Introduction to the Marine Environment
Oceans cover 71 percent of the earth’s surface and contain 99 percent of the habitable space on the planet.
The sea is not a uniform water world but a collection of highly variable habitats that change with distance from land, distance from the Equator, and depth below the surface. The biome concept is difficult to apply to this vast region of the planet and perhaps not appropriate at all. On land, the biomes are defined on the basis of dominant vegetation, while the plant life of the oceans consists mainly of algae, most of which are microscopic and provide no structural element to the environment. Marine biologists and oceanographers have attempted to formulate various schemes for delineating biomes in the world’s oceans, but to date no consensus has been reached.
One of the most common and indeed conventional ways of categorizing marine environments is to identify three main types: coastal, continental shelf, and deep sea, a classification that is used here. These three groups of habitats vary according to distance from land, depth below the sea surface, water temperature, salinity, density, ocean currents, seabed characteristics, nutrient sources, waves and tides. Each grouping varies internally according to latitude. Furthermore, different sides of the same ocean basin have varying physical conditions and assemblages of species. In general, the western boundary currents ̶ such as the Gulf Stream ̶ are warm; the eastern boundary currents ̶ such as the California and Humboldt currents ̶ are cool.
The Five Oceans
1. Pacific Ocean: The largest of the oceans, the Pacific covers 60,667,000 mi2 or 28 percent of Earth’s surface. It is essentially cut off from the Arctic Ocean, but connected to all other oceans.
2. Atlantic Ocean: The second largest ocean is roughly half the size of the Pacific at 29,937,000 mi2. The North Atlantic is dominated by a clockwise gyre with the warm Gulf Stream and its northern extension, the North Atlantic Drift, forming the eastern boundary. It has a direct connection to the Arctic Ocean.
3. Indian Ocean: At 26,737,000 mi2 this body of water is still larger than the largest landmass, Eurasia. Regions north of the Equator are strongly influenced by the changing wind directions of the Asian monsoon. From December to April winds and ocean currents flow from the northeast; from June through October both move in a southwesterly direction.
4. Southern Ocean: These cold waters, recognized as a separate ocean since 2000, encircle the Antarctic continent. Northern limits have been set by international convention at 60° S latitude. Its surface area extends over 7,927,500 mi2. The Southern Hemisphere’s strong, steady, prevail westerlies drive the Antarctic Circumpolar Current from west to east. Surface temperatures range between 50° F and 28° F. In winter, sea ice extends from the edge of the continent to 65° S latitude near the Pacific, but to 55° S latitude into the Atlantic. Due depression of the continent by the vast Antarctic ice cap, the continental shelf is unusually deep (1,300 to 1,600 feet below sea level).
5. Arctic Ocean: At 5,482,000 mi2, the Arctic Ocean is almost the same size as the Antarctic continent at the opposite end of the earth. Most of the ocean lies poleward of the Arctic Circle (66.5° N). Surface currents follow two gyres: the clockwise Beaufort Gyre north of Canada and the Transpolar Current from the Chukchi sea into the Greenland Sea. In winter, the Arctic Ocean is covered by drifting pack ice moving slowly within the Beaufort Gyre. In summer ice covers less than half the area. The Arctic pack ice is decreasing in thickness and areal extent today as a consequence of global climate changes.
For a more comprehensive treatment of all parts of this section on marine biomes, see Woodward, Susan L. 2008. Marine Biomes.Greenwood Guides to Biomes of the World (Greenwich, CT: Greenwood Press).