Marine Life

I

Introduction

Marine Life, plants and animals of the sea, from the high-tide mark along the shore (see Shore Life) to the depths of the ocean. These organisms fall into three major groups: the benthos, plants such as kelp and animals such as brittle stars that live on or depend on the bottom; the nekton, swimming animals such as fishes and whales that move independently of water currents; and plankton, various small to microscopic organisms that are carried along by the currents.

II

Range of Habitats

Benthic plants and animals inhabit distinct seafloor habitats. The shallow-bottom habitat that extends from the shore to the edge of the continental shelf 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 feeders such as the pycnogonid sea spiders and stalked crinoids (sea lilies).

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 a 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 near the water surface, provides food for grazing zooplankton and the fish life it supports. The deepwater and bottom life forms depend on a rain of organic matter from above.

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III

Environmental Factors

Life in the oceans is not uniformly abundant. Because of the low ratio of surface water to deep water and the lack of seasonal nutrient enrichment, much of the open ocean is a watery desert, especially the tropical seas. The most productive areas are the coastal regions, areas of upwellings (see below), and the Arctic and Antarctic oceans.

The intense cold that makes the Antarctic landmass 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 dinoflagellates. The phytoplankton is consumed by zooplankton and other grazers, including the enormous populations of shrimplike krill, which in turn serve as food for many fish species and whales.

The rich Antarctic waters are pulled away from the shore and become part of the Antarctic Circumpolar Current, also known as West Wind Drift. This current, the strongest ocean current on earth, is partially diverted by the southern tip of South America, forming the Humboldt Current off the coast of Peru. As the surface water is pulled out by winds, the nutrient-rich deep water replaces it, aided by the absence of a continental shelf. This is an important upwelling region, and it supports an enormous amount of life. Copepods and opossum shrimp replace the krill of the Antarctic. Feeding on these crustaceans are immense schools of small fish that, in turn, are consumed by seabirds, replacing whales at the top of the food web.

IV

Hydrothermal Vents

In the late 1970s, marine scientists studying the dark Pacific Ocean floor discovered ecosystems (see Ecology), the energy source of which is chemical synthesis by bacteria. One of these ocean oases is on the Galápagos Rift; another is on the East Pacific Rise at latitude 21° North. These ecosystems are associated with the midoceanic ridge and rift systems where the thin plates of the earth's crust are spreading, creating vents. Seawater seeps through, becoming saturated with minerals and hydrogen sulfide and heated to high temperatures. The water is then vented up through the cracks as dark black geysers and quickly cooled to a warm 23° C (73° F). Growing on rocks in this warm, fertile environment are clumps of primitive microorganisms, called Archaea, that use hydrogen sulfide as an energy source. Living upon the Archaea are a number of filter-feeding animals, some previously unknown, including a species of giant clam and giant tube worms up to 3.7 m (12 ft) long. The ecological productivity of these oases is four times greater than that of the surface water above. Marine scientists have also found such vents and oases on the flanks of the Mid-Atlantic Ridge, and believe they may exist on all oceanic rift systems (see Ocean and Oceanography).

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