Reasons for explosion of green slime along coast unclear

By Dave Schwab

Staff Writer

Green algae colors the water at WindanSea over the weekend. If you’re in the ocean and you run into green slime don’t panic - it’s just Tetraselmis, a harmless microscopic green algae.

“It’s a type of phytoplankton, the smallest of the plankton,” said Melissa Carter, a research associate at Scripps Institution of Oceanography, about the not-uncommon but little understood and recurring coastal phenomenon.

Tetraselmis, a green flagellate which is only 10 micrometers in size, has been found in concentrations as dense as 15 million cells per liter of seawater.

“This population of tetraselmis is exploding in some way,” said Carter. “Maybe they’re all reproducing, or there’s some sort of condition that’s bringing them onshore, probably a little bit of both. We don’t really know what’s causing this to happen. We’d like to know.”

The green foam, which invaded Orange and Los Angeles County beaches in late July, has gravitated down to La Jolla and other parts of San Diego County.

The foam has persisted this week, though it’s patchy distribution makes it visible only at some beaches and the foam becomes more apparent in the afternoon when the wind and waves mix the surface waters.

“There are no documented health hazards with swimming or fishing in areas of Tetraselmis blooms,” said Carter. “There are no documented cases of people getting sick from eating shellfish or fish that have been in touch with tetraselmis bloom, which is a good food source used in aquaculture.”

Carter added the seasonal algal bloom is “just a natural thing” that happens when a single species dominates for a brief time, like the more highly visible red algal tides.

There is one thing researchers do know about the tetraselmis bloom: It happened last year along the Southern California coast and in La Jolla at exactly the same time.

“Is there anything significant to that?” asked Carter. “I don’t know. I guess we’ll have to wait and see next year.”

For more information about the algae go to, click on “Data, Products and Modeling,” and then click on “Harmful Algae & Red Tides.”