SECT. 4] GEOGRAPHIC VARIATIONS IN PRODUCTIVITY 359 



between 20° and 40° in mid-summer, provided the lowest productivity values 

 encountered on the entire expedition. 



The results from the cruise by R.V. Crawford from 35° to 25°N latitude in 

 March, 1959, are interesting since they show the transition from semi-tropical 

 to tropical conditions during the peak of the spring flowering at the northern 

 end of the section. Fig. 5, from Ryther and Menzel (1960), illustrates how high 

 productivity (ca. 1.00 g carbon/m 2 /day) is associated with isothermal wind- 

 mixed surface water in the Bermuda region, and how low values (ca. 0.1 g 

 carbon/m 2 /day), typical of the tropics, were encountered to the south as soon 

 as thermal stratification of the euphotic zone became apparent. A similar 

 section in January-February, 1960, from 30°N, 53°W southwest to the West 

 Indies (11°N) confirmed the earlier results. Since these studies were made in 

 mid-winter, when cooling and mixing are presumably most pronounced, it 

 seems doubtful that higher production rates normally occur in that part of the 

 ocean. Unfortunately no measurements have been made in the equatorial 

 Atlantic, much of which is affected by African and South American coastal 

 influences. 



B. The Sargasso Sea 



Productivity values for the Sargasso Sea obtained by the Galathea section in 

 June were the lowest encountered on that expedition (ca. 0.05) and were at 

 variance with earlier estimates for the same area (Riley et al., 1949). The 

 reason for this became apparent when the seasonal cycle of organic production 

 for this area was considered. From 1957 to 1960 measurements were made of 

 primary production and related physical, chemical and biological features of 

 the Sargasso Sea a short distance from Bermuda but in typical oceanic condi- 

 tions. Observations were made at two -week intervals for a period of three 

 years, representing the only attempt of its kind to study intensively the 

 seasonal cycle and annual variability of primary production in the open sea. 

 Observations for the first half of the investigation have been described by 

 Menzel and Ryther (1960). 



That part of the Sargasso Sea (32°N) may be considered semi-tropical, 

 having characteristics intermediate between temperate and truly tropical 

 situations. In addition there are certain unique hydrographic features which 

 play an important role in controlling the productivity of the area, most im- 

 portant of which is the existence of a permanent thermocline beginning at 

 400-500 m. A typical summer thermocline develops in the upper 100 m, below 

 which, and above the permanent thermocline, there lies a large body of water 

 at a temperature of approximately 18°C. In winter the surface-water tempera- 

 ture may fall to 18°C, in which case the summer thermocline is completely 

 destroyed and the surface layers become isothermal and mixed to depths in 

 excess of 300 m. The 18° water is relatively poor in nutrients (0.1-0.3 fxg atom/ 

 1. P04 3_ -P, 1-3 fxg atoms/1. N03~-N) but richer than the euphotic zone where 

 these nutrients are almost undetectable in summer. Thus mixing of the 18° 



y 



