APHOTIC ZONE 



aphotic zone — That portion of the ocean waters 

 where light is insufficient for plants to carry on 

 photosynthesis. See euphotic zone. {See 

 figure for classification of marine environ- 

 ments.) 



apogean range — ( abbreviated An ) . The average 

 of all monthly tide ranges occuring at the time 

 of apogee. 



apogean tidal current — A tidal current of de- 

 creased sj^eed occurring at the time of apogean 

 tide. 



apogean tide — Tides of decreased range occurring 

 monthly near the time of the moon's apogee. 



apogee — That point on the orbit of the moon (or 

 any other earth satellite) farthest from the 

 earth ; opposed to perigee. ( 5 ) 



apparent contrast — The change in contrast that 

 depends upon the distance to an object, the 

 amount of light absorption, and the inclination 

 of line of sight to the vertical; expressed as 

 percentage. 



apparent freezing point — See freezing point. 



apparent luminance — With respect to the visual 

 range of a dark, distant object: luminance, 

 created by airlight, of that portion of the visual 

 field subtended by the object; that is, the light 

 scattered into the eye by particles, including air 

 molecules, lying along the optic path from eye 

 to object. (5) 



apparent velocity — The velocity with which a 

 fixed phase of a seismic wave, usually its front 

 or beginning, passes an observer. (35) 



appendicularian — One of a class (Larvacea) of 

 small, transparent, planktonic tunicates in 

 which the body is covei'ed by a large tunic and 

 is composed of a trunk and a long tail. Some 

 sjDecies are luminescent. 



approach — The area or space of indefinite extent 

 in immediate contact with an objective; par- 

 ticularly, that part of the sea adjacent to a 

 shoreline. (68) 



approximate absolute temperature scale — (ab- 

 breviated AA). A temperature scale with the 

 ice point at 273 degrees and boiling point of 

 water at 373 degrees. (5) 



apron — A slojDing underwater extension of an ice- 

 berg, or an outspread deposit of ice or rock ma- 

 terial in front of a glacier. (68) See ram. 



apsis — Either of the two orbital points nearest 

 or farthest from the center of attraction, the 

 perihelion and aphelion in the case of an orbit 

 about the sun, and the perigee and apogee in 

 the case of an orbit about the earth. The line 

 connecting these two points is called line of 

 apsides. (68) 



aquaculture — Fish, shellfish, and algae farming; 

 development of new sea foods, and methods of 

 rearing larvae of clams and oysters. Today 

 largely practiced in Japan and Southeast Asia. 

 (35) 



aquafact — An isolated boulder which has been 

 worn smooth on its seaward face by wave 

 abrasion. 



Aqua-Lung* — Self-contained underwater breath- 

 ing apparatus (SCUBA) of the demand or open 

 circuit type ; specifically, the Cousteau-Gagnan 

 apparatus. The trademark for SCUBA equip- 

 ment manufactured by a U.S. concern. 



aqueous desert — A marine bottom environment 

 in which there is little or no macroscopic in- 

 vertebrate shelled life, usually a bottom of 

 shifting sands. (2) 



aragonite — A mineral, orthorhombic CaCOs, a 

 major constituent of shallow muds on the 

 Bahama Banks and elsewhere. 



arch — See rise. 



arched iceberg — An iceberg with a large opening 

 at the water line, extending through the iceberg, 

 forming an arch. (68) 



archibathyal — See archibenthic. 



archibenthal — See archibenthic. 



archibenthic — (also called archibenthal, archi- 

 hathyal) . 



1. According to some authorities a subdivision 

 of the benthic division. According to other au- 

 thorities it corresponds roughly to the upper 

 half of the bathyal. 



2. Pertaining to a zone extending from a 

 depth of about 650 feet (200 meters) (con- 

 tinental shelf edge) to between 2,625 and 3,600 

 feet (800 and 1,100 meters) . See classification 

 of marine environments. 



Archimedean buoyant force — See buoyancy 

 (sense 2). 



Archimedes's principle — The statement that a 

 new upward or buoyant force, equal in magni- 

 tude to the weight of the displaced fluid, acts 

 upon a body either partly or wholly submerged 

 in a fluid at rest under the influence of gravity. 

 This force is known as the Archimedean 

 buoyant force (or buoyancy) and is independent 

 of the shape of the submerged body and does 

 not depend upon any special properties of the 

 fluid. (5) 



archipelagic apron — A gently sloping sea floor 

 with a generally smooth surface on the sea floor, 

 particularly found around groups of islands or 

 seamounts. (62) 



archipelago — A sea or part of a sea studded with 

 islands or island groups ; often synonymous with 

 island group. 



arc shooting — (or fan shooting). A method of 

 refraction; seismic prospecting, in which the 

 variation of travel time with the azimuth from 

 a shot is used to infer geologic structure. (35) 



arctic air — A type of air whose characteristics are 

 developed mostly in winter over arctic surfaces 

 of ice and snow. Arctic air is cold aloft and it 

 extends to great heights, but the surface 

 temperatures are often higher than those of 

 polar air. For two or three months in summer 

 arctic air masses are shallow and rapidly lose 



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