Plankton

Plankton is a living community of aquatic organisms that exist either suspended in the water column or floating close to the surface, with little or no power of locomotion. These organisms are at the mercy of the currents, and drift wherever the water flow takes them. (The word plankton comes from the Greek word planktos, which means wandering.) Although most planktonic organisms are very small, some are large enough to be visible to the naked eye.

Phytoplankton are planktonic plants and include microscopic algae such as diatoms, dinoflagellates, and blue-green algae. Diatoms are brown unicellular algae, which have a box-like, siliceous form — almost like a box with a lid when seen under the microscope. Dinoflagellates are photosynthetic, unicellular algae and are thus considered as phytoplankton by many scientists, although some scientists classify them as animals. Dinoflagellates sometimes produce toxins which may cause mass mortality of fish and invertebrates when the dinoflagellate population becomes extremely numerous. These occurrences are known as "red tides." You may have glimpsed the presence of dinoflagellates at night when you notice bioluminescence, a greenish glow in the water that lights up the wake of a boat, for example, when certain species of dinoflagellates are present in large numbers.

Zooplankton refers to planktonic animals. Zooplankton is made up of an extremely diverse group of animals, inlcuding many larval and adult forms. If you were to use a plankton net and then examine part of your sample under a microscope, you would see a rich soup-like mixture, including larval forms of many crustaceans, mollusks, jellyfish, starfish or sea urchins, sponges, fish eggs, and many others.

Together, zooplankton and phytoplankton are extremely important. In fact, the functioning of the entire ecosystem of the world's oceans is almost entirely dependent on the photosynthesis carried out by marine plants, and in particular, by phytoplankton. And because zooplankton contains so many larval forms of varied animal groups, it can be thought of as a vast nursery for the ocean's myriad species. Plankton's importance as the basis of the marine food chain cannot be overemphasized.