NATURE ADRIFT 



Gulf Stream work its way eastwards right across the Atlantic instead of going 

 due north; The answer is linked with the earth's motion, and currents in 

 the Northern Hemisphere tend towards the right and in the Southern 

 Hemisphere to the left; the oceanographers call this the 'Coriolis Force'. 

 Thus in the northern hemisphere the north-going warm currents will tend 

 further and further cast and the south-going cold current further west. 



These currents arc mentioned in this chapter on plankton because they 

 are responsible for carrying the plankton and they also provide the environ- 

 mental conciitions that enable some species to thrive and prevent others 

 from doing so. Other currents of importance in plankton distribution along 

 the Atlantic coasts of northern Europe are the low salinity outflow from 

 the Baltic, and the mixed sub-arctic current east of Iceland and north of 

 Faroe continuing towards the Norwegian Sea north and west of the warm 

 water. There is a complex but, in general, anti-clockwise circulation in the 

 southern North Sea. 



Strangely enough the British area is also affected by outflow from the 

 Mediterranean, which spreads out into the Atlantic at about i,ooo metres 

 depth west of Gibraltar. Some of this goes northwards to the Biscayan area, 

 to the English Channel and Celtic Sea, and west of Ireland. When it is 

 particularly strong its presence can be detected, much diluted, at the edge 

 of the continental shelf west of Scotland and even on occasion as far as the 

 North Sea and in the vicinity of southern Iceland. The part played by 

 plankton in the detection of these currents is dealt with in Chapter ii ; here 

 it is only necessary to emphasize that they are responsible for so much of the 

 geographical distribution of plankton. 



The ocean currents are, of course, very slow-moving. The sun main- 

 tains the warmth of the tropical surface waters, and also in the summer 

 warms the surface layers ofthe temperate waters, which brings about a stabiHty 

 of the water mass with little or no mixing between the warm and cold 

 layers. The junction may be very sharp indeed, and is then called a 'thermo- 

 chne'. To the phytoplankton this stability means that there is little replenish- 

 ment of nutrients in the illuminated surface waters where the plants utilize 

 them until there is a scarcity of plant food; little plant-growth means a 

 scarcity of food for the zooplankton so that plankton is poor, volume for 

 volume, in the warm water. In the tropics this condition continues through- 

 out the year (Fig. 29), although the number of different species is great. In 

 temperate waters there is a sharp rise in the phytoplankton in the early spring 

 as soon as there is sufficient warmth to stabilize the surface water. This 

 stabilization prevents the plants from being carried by convection currents 

 beyond the limit of light penetration so that the start of their growth will 

 depend on a relationship between light penetration and turbulence. The 



JIG 



