ECOLOGY OF ROCKY COASTS 343 



has been found that there are distinct spring and autumn maxima, 

 when some component species occur in great abundance. These 

 maxima appear to be associated with an interaction between Hght 

 supply and availabihty of nutrient salts. It has been found that at 

 the periods of maxima the nitrogen and phosphate are greatly re- 

 duced, and if the latter falls below a certain value it may limit 

 photosynthesis. In winter in temperate waters there is a rapid ver- 

 tical mixing of the waters bringing up fresh nutrients from the 

 lower layers, and this is responsible for the development of the 

 spring maximum. Continual vertical movement maintains a rich 

 medium, and as a result the cold upwellings (e.g. off South-West 

 Africa and California) are very rich in plankton. For certain species, 

 lack of trace elements, such as iron or manganese, as a result of 

 their absorption by other organisms, may bring about a fall in re- 

 production and later growth. 



An exception to the normal spring and winter maxima is pro- 

 vided by the enclosed waters of the Mediterranean where the 

 maxima occur in November-December and March, the one in 

 March being the larger. 



During the 1939-45 War attempts were made to increase the 

 plankton, and hence the fish, by adding artificial fertilizers to large, 

 more or less enclosed, bodies of water such as lochs. The addition 

 of such fertilizers did increase the plankton and also the fish and it 

 may be that a technique such as this can have a valuable applica- 

 tion^. In other cases a planktonic organism may have an adverse 

 effect on fisheries. One of the most outstanding examples is Phaeo- 

 cystis pouchetii, a coloured flagellate which, when present in 

 quantity, gives the waters of the North Sea a muddy appearance, 

 the so-called 'baccy juice' (Savage, 1932). Herrings are repelled by 

 this organism when it is present in mass, and its vernal maximum 

 off the Dutch coast turns the northward herring migration west 

 towards the coast of East Anglia, and thus brings about the spring 

 fishery there (Fig. 194 A, b). The occurrence of an abnormal autumn 

 maximum out of its usual station may completely change the 

 grounds of the autumn fishery during the southward migration : 

 such an abnormal maximum is known to have occurred in 1927 

 (Fig. 194 c). 



1 This technique has been used in recent years with the fish ponds made from 

 the mangrove swamps in the Philippines. 



