the sea. However, if one were to follow a given 

 parcel of water as it is brought to the surface and 

 subsequently is transported horizontally, one 

 would probably observe the same sequence of 

 events which we discussed above. Steemann 

 Nielsen and Jensen (1957) have described this for 

 the coast of Africa , pointing out that the freshly 

 upwelled water, though rich in nutrients, is poor in 

 phytoplankton . It is "new" water, in which the 

 plants have not had time to grow. As one moves 

 seaward, following the path of the surface current, 

 the plankton becomes more and more dense, passes 

 through a maximum, and then decreases ultimately 

 to a very sparse population by the time the water 

 has reached mid-ocean. The time course of this 

 sequence is probably not very different from that of 

 a perfectly stable water mass which is enriched by 

 winter mixing. The difference is that high produc- 

 tion in an upwelling area is maintained at a given 

 geographical location. Sette (1955) has described 

 a similar geographical sequence "downstream" from 

 the mid-Pacific equatorial divergence. 



In contrast to these dynamic situations, 

 which are comparatively rare in the oceans as a 

 whole, we described above a static system in 

 which the surface waters are enriched by winter 

 mixing. The term "static" refers here to the ab- 

 sence or minor effects of horizontal advection, not 

 to the absence of vertical water movements . Let 

 us now return to this situation and consider it in 

 more detail . 



During the winter in temperate and northern 

 regions, surface waters cool sufficiently to destroy 

 the summer thermocline, and the waters become 

 mixed to 300 - 500 meters, several times the depth 

 of the euphotic zone . Not only are the nutrients 

 from below the euphotic zone brought up and mixed 

 with the impoverished surface layers, but the 

 plankton algae are transported downward and spend 

 a considerable fraction of their time in darkness. 

 As a result, though nutrients are plentiful, pro- 

 duction is severely curtailed due to the limitation 

 of light. 



With the return of spring , the surface waters 

 begin to warm up, a seasonal thermocline develops, 

 and the euphotic zone becomes stabilized against 

 vertical mixing. At the same time, radiation in- 

 creases. Those phytoplankton which find them- 

 selves in the euphotic zone are held there and 

 suddenly have access to both light and nutrients . 

 The stage is set for the "spring bloom", a feature 

 characteristic of the temperate oceans. We have 

 shown on the previous pages the succeeding 

 events, terminating in the exhaustion of the nu- 

 trient supply . During a period of fine , calm 

 weather in March or April, the whole process may 

 run its course within a week or two. More typi- 

 cally, the formation of the summer thermocline is 

 interrupted by storms, periods of cold weather, 

 etc . and the spring flowering may then be pro- 



longed, at a lower level, for a period of one or two 

 months . But its days are numbered by the supplies 

 of nitrogen, phosphorus, and the other essential 

 elements which are limiting to plant growth in the 

 sea. As we have seen, the amounts of these sub- 

 stances brought to the surface by mixing are small 

 to begin with, and they are quickly consumed. 



What happens next is a matter of some con- 

 troversy. Some believe that most of the nutrient- 

 deficient plants sink, their density increasing with 

 old age. Evidence for this is the accumulation in 

 summer of relatively high concentrations of chloro- 

 phyll at or near the lower limit of the euphotic zone. 

 Others hold that the ultimate fate of the plants is 

 to be eaten by the animal members of the plank- 

 tonic community (i.e. Harvey e^t_al,, 1935; Gushing, 

 1958). Whatever happens , the spring maximum 

 soon gives way to a summer minimum during which 

 time production proceeds at a very low level which 

 is probably maintained by the complete recycling 

 (assimilation, death or consumption, and regenera- 

 tion) of a small fraction of the winter nutrient bud- 

 get within the surface layers. 



In the fall, when cooling again destroys the 

 seasonal thermocline, there may be minor and ir- 

 regular outbursts of plant growth as the surface 

 layers are alternatively cooled and mixed to a 

 slight degree and then restabilized . These small 

 blooms terminate with the final disappearance of 

 thermal stratification and the return of winter con- 

 ditions . 



At the semi-tropical latitude of Bermuda, in 

 the Sargasso Sea, the annual cycle of organic pro- 

 duction is much the same as that pictured above, 

 with the major difference that production persists 

 at a relatively high level throughout most of the 

 winter. This is due to the fact that winter cooling 

 and mixing is less pronounced than in more north- 

 ern waters, and never, in fact, extends below the 

 depth of the permanent tliermocline at 300 - 400 

 meters. In addition, incident radiation is higher 

 at these latitudes, and the water is exceptionally 

 clear. As a result of this combination of factors, 

 the plants are never carried down out of the light 

 for sufficiently long periods to prevent their growth. 

 Figure 2 shows the seasonal cycle of organic pro- 

 duction in the Sargasso Sea off Bermuda. The ac- 

 companying seasonal profile of temperature in the 

 upper 700 meters will illustrate how hydrographic 

 conditions influence plant growth . Note that high 

 production is correlated with a well-mixed, largely 

 isothermal layer in the upper 400 meters; low 

 production, with the thermally-stratified summer 

 conditions . 



The nutrient supply available to the plants 

 in this 400 meter deep reservoir in the Northern 

 Sargasso Sea is extremely low--an order of magni- 

 tude less than that present in the temperate seas. 

 Nitrate, for example, seldom exceeds 1.0 ug AN/1 

 in the euphotic zone. Consequently, plant 



77 



