrients up from below, and long summer days deliver plenty of light. Warm, fed, and sunny — the plankton go nuts. Then, as the bloom ages and gets stressed in that warm water, it goops up.

If this sounds familiar, it should: early-summer red tides have hit this stretch of coast three years running now. This is a seasonal pattern, not a freak event.

Is it harmful?

Mostly it's just gross, not dangerous. It can irritate skin and eyes for some people and it's unpleasant to swim through, but L. polyedra isn't a toxin-producer that shuts down beaches. The agencies keep an eye out for the genuinely toxic blooms; this is the gross-but-harmless variety.

The bonus round

Here's the payoff: if it is that dinoflagellate, the same bloom that slimed everyone by day will glow electric blue at night when the water is disturbed. Bioluminescent waves. Same organism, completely different vibe after dark — worth a sunset trip back to the beach.

What it means for divers

Viz-wise, the good news is that a bloom is largely a surface and near-surface layer. You'll often punch through the murk up top into cleaner water at depth — divers have been reporting 15 to 25 ft down low even while everything at the surface gets coated. If your usual spot keeps you shallow, expect to be in the soup; if you can get down, you may find a different day waiting below it.

Plan around it, keep an eye out for the nighttime light show, and know that this one clears out on its own once the heat backs off and the bloom runs its course.

Photo: Marine harmful algal bloom (red tide), NPS / NPGallery, public domain.