158 



Life 



NUMBER OF LIGHT SCATTERING OBJECTS 



500 OOO 1500 2000 ?500 3000 3500 



I I I I I M II I 11 I I I I II I I I I I I I II I I I M II 



I I I I I I I I I 



Figure 137. Vertical distribution of light-scattering ob- 

 jects as counted on photographic negatives exposed in 

 benthograph. Concentrations indicated as more than 

 2000 are probably even greater than shown because soupy 

 texture makes counts difficult. Night series above Santa 

 Catalina Basin between 0330 and 0500 on March 30, 

 1952; morning series above San Nicolas Basin between 

 0830 and 0930 on November 18, 1951; and afternoon 

 series above San Pedro Basin between 1300 and 1400 on 

 March 29, 1952. The top of the deep sound-scattering 

 layer is shown at the right for same times. From Emery 

 (1952fl, Fig. 8). 



A well-publicized but as yet poorly un- 

 derstood phenomenon of the mesopelagic 

 zone is the deep sound-scattering layer, 

 D.S.L., or phantom bottom. These terms 

 were given to describe the reflection of sound 

 by a layer or stratum of objects at one 

 or more depths between the surface and 

 about 1500 feet. It was first noted during 

 the war from the records of both echo 

 sounders and sonar detection gear, and, be- 

 cause of possible military importance, some 

 investigations were made of it at the Uni- 

 versity of California Division of War Re- 

 search at San Diego. The results of some 

 of these studies plus later fathograms made 

 in the Pacific and Antarctic Oceans were 

 summarized by Dietz (1948) and Johnson 

 (1948). The layer is now known to be of 



world-wide distribution. Fathograms show 

 not only the depth of the reflecting objects 

 but permit the drawing of several deduc- 

 tions about their nature. It was early shown 

 that the layer rises at dusk and sinks at 

 dawn. Such a cycle strongly suggests that 

 it is due to organisms rather than to some 

 physical discontinuity in the water, espe- 

 cially because many kinds of zooplankton 

 are known to undergo diurnal movements 

 in an apparent attempt to stay in water of 

 the same light intensity or to feed on the 

 abundant phytoplankton at the surface dur- 

 ing the night and to retreat from predators 

 into darkness during the day. The vertical 

 velocities of the layer are not incompatible 

 with the swimming speeds of some zooplank- 

 ton. Recent use of the precision depth re- 

 corder (Luskin, Heezin, Ewing, and Landis- 

 man, 1954) in the Pacific Ocean has revealed 

 that fact that the D.S.L. here sometimes 

 consists of more than ten separate layers. 

 Even casual examination of the records 

 shows that the layers do not rise and sink 

 together but that they sometimes cut across 

 each other, supporting the view that each 

 layer consists of a diff'erent kind of animal 

 which responds to hght of a diff'erent 

 intensity. 



Early efforts at sampling the deep sound- 

 scattering layer were not very fruitful of 

 zooplankton, but a later and more intensive 

 study by net hauls at various depths above, 

 in, and below the layer (Boden, 1950) showed 

 that it has a definite concentration of zoo- 

 plankton consisting mostly of large cope- 

 pods, euphausids, sagittae, amphipods, 

 pteropods, ctenophores, and other animals. 

 The numbers of large individuals ranged up 

 to about 100 per 100 cu meters of water, and 

 the number of small pteropods and cope- 

 pods was sometimes greater. It was believed 

 that many of the euphausids in the path of 

 small nets may have avoided them or es- 

 caped and that a higher concentration prob- 

 ably exists than was indicated in the 

 sampling, even with large nets. 



Lyman (1948) suggested that zooplankton 

 are too small to be effective sound reflectors 

 and that instead the objects may be squids. 

 Net hauls described by Tucker ( 1 95 1 ) showed 

 the presence of some squids, but perhaps 



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