A

Zooplankton

The word "plankton" refers to the floating marine organisms that live on the surface of oceans. These organisms can be plants or animals. The plant forms are microscopic algae whose photosynthesis reactions provide the Earth's atmosphere with the majority of its oxygen. The other type of plankton, composed of tiny animals, is called zooplankton.

Zooplankton is made up of hundreds of thousands of different species of animals. Some are baby or larval forms of the animals while others spend their whole life as free-floating organisms. The entire scope of species of zooplankton is enough for scientists to have identified whole communities of these organisms. These communities are very dynamic in that they change their structure and populations on a seasonal basis.

Many members of a zooplankton community begin their lives in estuaries where crabs, fishes, and a whole host of various invertebrates come to breed. The calm and relatively shallow waters of an estuary provide a safe place for eggs to survive and hatch. Upon hatching, the tiny larvae are too small to succumb to the effects of gravity in the water and so begin their journeys as floating animals.

In many species of zooplankton, the larval forms look nothing like the adults they will become. A remarkable example is the flounder fish. It starts its life as a small larval floating form that looks very much like a common fish. It drifts in the water for about forty or forty-five days until it begins its transformation into a bottom-dwelling flat adult. While drifting in the ocean it feeds on other plankton and begins to grow until it begins its juvenile phase. As a juvenile, it drifts to the ocean bottom and flattens out, with one of its eyes rotating around its head to sit next to the other eye. As a bottom-dwelling adult it becomes a flat fish with eyes on the upper surface of its body. Then it is ready to produce more planktonic larvae.

Many members of the zooplankton community feed on other members of the population, and in turn become the meals of other larger predators. Eventually, the whole zooplankton community becomes the bottom of a food chain for an entire food web stretching from the smallest fish to the largest whale. Many of the ocean's largest animals feed on zooplankton. Many whales have feeding structures called baleen that filter the zooplankton from the water. In the polar regions, a small component of the zooplankton community called krill is the basic diet of the many summer-feeding whales.

One of the benefits of becoming a pelagic, or open ocean-dwelling, organism for a specific population is that the drifting currents move the offspring from one place to another. This ensures species distribution, which is critical to the survival of many species. It keeps genetic diversity high and populations healthy.

The waterborne distribution of zooplankton helps its population survive harsh environmental conditions such as freezing, high heat, large storms, and other severe natural phenomena. Because riding the ocean currents distributes many species worldwide, only small portions of a population may be seriously affected by these conditions.

For many planktonic forms, their lifestyleas organisms suspended on the ocean surface, means that they can avoid unfavorable conditions in the deeper regions of the water column. Those species that eventually drift to the bottom and complete their metamorphosis avoid some forms of predation until they are of a suitable size or form to avoid attack.

One of the concerns raised by the increasing depletion of the ozone layer is how the increased influx of ultraviolet radiation that it causes will affect zooplankton. Because they are the basis of the food chain for a great many animals in lakes and oceans, many scientists and others are concerned that the tiny covering around these larval animals may not be strong enough to withstand the impact of increased radiation. As in all food web chains, the zooplankton provide a foundation for so many other and larger food species that some forms of higher predators may be seriously impacted.

Until the increasing ozone loss is curbed, the protection of estuaries, deltas, and other coastal planktonic breeding grounds is crucial for the continued production of zooplankton. More and more, scientists are discovering how important zooplankton are for the health of the marine ecosystems.