LIMITING FACTORS 193 



low temperatures favouring the persistence of a high proportion of trihydrol. This may 

 well be a partial explanation of the poverty of the phytoplankton observed during the 

 abnormally warm season 1929-30, and of the fact that the positions of the richer diatom 

 hauls frequently showed a closer correlation with low temperatures than any other 

 hydrological feature, particularly during the South Georgia survey of that year. 



Apart from these physiological considerations, pack-ice may also exert a locally 

 beneficial effect by damping down the sea and swell. Atkins (1926, p. 456) has pointed 

 out that in rough water much of the light available is scattered and reflected at the sur- 

 face and the quantity entering is correspondingly reduced. 



These considerations give the impression that such seasonal variations and yearly 

 fluctuations in phytoplankton production, as have been observed within the area dis- 

 cussed, are influenced mainly by the following four factors (or complexes of factors) : 



(i) Stabilization of the upper layers when the ice melts, a factor which is markedly 

 beneficial ; and conversely, instability, whether due to convection or wind action. 



(2) Transportation by surface currents and the possible return of resting spores by 

 sub-surface currents followed by upwelling. The probable importance of the trans- 

 portation factor (in the Norwegian Sea) has been well expressed in a recent paper by 



Gran (1930, p. 72). 



(3) The action of ice, which may promote phytoplankton production (i) by harbour- 

 ing resting spores, (ii) by assisting stabilization, (iii) by shelter from sea and swell, and 

 (iv) by the increase in viscosity and (possibly) by the increased physiological value of 

 water rich in trihydrol. This last is obviously bound up with the temperature eflFect. 



(4) Light intensity and duration, which is doubtless limiting during the southern 

 winter and will determine the vertical as well as the horizontal range over which pro- 

 duction is possible so long as the other factors remain favourable. 



It cannot be too strongly emphasized that in all probability phytoplankton production 

 is always governed by a complex of inter-dependent factors, rather than by one or two 

 which are clearly definable. To this there are two obvious exceptions. In latitudes 

 higher than about 45° light will be the main limiting factor during winter, and in some 

 north temperate waters the complete exhaustion of the small quantities of nutrient salts 

 present, combined with increasing stratification of the upper layers in summer, un- 

 doubtedly leads to the temporary stoppage of production, more or less prolonged, after 

 the great vernal increase. At the two more critical periods, spring and autumn, it would 

 appear that a combination of several factors has to be taken into consideration even in 

 north temperate waters. 



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