292 POPULATIONS OF THE SEA 



tricornutum, the former Nitzschia closterium f. minutissima, there 

 is a host of unsolved problems calling for studies by means of ex- 

 periments with the use of cultures. The mention of a few problems 

 may be allowed. 



The annual cycle of the phytoplankton, which has such char- 

 acteristic features in higher latitudes, has been investigated in all- 

 year surveys and, in many cases, the general trend has been 

 found to relate well with changes in the supply of nutrients, 

 stability conditions, and light supply, but we cannot yet give any 

 substantiated explanation of the succession of species. Several 

 theories have been advanced, but these have a common lack of 

 background knowledge regarding the actual beha\'ior of the organ- 

 isms in question. We have already seen how doubtful the informa- 

 tion is on the temperature response on the few species which have 

 been studied. The situation is no better in the case of the mutual 

 effect of species growing together, for instance by means of 

 ectocrine substances. This factor has been suggested as playing 

 a part in producing succession, but this has still to be proved. 

 The different rate of growth in the various members of a society 

 is obviously of prime importance for their relative abundance, 

 but actual knowledge of the rate of reproduction in diatoms, dino- 

 flagellates, and coccolithophorids is so fragmentary that it cannot 

 be used for estimating with any certainty the effect of this feature 

 in individual cases. 



Extreme mass occurrence of certain species, as for instance in 

 red tides, represents an interesting and spectacular example of 

 societies presumably resulting from hitherto obscure conditions 

 favoring growth in certain species. These are normally members 

 of societies of a more balanced composition. 



Form variation is observed in every species. In some cases there 

 seems to be a phenotypic variation which has been tentatively 

 related to the influence of temperature, salinity, nutrient salt con- 

 centration, and day length. Experimentally little has been done 

 in this field. For Ceratia, Nordli (1957) observed variation in size 

 with changes in temperature and, in the case of rather abnormal 

 forms, he found their occurrence to be related to an ample supply 

 of nutrients and the absence of a marked day-night rhythm. How- 



