282 



POPULATIONS OF THE SEA 



10* 



20° 



30°C 



20° 



30° C 



Fig. 8. Temperature response in strains from different geographical regions 

 {left, Barker, 1935, and Bakken, unpublished; right, Bakken, unpublished, and 

 Braarud, unpublished). 



A similar displacement of the optimum is known from tempera- 

 ture experiments on Prorocentrum micans. The Oslofjord strain 

 of this species showed a definite optimum at 20°C, whereas Barker 

 (1935) in experiments with cultures from the coast of California 

 observed an optimum at 25°C (Fig. 7). 



In the coccolithophorid Coccolithus hiixleyi Mjaaland (1956) 

 observed a temperature optimum at about 20°C and growth at 

 as low a temperature as 7°C (Fig. 5). This reaction is in fair agree- 

 ment with observations from surveys, although these indicate 

 that growth may even take place at a considerably lower tem- 

 perature. 



For the available temperature experiments the comparison with 

 field observations gives a less coherent picture than the salinity 

 experiments. There are examples of a general agreement between 

 the experimental results and the field data, as for the dinoflagellates 

 from the Oslofjord, and there are instances of pronounced dis- 

 agreement as for Asterionella japonica and Thalassiosira nor- 

 denskioeldi. Two explanations may be suggested for these dis- 

 crepancies and both may be involved : that the experimental tech- 

 nique has been unsatisfactory and given erratic results, or, that 

 the deductions from biogeographical data are erroneous and have 

 blurred the ecological picture for the species in question, as sug- 



