NATURE ADRIFT 



is a 'deep scattering layer', a layer of 'something', not yet thoroughly in- 

 vestigated, which returns echoes of sound from the midwater layers of 

 all the oceans of the world. It also showed that this deep scattering layer 

 moves up and down every day, being as low as 400 fathoms or so in the 

 day and coming up to 100 fathoms or less at night, sometimes reaching 

 the surface. Because of its regular diurnal movement it is obviously a 

 'living layer' and not some physical difference of temperature or salinity. 

 Because most planktonic organisms are not good sound reflectors, the 

 actual sound scattering is thought to be mostly due to small fish wdth swim 

 bladders, but euphausids and other larger creatures may also be partly 

 responsible ; these may of course be associated with large numbers of non- 

 reflecting organisms. What is probably happening is that the associated 

 plankton is regularly moving up and down according to the light con- 

 ditions, and that the sound-scattering fish etc. are following the plankton 

 to feed. 



When the after effect of the atomic explosion at Bikini was being in- 

 vestigated, a curious problem arose. During the first day the radiation level 

 of ships in the vicinity was not very great, but it gradually increased during 

 the night. The key to the problem lies in this diurnal migration of the plank- 

 ton. The micro-organisms in the lagoon were affected, bvit, being so small, 

 each carried only a small dose. At night they followed their normal be- 

 haviour of climbing to the surface. Here they formed the food of the 

 barnacles attached to the ships and, as each barnacle could capture a large 

 number of the affected plankton animals, their radiation level increased 

 sufficiently to affect the value given the ship as a whole. 



Vertical movement of plankton in the sea raises the question of buoyancy. 

 Protein, and skeletal parts, often the main body constituents of plankton, 

 are heavier than water and dead plankton sinks fairly rapidly. How does 



Plate XL. 

 {Top) The echo-sounder (Kelvin Hughes MS24G) fitted in the laboratory of the research sliip 

 Explorer (see Plate I). The unit to the right contains the sensitivity control. A roll of special paper 

 is being marked by the echo as it appears immediately below the transparent disc graduated in 

 depths in fathoms, and the paper is being wound on to a storage spool below. The regular ticks 

 to the left of the paper are at two-minute time intervals, and the dark hne next to them represents 

 the position of the surface of the sea. The dark marking near the centre depicts the sea bed and a 

 'double echo' is seen at twice the depth and is due to the sound impulse being reflected from the 

 sea bottom to the ship's hull and back to the bottom again before being picked up on board. 

 Echo traces from organisms (fish or plankton, see p. 157) near the surface of the sea can be seen 



on the paper close to the storage spool and also through the glass panel of the instrument. 

 (Bottom) Part of the echo-trace taken by Scotia at the edge of the continental shelf west of Scot- 

 land, 56° 46' N 8° 52' W on 14 July 1955. The depth of water on the shelf is about 60 fathoms, 

 but the edge is very steep and the depth increases quickly to about 1,000 fathoms, and so beyond 

 the Hmits of the scale shown here. The 'double echo' is seen in the shallow water and the two- 

 minute time marks are also shown. There are two distinct traces, one near the surface to about 

 16 fathoms depth, and a deeper one which is seen upwelling at the edge of the shelf and then 

 settles down at about 30 fathoms. 

 Photographs by James Frascr (Crown Copyright reserved on the echo-trace) 



158 



