Looking out at the ocean today from Bodega Head, there were bright, almost glowing, orange bands everywhere you looked.
Here's a view to the north along the Sonoma Coast (below). Some of the patches were quite concentrated, while others were more diffuse.
The color was so intense that it was sometimes hard to believe. The next two photos show waves approaching Salmon Creek Beach.
And here's a close-up of the color in a wave approaching the outer coast of Bodega Head. (Although I used a polarizer, I didn't enhance the color in any way.)
These are accumulations of Noctiluca, a dinoflagellate. Dinoflagellates are protists, single-celled animal-like organisms that are often motile. Noctiluca is heterotrophic — they can't make their own food like algae or plants can, but instead eat other organisms (such as diatoms, other dinoflagellates, and bacteria).
You might see these events referred to as "red tides," although in this case I'm not sure it's the most accurate term. It's not really red, and it's not really related to tides.
It's important to remember that not all of these events are toxic. I've read that Noctiluca does not produce toxins...although at times the dense concentrations might produce anoxic conditions or high levels of ammonia.
Some species of Noctiluca are famous for their bioluminescence at night. Noctiluca means "night light" and is commonly called "sea sparkle." I'd be very interested in hearing about any signs of bioluminescence at local beaches! [ADDENDUM (4 October 2012): I've since learned that Noctiluca might not be bioluminescent in the Northeast Pacific Ocean.]
While taking these photographs, I noticed a few Gray Whales among the patches. I wondered if the whales would avoid the Noctiluca, or if they would be attracted to it or other animals feeding on or associated with it? Sometimes they surfaced right in the middle of the patches.
Noctiluca is microscopic, but may reach 1-1.5 mm in length. If you'd like to see what the actual organism looks like, there's a nice illustration here (top three drawings).