DISTRIBUTION 



catastrophe for all concerned, known as El Nino, until conditions return to 

 normal again. A similar but perhaps less spectacular situation is found off the 

 west coast of South Africa. 



In our northern hemisphere an important cause of up welling is the 

 impact of currents against the continental shelves, and of course the break- 

 down of the thermocline in the colder regions. A chart of the distribution of 

 plankton in the North Atlantic, taken from the book by Russell and Yonge, 

 is given in Fig. 30. 



The seasonal distribution of phytoplankton in temperate waters, following 

 the stability of the water, has been mentioned (p. i 10). One of the 

 first diatoms to bloom in early spring in coastal waters is Skclctoiieina 

 followed by Tlialassiosira (Frontispiece; 7 and 4) which blooms further 

 offshore. These two species alone usually form about 90 per cent of the 

 spring plankton in the shelf waters of the north temperate Atlantic. Later a 

 more mixed flora is found, depending a good deal on the amount of 

 admixture of oceanic water. The autumn dominants are usually species of 

 the needle-shaped Rhizosolciiici, and when these are very abundant they cause 

 a distinctive smell which the fisherman call 'baccy juice', and water so 

 affected has an adverse effect on herring fishing (p. 133). 



This richer plant growth in the shelf water compared with the open 

 ocean results in a marked difference in colour easily distinguishable from a 

 ship. Shelf water, particularly in spring, has a distinctly green appearance, but 

 oceanic water with its paucity of surface phytoplankton is blue, and crystal 

 clear. The blue Mediterranean is famous, its blue being derived from the 

 extreme clarity of the water and the lack of plankton due to the warmth of 

 the surface waters, and the consequent lack of nutrients there. 



The spring outburst of plant growth is the new season's food supply and 

 leads to a corresponding but slightly delayed increase in the zooplankton as it 

 starts its multiplication. At this season many of the truly planktonic species 

 reproduce, and there is a spate of spring and summer planktonic larvae, from 

 the bottom-living and attached species as well as from the plankton itself. 

 Most of the species that reproduce once a year do so in spring ; less do so in 

 summer although there is a group that utilize the more chancy autumn out- 

 burst. Others will reproduce at any time or fairly continuously throughout the 

 summer six months. Survival depends on food; very few young animals ap- 

 pear in the plankton during the winter, and those are either carnivorous from 

 the start or detritus feeders — but more of the food cycle in the next chapter. 



The distribution of the planktonic stages of bottom-living and attached 

 species is thus closely related to the current systems, and — need so obvious a 

 factor be stated; — on where the adults are living. The spring plankton in 

 water bathing a rocky coast will thus be different from that in a sandy area, 



113 



