FACTORS AFFECTING HORIZONTAL DISTRIBUTION 



latitudes lower than 40° where the number of species 

 per station dropped below ten. 



The relation between the richness of species and 



environmental conditions will be taken up in the follow- 

 ing sections. 



FACTORS AFFECTING THE HORIZONTAL DISTRIBUTION OF CERATIUM 



Salinity 



It is probable that the slight variations in salinity 

 found in the oceanic waters have no influence on the dis- 

 tribution of Ceratium species. Peters (1934) coulddem- 

 onstrate no effect in the South Atlantic nor could Nielsen 

 (1934) in the South Pacific. Nielsen did find that in the 

 Panama region, where the salinity is low, the total num- 

 ber of species was low. Conditions were neritic there, 

 however, so it was impossible to determine which was 

 the influencing factor. In the Carnegie investigations no 

 correlation could be found between salinity and the dis- 

 tribution of any species. In this connection it is worthy 

 of note that the boundary between the distribution of the 

 warm and cold water species of the North Atlantic is ap- 

 proximately at the 36 per mille isohaline; in the North 

 Pacific the limits of the same species occur at 34 per 

 mille off Japan and 33 per mille off California. Thus, 

 apparently these species are not affected by such slight 

 variations in the salt content as are found In the open 

 ocean. 



Nutrient Salts 



Since the depletion of phosphate and nitrate is known 

 to limit the total production of phytoplankton in some lo- 

 calities, it is important to examine these factors in rela- 

 tion to the world distribution of Ceratium species. Ni- 

 trate determinations were not made on the Carnegie but, 

 since the fluctuations in nitrate and phosphate run in a 

 parallel manner, it is sufficient to discuss the phosphate 

 distribution as representative of the inorganic nutrient 

 materials. 



In general, regions of very low phosphate content 

 are characterized by sparse populations of all species 

 but by a larger number of species. This led Peters 

 (1934) to suggest that nitrate and phosphate inhibited the 

 growth of certain species. Thus in region I of the North 

 Atlantic where the phosphate content of the upper 50 me- 

 ters is above 25 mg P04/m3, the number of Ceratium 

 species is less than ten per station (see charts 52 and 

 53). In region n the phosphate content is under 10 mg 

 and the number of species of Ceratium per station every- 

 where above ten and, usually above twenty, and at one 

 station above thirty. Likewise in the Pacific the phos- 

 phate-rich regions are characterized by a poor Ceratium 

 flora. Region HI, where the concentrations run above 25 

 mg and at most stations above 100 mg, the number of 

 species per station is less than ten. The correlation is 

 not quite so good In region IV, the warm Pacific region. 

 The number of species per station is between twenty and 

 thirty throughout most of this region although the phos- 

 phate content of the northern part is under 10 mg and in 

 the southern part it varies from 10 to 50 mg. The south- 

 east Pacific region has many values above 50 mg, that 

 Is, higher than the warmer region to the west. Accord- 

 ingly the number of species per station is less, namely, 

 between eleven and twenty. 



Thus, within any given area there seems to be a 

 correlation between high phosphate content and small 

 number of species. It is important to note, however, 

 that the limit between any particular number of species 

 does not occur at the same concentrations of phosphate. 

 Thus, in the northern Atlantic less than ten species are 

 found in water with 25 to 50 mg of phosphate, whereas 

 in the South Pacific the area with more than twenty spe- 

 cies includes water with over 25 mg P04/m3. Similar- 

 ly, in the North Pacific all the water containing more 

 than 50mgPO4/m3 has less than ten species per station, 

 whereas in the southeastern Pacific there are areas 

 where the phosphate content is above 50 and the species 

 per station are between ten and twenty. 



The same sort of correlations can be found between 

 the Ceratiimi life zones and phosphate content. Each re- 

 gion has its characteristic phosphate content. Thus re- 

 gions I, ni, and V have high concentrations, whereas 

 region II has uniformly low concentrations, and region 

 IV is low in the north and has medium values in the 

 south. As in the case of number of species, the correla- 

 tion with floras is not exact. Thus the flora of region I 

 has little in common with the flora of the southern part 

 of region IV which has the same phosphate content. 



These studies tend to indicate that the phosphate 

 content of the water has no direct effect on the horizon- 

 tal distribution of Ceratium species, at least not as re- 

 gards absolute values. There are indications, however, 

 that the relative values in a given region bear some re- 

 lation to the Ceratiimi flora. Perhaps some factor as- 

 sociated with an increase in phosphate is significant in 

 the distribution of Ceratium species. 



Temperature 



Peters (1934) came to the conclusion that water 

 temperatures between 15° and 27.°5 C had no influence 

 on the distribution of Ceratia in the South Atlantic. How- 

 ever, he was considering the yearly range of tempera- 

 ture at each locality. Peters was able, on the other hand, 

 to group his species according to thermal environments. 

 Of his fifty-five species, thirty-three were limited to 

 warm water, twenty-one to warm and cool water, and 

 only one was observed in the southern cold water. Niel- 

 sen (1934) was inclined to consider 15° C too low for 

 noneffective temperatures. In the. regions which he in- 

 vestigated in the South Pacific, however, he did not find, 

 many cases of temperature correlation. He designated 

 only two temperate forms-- C. petersii and C. tripos at- 

 lanticum. He also found that C. filicorne is restricted 

 to high temperatures in the South Equatorial Current. 

 Nielsen, however, did classify his species according to 

 temperature zones with the following categories: tropi- 

 cal, tropical-subtropical, tropical-subtropical-temperate, 

 and temperate. 



In the Carnegie collections it is possible to study 

 the transition from tropical to cold water regions infour 

 different places: In the western North Atlantic, western 



