272 POPULATIONS OF THE SEA 



study of marine phytoplankton in general. In this way the scope 

 of the following discussion will mainly be a consideration of ex- 

 periments dealing with specific differences among the members of 

 the marine phytoplankton in their reaction to the environment. 



Field observations have revealed a \'ariety of patterns in hori- 

 zontal and vertical distribution, and pronounced annual cycles 

 of species, leading to a succession of plankton societies. It is char- 

 acteristic of the marine phytoplankton that the details in this 

 succession may change from year to year in the same habitat. 

 An analysis of these striking events requires knowledge of the re- 

 action of each species or group of species to the environmental 

 conditions. 



The reason for my taking this opportunity to re\"iew the auteco- 

 logical aspect especially is not that there are many spectacular 

 results to present. On the contrary, the scarcity of available ex- 

 perimental data forces me to stress mainly the need for an intensi- 

 fication of effort, in order to establish a harmonic progress in the 

 study of the producers in the sea. 



Let us first re\'iew briefly what information studies on cultures 

 have afforded as to the influence upon \-arious marine phyto- 

 plankton species of such environmental factors as salinity, tem- 

 perature, light, and inorganic nutrients, which are subject to 

 routine recording in oceanographic investigations. Salinity and 

 temperature are generally accepted as important for the distri- 

 bution of species, while light and inorganic nutrients are recog- 

 nized as controlling factors for growth of the population in general. 

 As the first of these classical ecological factors we may consider 

 salinity. 



Salinity 



For the diatoms, which form such an important part of the 

 planktonic vegetation, even in tropical waters (Hasle, 1959), the 

 only records on how salinity affects growth are from Kain and 

 Fogg's (1958) experiments on Asterioiiella japonica (Fig. 1). They 

 indicate an optimum at a salinity of 30 %o and no growth at 15 %o 

 and below. In Danish waters, with a wide range in salinity, the 



