10 



CERATIUM IN THE PACIFIC AND NORTH ATLANTIC OCEANS 



influence on the chemical and physical conditions of the 

 water are far-reaching. Associated with this we find 

 that the biological conditions have been affected for 

 hundreds of miles offshore. In the first place there is 

 a high production of total plankton; secondly, there is a 

 reduced number of species per station in the region 

 (see chart 53); and thirdly, the development of some of 

 the tropical species apparently has been suppressed. 



Many of the tropical species common in the waters 

 to the west were absent in at least certain parts of the 

 southeastern Pacific. Sometimes this distributional hia- 

 tus was confined to the Peruvian coast and Galapagos 

 area as in C. extensum (chart 16) and C. tenue (chart 

 45). Other species seemed to show a more or less gen- 

 eral avoidance of the area, as for instance, C. pulchel- 

 lum (chart 20), C. carriense (chart 40), C. vultur (chart 

 43), and C. teres (chart 11). 



Here again we probably are not confronted with a 

 problem of simple current displacement, but rather with 

 current effects brought about through a modification of 

 the environment. What these peculiar modifications 

 might be, will be discussed in the next section. 



Other Factors 



Although the above geographic classification of the 

 Ceratia (p. 8) has a temperature connotation, it is not 

 intended to mean that the controlling factor in Ceratium 

 distribution is necessarily temperature. There are stlU 

 other factors which are associated with temperature 

 which we have not yet discussed. 



Nielsen (1934) is of the opinion that the concentra- 

 tion of plankton organic metabolic products is the most 

 important factor in the distribution of Ceratium species 

 in the warmer seas. Since no measurement of such 

 substances are available, it is impossible to test this 

 theory. Nielsen based his opinion principally on the 

 distribution of neritic and oceanic species and on the 

 relative number of species in these two environments. 

 He emphasized the fact that In neritic waters only a 

 small number of Ceratia occur. That this is not owing 

 to higher concentrations of nutrients alone was shown by 

 the Great Barrier Reef region where the nutrients are 

 low as well as the Ceratium flora. Nielsen found higher 

 concentrations of nutrients in the open ocean (eastern 

 South Equatorial Current) and a diminution in species. 

 He contended that the metabolic products require con- 

 siderable time for decomposition, that the fertility of 



neritic water is replenished from shallow depths where 

 these products have accumulated, and that in oceanic re- 

 gions no such accumulation can occur. What these met- 

 abolic products are, remains to be demonstrated. 



Even If the presence of such products is demon- 

 strated and correlations found, their causative nature 

 must be proved and this probably will have to be done 

 experimentally. 



The difficulties In establishing the important envi- 

 ronmental conditions are accentuated by the fact that 

 many water conditions change together. The agency 

 which enriches the photic zone to any great degree is a 

 movement of water from deeper levels to the surface 

 layers. This one agency, whether near shore or in the 

 open ocean, brings about a whole set of new environ- 

 mental conditions. Not only is the phosphate and nitrate 

 content of the water Increased, but the temperature is 

 lowered and the hydrogen-ion concentration Is increased. 

 Undoubtedly other changes are effected also. If some 

 particular metabolic product Is rather stable, as Nielsen 

 postulated, no doubt higher concentrations of this accom- 

 pany any enrichment of the photic zone. A point worth 

 considering in this connection, however, is thatNlelsen's 

 postulated metabolic product poisoning may be purely of 

 plant origin. In this connection Hardy's (1935) discus- 

 sion of the "exclusion effect" of phytoplankton for zoo- 

 plankton is interesting. Considerable evidence is being 

 acciunulated that Indicates that phytoplankton exerts an 

 exclusion effect on many animals. Is it possible that, in 

 a similar way, it may exclude certain oceanic species of 

 Ceratium? 



The association of all these factors makes it diffi- 

 cult to select the particular one which might be influenc- 

 ing the dlstrlbutlpn of any particular species. Thus the 

 floristlc zones outlined above are separated not only by 

 certain temperature differences, but also by certain as- 

 sociated differences in phosphate, pH, plankton content, 

 etc. 



The difficulty Involved In determining causative fac- 

 tors is well illustrated by the waters of the southeast 

 Pacific where, obviously, some condition associated with 

 the Humboldt Current and upwelllng limits the distribu- 

 tion of certain Ceratium species. This condition here is 

 probably not temperature, as many of the temperature 

 records were over 20°. Which of the conditions associ- 

 ated with cold currents and upwelllng is the determining 

 one. however, cannot be pointed out at this time. 



COMPARISON OF THE ATLANTIC AND PACIFIC WATERS 

 ON THE BASIS OF THEIR CERATIUM FLORAS 



There Is no known difference in the environmental 

 conditions of the waters of the Atlantic and Pacific 

 oceans which might operate to favor or hinder certain 

 species in one ocean and not in the other. Thus it would 

 be expected that, given the same plankton commimlties, 

 the same species would survive in each. The two oceans 

 are completely Isolated from each other, however, except 

 in the south, where they both merge Into the Antarctic 

 Ocean, and In the far north, where they are connected by 

 way of the Arctic Ocean and the narrow Bering Strait. 

 Since temperature or some correlated factor Is probably 



responsible for the distrlbirtion af Ceratium species, 

 temperature will be discussed in connection with the 

 comparison of the floras of the two oceans. The surface 

 temperatures In the latitude of Cape Horn In summer 

 are under 8° C. Whether 8° is sufficiently low to con- 

 stitute a barrier to tropical Ceratium species or their 

 spores, is unknown. The continental barrier between 

 the Atlantic and bdlan oceans certainly must be less 

 effective since the temperatures at the Cape of Good 

 Hope In summer may be above 20° C. Thert Is no con- 

 tinental barrier between the Indian and Pacific oceans. 



