BIOLOGICAL RESULTS OF LAST CRUISE OF CARNEGIE 



tween the abundance of phytoplankton and the abundance 

 of nutrients, which are drawn from observations made 

 at any particular instant during a phytoplankton pulse, 

 are meaningless unless the stage in the development of 

 the pulse is considered, for, during a single pulse, every 

 possible combination of the two factors occurs. Such a 

 phytoplankton pulse as described, however, can run its 

 course only when there is a single supply of nutrients 

 which is not augmented during the pulse. Such a situa- 

 tion is characteristic of cold-temperate regions. In 

 these latitudes a supply of nutrients is brought to the up- 

 per levels during the winter when the absence of strong 

 thermal stratification permits considerable vertical cir- 

 culation. In the early spring a phytoplankton pulse prob- 

 ably is initiated by an increased amount of light. At the 

 same time the temperature of the upper strata begins to 

 rise and the body of water attains a more stable condi- 

 tion so that nutrients are no longer supplied in great 

 quantities from lower depths. It must also be remem- 

 bered that the abundance of plankton in high latitudes is 

 of a higher order of magnitude than that in lower lati- 

 tudes so that the phytoplankton still existing after the 

 close of a typical northern pulse or "bloom" may be 

 large as compared with that ordinarily found in other 

 regions. 



The vertical stability of the water probably has 

 more to do with the abundance of plankton than any other 

 factor. Whatever is the chemical substance limiting 

 growth at any particular time, it probably occurs in 

 abimdance in the deep water where organic remains are 

 decomposing and any vertical circulation of the water 

 that brings the deep water to the photosynthetic zone 

 will increase the fertility of that zone. Various inves- 

 tigators, viz.. Gran (1912 and 1928), Ruud (1932), Mar- 

 shall and Orr (1927), and Atkins (1928) hive shown that 

 the development of the phytoplankton is dependent on a 

 replenishment of the supply of such nutrient salts as 

 nitrates and phosphates in the photosynthetic zone, and 

 that such a replenishment can usually take place only by 

 some type of vertical circulation which carries quanti- 

 ties of these salts from the rich stores in the deeper 

 water. 



At Carnegie stations the richer plankton samples 

 were obtained in regions where the surface water had 

 characteristics of subsurface water, whereas the poor- 

 er samples were obtained mostly in regions where a 

 strong stratification of the water caused the conditions 

 in the surface water to be considerably different from 

 those in the deep water. Figure 2 shows the vertical 

 distribution of temperature, salinity, density, phosphate, 

 and pH in the upper 1000 m at station 137 in the North 

 Pacific where there was little circulation of any nature. 

 Conditions here were typical of regions in which the 

 plankton population was extremely scanty. It is evident 

 in fig\ire 2 that any vertical circulation which would re- 

 new the phosphate at the surface would have to extend to 

 more than 200 m, for there was practically no phosphate 

 in the water to that depth. It is equally evident from the 

 temperature and density curves that there was a very 



pronounced thermocline and stratification of the water 

 above 700 m so that there would be considerable resist- 

 ance to any such vertical mixing. 



We may now inspect conditions in two regions where 

 the phytoplankton was relatively abundant, namely, south 

 of the Aleutian Islands and off the coast of Peru. Condi- 

 tions at the former region are represented by station 

 122 in figure 3 and at the latter by station 70 shown in 

 figure 4. At station 122 there was an admixture of cold 

 water of low salinity from the Bering Sea where vertical 

 mixing is common. At station 70 there was an upwelling 

 of water from a depth of about 500 to 700 m. At these 

 two stations the difference in density between the sur- 

 face water and the deep water is relatively not so great 

 and the most rapid gradient is in the upper 100 m. Con- 

 sequently, there is less resistance to vertical mixing 

 and this fact is reflected in the higher concentrations of 

 phosphate at the surface and in the correspondingly hieh 

 concentration in the upper 500 m. At station 70, how- 

 ever, this is owing partly to the upwelling of subsurface 

 water. Any vertical circulation which extended to the 

 100-m level at these stations would carry great quanti- 

 ties of phosphate to the surface layer for there were high 

 concentrations at that depth. These observations were 

 made in the summer when the most stable conditions oc- 

 cur. In the winter there would be an even greater re- 

 plenishment of the nutrients in the upper levels. Al- 

 though phytoplankton pulses may occur in these regions, 

 it is probable that there is always a greater production 

 here than in such regions as represented by station 137. 



To return to the correlations discussed above, the 

 richer phytoplankton samples were found in water of 

 lower temperature, salinity, and pH, and of higher phos- 

 phate content. A glance at figures 2, 3, and 4 will show 

 that any vertical circulation that carries phosphate-rich 

 water to the surface and thus increases the fertility of 

 the surface water will also lower the temperature and 

 pH of that water. Under such conditions the salinity of 

 the surface water may be increased or decreased. For 

 example, in figures 2 and 3 there is an increase in salin- 

 ity with depth, but in figure 4 there is a decrease. It is 

 also evident in figure 2 that a transport of water from 

 lower levels should increase the silicate content of the 

 surface water, and it is probable that a correlation be- 

 tween silicate content and abundance of phytoplankton 

 similar to that between phosphate content and abundance 

 of phytoplankton would be found if the silicate data were 

 as extensive as those for phosphate. 



It is well known that the cold waters of high lati- 

 tudes support the greatest production of plankton. It is 

 probable that low temperature, per se, is not particu- 

 larly favorable for plankton growth except as it affects 

 the particular species which are now adapted to low 

 temperatures. The more abundant life of the colder 

 seas is probably only indirectly related to low tempera- 

 tures. Lower surface temperatures cause less stable 

 conditions and thus permit a more rapid renewal of the 

 fertility of the photosynthetic zone. 



DISSOLVED OXYGEN IN THE PHYTOSYNTHETIC ZONE 



A study of the dissolved oxygen in the photosynthet- 

 ic zone reveals an interesting situation. Of the twenty 

 stations (stations 130 to 149) in the northern region at 

 which oxygen determinations were made, all but two 



showed supersaturation in the upper 100 m. Figure 5 

 shows the vertical distribution curve of percentage sat- 

 uration of oxygen at station 145 which is characteristic 

 of the region. Supersaturation occurred here between 



