360 



RYTHER 



[CHAP. 17 



water to the surface provides a definite though limited stimulus to plant 

 production. At the same time, because nutrient concentrations are not great 

 enough to permit the development of dense plant populations and because 

 incident radiation is relatively high in winter, the euphotic zone remains deep. 

 It is an interesting fact that, although chlorophyll values vary seasonally by 

 an order of magnitude (0.1-1.0 fAg/L), the transparency remains almost un- 

 changed, presumably maintained by a balance between living and non-living 

 material. Thus the phytoplankton is not mixed out of the euphotic zone in 

 winter for long enough periods to prevent its growth, and production is 

 maintained at relatively high levels throughout the winter. In spring, at the 

 onset of thermal stratification, there is a population maximum, but it is more 

 the climax of a gradual build-up than the sudden outburst of the spring 

 flowering typical of more temperate regions. 



Fig. 6. Primary production and the thermal structure of the upper 700 m in the Sargasso 

 Sea off Bermuda. (After Menzel and Ryther, 1960.) 



The seasonal cycle of primary production and the vertical temperature 

 profile of the upper 700 m for 1958 (Fig. 6) reveal that production increased 

 with the breakdown of the summer thermocline, built up during the winter and 

 early spring to a maximum of 0.89 g carbon/m 2 /day in April, and then quickly 

 fell off to low summer rates of 0.1-0.2 g carbon/m 2 /day, as soon as thermal 

 stability returned to the euphotic zone. Annual production for the period was 

 72 g carbon/m 2 , perhaps twice the value which could be expected in the tropics. 



The winter of 1957-1958 was severe enough to destroy completely the 

 summer thermocline by March (Fig. 6). The winters of 1958-1959 and 1959- 

 1960 were milder, surface temperatures did not fall below 19°-20° and a slight 



