Q|.o 



Figure 2. The seasonal profile of temperature to 700 meters and net organic production in the Sargasso 

 Sea off Bermuda for the period January - July, 1958 (from Menzel and Ryther, in press) . 



production per unit volume is never high — the 

 process cannot build up towards its biological po- 

 tential . In contrast to a dense flowering in the up- 

 per five meters of temperate seas, the Sargasso 

 Sea plankton grow over a euphotic zone which is 

 never less than 50 - 100 meters deep. What main- 

 tains this organic production in the face of such 

 poverty of nutrients? Low as they are, the con- 

 centrations of nitrate, phosphate, and silicate (to 

 name a few essential elements which have been 

 studied) never change appreciably in these surface 

 waters. We don't yet understand this, but it ap- 

 pears that in these relatively warm waters the 

 whole cycle of assimilation, consumption, death or 

 excretion and remineralization occurs very rapidly. 

 The amounts of plants and animals and minerals 

 are small, but the metabolic wheels turn fast. 



No study of the seasonal cycle of organic 

 production has been made in tropical waters, but 

 we can imagine what it must be like. In the true 

 tropics there is no winter cooling of the surface 

 waters, no mixing, no replenishment of the eu- 



photic layer with rich aphotic waters . The sea - 

 sonal thermocline of temperate and semi-tropical 

 latitudes becomes a permanent, thermal barrier to 

 vertical mixing . 



On March 1, 1959, a short oceanographic 

 section was made by Research Vessel Crawford 

 between 24° and 35° North latitude at the longitude 

 of Bermuda. At that time of year, the surface water 

 temperatures in the North Atlantic Ocean are at 

 their seasonal minimum; vertical mixing is most 

 pronounced. Actually the 1959 season was some- 

 what early. Our seasonal study at Bermuda re- 

 vealed the beginnings of warming and stratification 

 by March 1 , and the spring phytoplankton pulse 

 was in full bloom on that date. Figure 3 shows the 

 temperature profile and values of organic produc- 

 tion for this section running from North to South. 

 It is remarkably similar to the seasonal cycle at 

 Bermuda. The well-mixed, almost isothermal , 

 highly productive stations at the northern end of 

 the section merge into stratified, low productive 

 stations to the South much as the spring flowering 



78 



