148 THE DISTRIBUTION OF THE PLANKTON [PART II 



lower layers will be accentuated. Chilling of the surface layers of the 

 sea will also produce convection currents when the force of the 

 Avind is not strong enough to set up very decided surface drifts. 

 The cold Avater at the surface becomes denser and sinks down and 

 warmer water from underneath rises up to replace it. No doubt 

 the cooling of the surface water must affect the organisms living 

 there, merely because many of them must be susceptible to changes 

 in temperature, but the main effect is probably the establishment 

 of convection currents, and it is these which lead to the vertical 

 movement of the plankton, and ultimately to a more or less com- 

 plete mixture. 



Viscosity changes. Ostwald^ has made the interesting 

 suggestion that vertical movements of the plankton are due to 

 changes in the viscosity of the sea water. Small organisms, such 

 as those of the plankton, are particles in the physical sense and 

 behave as such. When the temperature increases the viscosity of 

 the water decreases and then the plankton organisms sink towards 

 the bottom. Diffusion currents will bring them up again. Ostwald 

 traces the annual spring maximum of plankton to this cause and 

 attempts to shew that the daily maximum is also due to the changes 

 in the viscosity of the sea water. Such movements are quite 

 conceivable and one may accept them as one of the causes of the 

 observed irregularity in the vertical distribution of the plankton, 

 but before we can attribute the annual maxima of abundance to 

 these causes much experimental work remains to be done. 



Inshore and offshore plankton. Kow such irregularity in 

 the vertical distribution of the plankton affects only the hauls 

 made by nets which are dragged through the water in a 

 horizontal direction, or the catches of nets which fish only in 

 definite layers or through restricted strata, and it can largely be 

 avoided by using vertical nets which are hauled up through the 

 water from the bottom to the surface and so fish equally through 

 all layers of the water-. If then we find that vertical hauls made 



1 " Zur Theorie des Planktons," Biologisches Centralhlatt, Bd. xxii. Nr. 19. 



- Not entirely avoided because the migrations of the plankton due to the causes 

 mentioned may not be entirely vertical. Where there are strong tidal streams or a 

 surface drift due to wind, oblique movements of the plankton must be produced at 



