The word, plankton is from the Greek word for "wandering". Ocean life is sometimes classified by its role in the food chain (producer, consumer, decomposer), phylogentically (kingdom, phylum, class, order, family, genus, species), or by its general lifestyle (benthos, nekton, plankton). The benthos or bottom dwellers are comprised of seaweed, crabs, tube worms and the like. The nekton are free swimmers, such as adult fish, whales, dolphin, squid, and so on. The plankton are small floating or weakly-swimming plants and animals. They are at the mercy of winds and tides. Since they need to stay near the surface to survive, they often have oil, spikes, air chambers, or flagella to increase their buoyancy.
The two main types of plankton are phytoplankton ("light-loving plankton") and zooplankton. The phytoplankton are tiny plants which need to be near the light (or euphotic zone) to survive. The zooplankton are tiny animals which need to be near the phytoplankton to feed.
Phytoplankton comprise 80% of all plant life on the planet and are responsible for 40% - 80% of the world's oxygen (depending on which author you believe). Perhaps you thought that the rain forest or old growth forests were the main contributors of the world's oxygen. Not so. In fact old growth forests consume more oxygen than they produce, and while trees do contribute to the world's oxygen supply, their contribution is less than the phytoplankton and other plants on the planet.
There are three main types of phytoplankton: Diatoms, Dinoflagellates, and Cocolithophores. The diatoms are single cell, yellow-green algae. They exist in single units or in long chains. They have cell walls made of silica (glass plants). The Greek meaning for their name is "cut in two". That is because under a microscope, you can see that half of their cell wall fits over the other half. They are the most abundant form of phytoplankton.
Dinoflagellates resemble both plants and animals. They have cellulose cells that act like armor, but use flagella to swim. There are two species of dinoflagellates that are responsible for red tide (a deadly bloom of these organisms results in a huge release of their toxins into the water, which kill fish and sicken humans). Cocolithophores are the smallest of the phytoplankton and are made of calcium carbonate.
Not only are the phytoplankton vital for the release of oxygen and the base of the food web, but they have important industrial uses as well. Crude oil and natural gas ar remnants of the photosynthetic products of ancient algae, which were subsequently modified by bacteria. There is a brown algae in a lake in Russia that produces so much oil, that they simply skim it off the surface to collect it. Some companies are now producing special algae in high salinity tanks to create oil directly, without using fossil fuel.
Over 500 species of algae are eaten by humans, some in the form of instant puddings and pie fillings, artificial dairy toppings, fillers in candy bars, and syrups. The fossilized cell walls of diatoms mix with sediment on the ocean floor to become diatomite. This is mined and used in a huge number of products from toothpaste and paint to welding rods and dynamite.
Perhaps now, you have a little better appreciation for the ucky brown-green water in the marsh. Nature's soup is hard at work building many of the vital components for life on this planet.