THE PHYSICS AND BIOLOGY OF VERTICAL MIGRATIONS 87 



While sunlight cannot penetrate far (if at all) below this depth, there is still light from the luminous 

 organs of deep-sea animals (Clarke, 1958). At depths between 1000 and 4000 m., temperatures range 

 from about 6° to i° C. in temperate and tropical regions, while the quantities of dissolved oxygen 

 are generally above 2 ml./l. 



Considering now the biological structure of temperate and tropical oceanic waters, the well 

 illuminated surface waters (down to about 100 m.) form the living-space of the primary producers, 

 the phytoplankton. And, except in the tropical eastern Pacific, these upper layers (from the surface 

 down to about 200 m.) contain far greater quantities of zooplankton than the waters below (Jespersen, 

 1935 ; Vinogradov, 1955; Foxton, 1956; Zenkevitch and Birstein, 1956; Bogorov, 1958). Riley (1951) 

 has estimated that the total oxygen consumption and phosphate regeneration below the density 

 surface sigma t 26-5 (average depth 200 m.) is equivalent to a utilization of about one-tenth of the 

 surface production of organic matter by the plants. Thus, about nine-tenths of this organic production 

 will be consumed in the upper 200 m., and, compared to the average depth of the ocean, this layer is 

 little more than a ' surface film '- 1 



While the waters below 200 m. support sparse populations of zooplankton, there is good evidence 

 of a secondary maximum in the intermediate thermocline layer. In the western North Atlantic 

 Leavitt (1938) found a mid-depth maximum centred at a depth of 800 m. (there was also a lower 

 lying, but lesser concentration at 1600 m.). Jespersen (1935) also found a mid-water maximum of 

 zooplankton (at about 1000 m.) in the tropical Pacific Ocean, whereas in the north-western part it is 

 centred higher in the water at about 500 m. (Bogorov and Vinogradov, 1955). It would seem that 

 the rapid increase of density and viscosity in the thermocline layer slows down the fall of detrital 

 material, so that it tends to accumulate at mid-depths. Such concentrations of suspended material 

 will support detritus-feeding animals and their predators. As Miyake and Saruhashi (1956) have 

 pointed out, oxygen-minimum layers in the Atlantic and Pacific Oceans occur most frequently along 

 the same sigma-? surface (27-2-27-3) and are most marked in the more productive regions. However, 

 in the equatorial Pacific (Albatross stations III— 133, between o° and 15 N.), Jerlov (1953) found a 

 mid-depth maximum of suspended particles between 700 and 800 m., a level 150 m. below the centre 

 of the oxygen-minimum layer. 



Below these rather slight mid-water concentrations, the biomass of zooplankton is small until the 

 bottom waters are reached. The concentration of plankton near the bottom has been repeatedly 

 observed from bathyscaphes (Bernard, 1955; Peres, Picard and Ruivo, 1957; Peres, 1958 and Tregou- 

 boff, 1958). Apart from ' microplankton ' (some of which is detritus), aggregations of larger forms, 

 such as euphausiids, sergestids and chaetognaths have been seen. (The observations were made in the 

 Atlantic (off Portugal) and in the Mediterranean at depths ranging from 600 to 2200 m. and the layer 

 seems to be about 100 m. in depth.) Such near-bottom concentrations of plankton are likely to be 

 widespread and may at least partly explain Riley's (1951) finding that the rates of oxygen consumption 

 near the deep-sea floor appeared to be larger than those in the main body of Atlantic deep water. At 

 all events, these bathyscaphe observations are a valuable contribution to deep-sea biology. 



The time and effort given each day to vertical migrations by countless oceanic animals is one of the 

 most striking features of their biology. But the plants can only actively assimilate carbon in the 

 surface-waters, and the resulting marine pastures support not only the animals that occupy the same 

 living-space, but also those in the underlying waters. Considering the fishes, there can be little doubt 

 that their daily climb towards the surface is a feeding migration. In the western North Atlantic, to 



1 'The autotrophic zone has a depth of 200 metres at most and includes less than 5 per cent of the volume of the ocean. 

 Below this zone, life depends on organic matter carried down by organisms sinking from above or by the vertical migrations 

 of animals back and forth between the depths'. Redfield (1958). 



