Plankton Station

Plankton is the term for all the plants and animals that float and drift in the sea. Most are microscopic in size, and they can be incredibly abundant. Planktonic organisms include zooplankton (tiny floating animals) and phytoplankton: tiny, single-celled creatures that are the most important life forms in the sea because they create most of the food and oxygen used by other ocean life!

We catch plankton using a plankton net—a very fine mesh net that is towed behind the boat (see pictures below). We then transfer what we catch into small dishes that can be viewed under a microscope. If the ocean is particularly productive, we may catch millions of specimens. Here are some of the types that you are likely to observe on the boat trip.

Phytoplankton

Phytoplankton are tiny, single-celled plant-like creatures that, through photosynthesis, create most of the food and oxygen used by other ocean life. Phytoplankton are the most important primary producers in the ocean, and when they are growing and reproducing in great abundance, we say that the ocean has high primary productivity. Phytoplankton need sunlight, and therefore live only in the sunlight waters of the euphotic zone. The two most common forms of phytoplankton in our area are diatoms and dinoflagellates.

o Diatoms are the most important type of phytoplankton, and responsible for most of the primary productivity in our coastal ocean. They have a siliceous “test” (small internal shell) and prefer cooler water with abundant nutrients brought up by upwelling. When present in large numbers, they will give you a green Secchi disk and usually low to moderate water clarity.

o Dinoflagellates are usually present in smaller numbers than diatoms. When present in large numbers, they will give you a brown Secchi disk and usually very low water clarity. Some species of dinoflagellates produce toxins, and when these species reproduce in large numbers, they cause a harmful algal bloom (HAB), commonly—though inaccurately—called a “red tide.”

Zooplankton

Zooplankton are animals, usually tiny, that eat phytoplankton or other zooplankton. We classify them into two broad categories: temporary zooplankton (also called meroplankton) and permanent zooplankton (also called holoplankton).

o Temporary zooplankton are creatures that spend the larval stage of their life cycle floating and drifting. These include fish eggs, and the larvae of many benthic (bottom-dwelling) animals including clams, snails, crabs, lobsters, sea stars, barnacles, sea urchins, etc.

o Permanent zooplankton are animals that spend their entire lives floating and drifting. There are many types, but one of the most abundant and important locally are copepods. Copepods are microscopic crustaceans (shrimp-like animals) that eat phytoplankton. Copepods are sometimes called the “cows of the sea” because they are herbivores, grazing on plant-like diatoms like cows grazing on grass. Because copepods depend entirely on phytoplankton for food, their abundance tracks the abundance of phytoplankton. As phytoplankton numbers increase, copepod numbers increase in response, leading to over-population of copepods which then consume most of the phytoplankton. As the phytoplankton numbers crash, the copepods are left with little food, so their number crash too, producing a “boom-and-bust” cycle of phytoplankton – zooplankton abundance.

Phytoplankton and zooplankton form the base of marine food webs; all other ocean life depends on them!

(From: Steven M. Stanley, Earth System History)

Deploying the plankton net.

The plankton net in tow.

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Modified from pages developed by John Turbeville.