362 



[chap. 17 



and four times the annual production of the tropics. Noteworthy are the low 

 winter and high summer rates of production relative to the Sargasso Sea. 

 Apparently the combined effects of vertical turbulence, greater turbidity and 

 reduced radiation are sufficient at this latitude (ca. 40°N) to inhibit winter 

 production markedly. On the other hand, the higher summer rates may result 

 from regeneration within the euphotic zone of nutrients from a considerably 

 larger spring flowering than occurs in the Sargasso Sea. The higher production 

 during the spring flowering (maximum observed value of 1.93 g carbon/m 2 /day), 

 in turn, resulted from the enrichment of the surface layers through winter 

 mixing with water containing nitrate and phosphate at concentrations 

 approaching 10.0 and 1.0 \j.g atoms/1, respectively. These levels are nearly an 

 order of magnitude greater than ever are encountered in the surface water of 



2.0 



1.8 



l.6h 



1.4 



1.2 



1.0 



0.8 



0.6 



0.4 



0.2 

 



J J 



Month 



Fig. 7. Primary production as determined by in situ 14 C measurements at offshore stations 

 (solid line, filled circles) and shallow stations (broken line, open circles) off New York. 



the Sargasso Sea, where sub-surface concentrations down to the level of mixing 

 are low to begin with, and where the nutrients are continually assimilated as 

 quickly as they become available throughout the winter. 



Productivity studies in the North Sea were made by Steele (1956, 1958), 

 first between the years 1951 and 1953 at a single location, the Fladen Ground, 

 and later between 1954 and 1956 over the entire northern part of the sea. The 

 method used was a refinement of the technique employed earlier by Kreps and 

 Verjbinskaya (1932), Cooper (1938) and Riley et al. (1949) which involves the 

 indirect estimation of plant growth from the rate of removal of phosphate from 

 the water. Steele's treatment considered the vertical transport of phosphate 

 within the water column but not its rate of regeneration within the euphotic 

 zone. For that reason conservative estimates might be expected, but the author 

 found close agreement between values obtained by that and the 14 C method 

 (Steele, 1957). Therefore the North Sea data are assumed to be comparable to 

 others reported here. 



