356 KYTHER [CHAP. 17 



wind-mixing and eddy diffusivity cannot be estimated quantitatively, it is 

 clear that the process is minimal and seasonally invariable at the equator, 

 maximal at 60° and most pronounced in the temperate and northern seas in 

 winter. On the basis of nutrient availability alone, one would predict a low and 

 nearly constant rate of organic production in the tropics, low summer but 

 relatively high winter rates at 40° and appreciably greater production through- 

 out the year at 60°. 



Turning to light as a limiting factor, we must keep in mind three aspects of 

 this factor: (1) incident radiation, (2) the transparency of the water and (3) 

 the depth of the wind-mixed layer. In the tropical latitudes, radiation is 

 relatively high and constant throughout the year, the water is exceptionally 

 clear and thermal stability prevents the plants from being mixed out of the 

 euphotic zone. At 40°, however, and more strikingly at 60°, radiation is appre- 

 ciably lower in winter, the waters tend to be more turbid and mixing may 

 carry the plants to depths 5-10 times as great as the euphotic zone. To the 

 extent that isothermal water is indicative of such mixing, plant growth is 

 clearly inhibited during the winter at 40°, and for half the year or more at 60°. 



By now it is apparent that the two factors which control plankton growth in 

 the sea, though seldom limiting at the same time, are also seldom available 

 simultaneously in amounts capable of sustaining high production. When and 

 where, in the four situations we have discussed, are both light and nutrients 

 present in non-limiting quantities? In tropical seas probably never except in 

 those peculiar regions to be discussed later where hydrodynamic features bring 

 rich water to the surface ; in temperate and northern seas, only for that brief 

 period in spring when surface waters, enriched by winter mixing, become 

 thermally stratified. Possibly again in the fall, when the thermocline begins to 

 break down and periods of mixing alternate with periods of stability, conditions 

 may again become favorable for moderately high production. The timing of 

 the spring and fall maxima varies from year to year, depending upon weather 

 conditions, but generally occur respectively later and earlier at progressively 

 higher latitudes and may merge together as a single summer flowering in boreal 

 and austral regions. 



The low level of production which persists in most of the tropics and in 

 temperate waters during the summer is maintained by the slow upward diffu- 

 sion of nutrients through the thermocline and, probably more important, by 

 the complete cycling of these materials within the euphotic zone itself. There is 

 at present no suitable way of evaluating either of these processes. It seems 

 probable, however, that the bacteriological remineralization of organic matter 

 would be enhanced by the relatively high temperatures which prevail in the 

 tropics and in summer temperate conditions. 



There are, of course, many exceptions to the general picture presented above. 

 One in particular deserves mention here. A column of water of the same density 

 is unstable, but instability is a non-dynamic characteristic which requires a 

 force such as wind stress to produce vertical turbulence. There are probably 

 many interludes of fine, calm winter weather when an unstable water-mass is 



