Rhonda Ovens, age 14, of Monroe, La., for her question:
WHAT TYPES OF MARINE LIFE DO WE HAVE?
Marine life includes all of the plants and animals of the sea, from the high tide mark along the shore to the depths of the ocean. These organisms are divided into three major groups: the benthos, or plants such as kelp and animals such as brittle stars that live on or depend on the bottom; the nekton, or swimming animals such as fishes and whales that move independently of water currents: and plankton, various small to microscopic organisms with limited powers of locomotion that are carried along by the currents.
Benthic plants and animals inhabit distinct sea floor habitats.
The shallow bottom habitat extends from the shore to the edge of the continental shelf and it supports mollusks, polychaete worms and attached algae and sponges.
The continental slope and beyond make up the benthic zone which includes the deepest part of the ocean floor. It is sparsely populated with deposit feeders and filter feeds such as the pycnogonid sea spiders and stalked crinoids or sea lilies.
The nekton or free swimming fishes and other animals are found in greatest abundance in the relatively shallow and well lighted strata of water above the continental shelf, where most commercial fishing takes place.
In the lower and darker water stratum, beginning at the continental slope, giant squid may be found. Below that stratum is the abyssal zone, the cold, dark waters inhabited by bioluminescent fishes.
Plankton is the dominant life and food source of the ocean. Phytoplankton, which carries on photosynthesis on the water surface, provides food for grazing zooplankton and the fish life it supports. The deep water and bottom life forms depend on a rain of organic matter from above.
Marine life in the oceans is not uniformly abundant. Much of the open sea is a watery desert.
Much of the open sea can be described as a watery desert because of the low ratio of surface water to deep water and the lack of seasonal nutrient enrichment. This is especially true with the tropical seas.
The most productive areas are the coastal regions, acres of up wellings and the Arctic and Antarctic seas. Here you'll find lots of marine life.
The intense cold that makes the Antarctic land mass so forbidding also influences the great productivity of the Antarctic waters. Cold water, made denser by the freezing of ice (which removes fresh water from salt), sinks to the bottom and moves northward from the continental shelf, as does the surface water.
The northward moving water is replaced by a deep mass of water flowing southward between the surface and bottom layers. That water, rich in nutrients, rises to the surface in an upwelling that stimulates a heavy growth of phytoplankton in the form of diatoms and dinofiageilates.