If you are not near a river or stream transporting material from land to sea, and if you are not in very shallow water where the bottom is visible or bottom sediments are stirred up, you can bet that the color of the ocean water is determined by the microscopic plants -- phytoplankton -- living in it. The color that you perceive depends on the color of the phytoplankton and on their abundance. Ocean waters containing low concentrations of phytoplankton appear blue, whereas ocean waters containing high concentrations of phytoplankton may have a wide range of colors - often green, brown, golden brown, or reddish.

The colors you have noticed, red-brown and yellow (here's a photo from Southern California from the Surfrider Foundation site), usually indicate abundant populations of a particular group of phytoplankton called dinoflagellates. Dinoflagellates are particularly effective at coloring the ocean surface because they tend to aggregate near the surface during the day, in order to get the light they need for photosynthesis (they can swim!). Ocean circulation can also infuence how densely packed the dinoflagellates are, and thus the color of the water. For example, in a place where ocean waters flow together at the surface (a convergence), the waters flow down. Many plankton flow down in the convergence. However, dinoflagellates can swim up (toward the sunlight), and this causes them to aggregate as the population is swept into the convergence zone by the ocean circulation.

What's interesting is that our perception of red or red-brown does not depend only upon the color and concentration of the phytoplankton, it also depends on the physiology of our eyes. Because of the overlapping responses of the green and red cones in our eyes, we see red-brown when the actual color measured by an instrument is yellow.

Dense blooms of dinoflagellates that change the color of the ocean surface are often called "red tides", though their occurrence is not caused by the tides. Monitoring programs indicate that in recent years along the central California coast, the phytoplankton community shifted toward more dinoflagellates. So, sightings of strongly colored surface waters, "red tides", has been more common.