Voyager: Why do we have red tides?

This is a surprisingly difficult question to answer. A “red tide” is an accumulation of plankton so concentrated that it colors the water. Many different types of plankton can create red tides, and they can have little in common with each other. Some of these organisms are relatively big, some are tiny. Some are photosynthetic, some are not. Some swim, some can’t. Some produce toxins, some don’t.

To form a red tide, plankton either has to grow to very high concentrations or somehow accumulate at a location – by swimming, for example. There are two ways of growing to high concentrations: grow faster or die slower.

Some red tide organisms can reduce their death rate by exuding toxins that repel predators or by forming groups so concentrated that predators avoid them. They may also use chemicals to inhibit the growth of other plankton, leaving more resources for themselves.

Plenty of nutrients are required to support the growth and high concentration of red tide organisms. Some red tide areas contain more nutrients than the waters the plankton originally grew in, which implies that they had to get their nutrients from somewhere else. This usually involves the organisms swimming into deeper, nutrient-rich water to take up nourishment and then swimming back up toward the surface. For these tiny organisms, this trip would be like us walking from San Diego to Los Angeles each night for dinner and then back again in the morning.

Many red tide organisms can swim. If they all swim to the same depth (such as the surface of the ocean), the concentration of organisms will increase to the point that they color the water. This is why red tides are often most apparent at midday – the organisms all swim up to the surface to photosynthesize, or get energy from the sun. Many red tides begin forming in thin, dense layers below the ocean’s surface where they are invisible to humans. Suddenly one day the plankton can appear at the surface even though most of their growth probably happened during the previous weeks.

-- Peter Franks, professor of biological oceanography, Integrative Oceanography Division