DYNAMICS OF DISTRIBUTION 



353 



plankton under AB be such that these organisms make only a small migration vertically 

 into water travelling just a little faster than the surface water, so that at the end of 24 

 hours they occupy the positions shown in the second line of the figure. The organisms 

 mnop have been carried into the zone BC of denser phytoplankton ; when they next go 

 down they would penetrate much deeper than before into water travelling faster to the 

 right, so they would be carried along as a group together, and will come up to the surface 

 at the end of the second day in a group separated from the organisms abc. ..I. At the next 

 vertical migration the group ijkl would become separated from abc. ..h. Thus we shall have 

 a series of parallel belts of plankton at intervals at right angles to the current. Now this 

 is a phenomenon which we have observed from the ' Discovery ' on a number of occa- 

 sions ; particularly striking were parallel belts of Pyrosoma stretching across the sea, and 

 possibly the patches of Enphausia superba, Parathemisto, etc., observed in the consecu- 

 tive net series (Figs. 133 and 134), were in reality belts at right angles or making an 

 angle with our direction of steaming. These consecutive net series were made off the 

 north-east coast of South Georgia, where contrasts between zones of moderate and 

 heavier phytoplankton may have been sufficiently sharp to produce this effect. 

 If the organisms migrated upwards at 2100 o'clock and downwards at 0300 o'clock 

 (as is more usual than 1800 and 0600 taken in our hypothetical examples previously), 

 and the water into which they penetrated under moderate phytoplankton was travelling 

 200 yards a day faster than the surface water, and that into which they penetrated under 

 the heavier phytoplankton 1 mile a day faster than the surface, then the belts of plankton 

 would be 150 yards across and three-quarters of a mile apart. Such conditions would 

 explain the type of patchiness met with in our consecutive net experiments. 



Start of observations 



After 24 hours 



After 48 hours 



After 72 hours 



After 96 hours 



abcefgijkmnop 



d h 1 



Fig. 190. 



A series of such parallel belts of plankton passing from a region of dense phyto- 

 plankton to one of low phytoplankton would become closed up again ; but if the speed 

 at which they travelled under this new low phytoplankton concentration was less than 



