THE PHYTOPLANKTON 



9 



20 and 80 m. Now the question arises as to the source 

 of this oxygen. It cannot have been brought up from low- 

 er levels because the adjoining water was deficient in 

 oxygen. It is very unlikely that a thin horizontal layer 

 could move in from some region rich in oxygen along 

 this high level where vertical mixing so often occurs. 

 Furthermore, the temperature and salinity data do not 

 permit such a conclusion. The only other possible 

 source of oxygen is the photosynthetic organisms inhab- 

 iting the upper strata of water. Of the twenty oxygen 

 stations, only six produced samples with more than 100 

 diatom and dinoflagellate cells per liter, and at only one 

 of these were samples collected giving over 1000 cells 

 per liter. Zero counts were obtained at a subsurface 

 level at four of these stations. It does not seem possi- 

 ble that such a low population of diatoms and dinoflagel- 

 lates could produce such an excess of oxygen. There 

 are two possible explanations for this condition. Either 

 there is present in the upper 100 m a large population of 

 photosynthetic organisms which pass through the ordi- 

 nary filter net, or else it is possible for a layer of water 

 to exist in the ocean at this level and maintain its super - 

 saturation with respect to oxygen after the phytoplankton, 

 which has produced the oxygen, has disappeared. The 

 latter explanation is hardly tenable except for regions 

 where the water strata are in a very stable condition. 



Gran (1912) found in the Atlantic that the Coccolitho- 

 phoridae, which readily passed through the finest net, 

 far outnumbered the diatoms and dinoflagellates which 

 were retained in the net. It was doubted, however, if 

 the photosynthetic activity of such small organisms, 

 even in such large numbers, could surpass that of the 

 larger forms occurring in smaller numbers. The Car- 

 negfie data lead one to suspect very strongly that in some 

 instances the photosynthetic activity in the sea is con- 

 fined principally to such minute organisms. Much more 

 work must be done in the Pacific before such problems 

 as this can be satisfactorily solved. 



At the fifteen oxygen stations in the tropical region 

 only one observation showed complete saturation. This 

 is in strong contrast with the northern group of stations 

 among which were only two that did not show saturation 

 at some level. Moberg and Graham (1930) have pointed 

 out that this "can be accounted for only by assuming 

 that at these stations at least part of the water had re- 

 cently come from depths at which oxygen content is 

 normally low." Unfortunately the quantitative plankton 

 samples for most of these stations were lost, but the 

 samples from six of them and from other tropical sta- 

 tions indicate that the plankton flora in this region is 

 much scantier than in the north, which condition also 

 contributes to the low oxygen content of the upper strata. 



SUMMARY 



1. There was a very sparse population of diatoms 

 and dinoflagellates in the open waters of the Pacific in 

 1928 and 1929 at 0-, 50-, and 100-m levels. 



2. Although dinoflagellates never reached the large 

 numbers attained by the diatoms at some stations, they 

 outnumbered the diatoms in 25 per cent of the samples. 



3. When a rich sample is defined as one with more 

 than 100 combined diatom and dinoflagellate cells per 

 liter, the richer samples occurred in waters of lower 

 temperature, lower salinity, higher phosphates, and 

 higher hydrogen-ion concentration. No definite corre- 

 lations were found between either dissolved oxygen or 

 silicate and number of plant cells. The richer areas 

 occurred in regions where subsurface water is brought 



to the surface by vertical circulation. 



4. A comparison of the Carnegie data with results 

 obtained by Bigelow and Leslie in Monterey Bay shows 

 the same correlation between the above factors and 

 quantity of phytoplankton. 



5. Supersaturation of oxygen in the upper 100 m in 

 regions where the diatom and dinoflagellate populations 

 were very low suggested the presence of photosynthetic 

 organisms not captured in the ordinary filter net. 



6. The Carnegie investigations have shown the ne- 

 cessity of making quantitative studies of the phytoplank- 

 ton that pass through the ordinary filter net and of se- 

 curing more complete vertical series over the whole 

 ocean. 



