DIATOMS 



By 



W. E. ALLEN 



Throughout the six cruises of 1938 (February, 

 April, June, August, October, December) not only 

 was the group of diatoms far the most prominent 

 in catches of phytoplankton, but the representa- 

 tion of other groups was negligible to about the 

 same degree. Even though such forms as cocco- 

 lithophores and smaller dinoflagellates may have 

 been lost excessively through the meshes of the 

 filtration net (200 meshes to linear inch), their 

 presence should have been observable if their 

 abundance had been great at any time. Therefore, 

 it appears reasonable to assume that attention 

 to diatoms is sufficient for present purposes and 

 that other groups represented in the phytoplank- 

 ton may be neglected. 



After microscopic examination of the 1130 

 catches of phytoplankton I felt most impressed 

 with the fact that diatoms were represented in 

 all sections at all seasons. Even in those 

 catches yielding numbers too small for statisti- 

 cal significance there were enough specimens to 

 constitute an important source of supply for 

 production or renewal of large populations under 

 favorable conditions. In the region investi- 

 gated, taken as a whole, nearly two-thirds of the 

 catches showed numbers of 500 diatom cells per 

 liter, or more. The line about one hundred and 

 forty miles southwest from the vicinity of Santa 

 Barbara showed greatest consistency in producing 

 significant numbers, more than two-thirds of the 

 catches yielding 500 cells or more; but the 

 northern line was nearly as good, with almost 

 exactly two-thirds containing such numbers. The 

 poorest line was the one southwest from Los 

 Angeles Harbor, only a few more than one-third 

 of the catches reaching or exceeding the 500 mark. 



There is no possibility of determining from 

 existing data the true relationships between 

 alongshore sind offshore populations, but it is 

 natural (and reasonable) to suppose that there 

 is a fairly close correspondence of periods of 

 increase and decrease in abundance. Uncompleted 

 manuscript records of surface catches made daily 

 at two shoreline stations (Point Hueneme near 

 Santa Barbara and La Jolla near San Diego) indi- 

 cate that all but one (November) of the six 

 cruises of 1938 were made in periods of decline 

 of abundance or of a minimum abundance at these 

 two stations. Obviously, the noteworthy or large 

 numbers found in many catches offshore may have 

 been remnants of still larger numbers in process 

 of reduction, or they may have been derivatives 

 of inshore populations in process of increase, 

 or they may have been entirely independent in 

 origin. In consideration of such wide differ- 

 ences in possibilities no positive conclusion 

 can be completely acceptable at present. How- 



ever, a tentative suggestion may be made that 

 constancy of representation of diatom popula- 

 tions is to be more generally expected offshore 

 throughout the whole year, irrespective of sea- 

 sons. 



At most offshore stations the abundance at the 

 surface level was closely indicative of the size 

 of the population total at any particular station. 

 Therefore, comparison with shoreline surface 

 catches is surely permissible. But, in any event, 

 the clear showing of declining production at two 

 stations inshore at the time of five out of six 

 cruises constitutes warning that the problems of 

 the range of either actual or potential annual 

 productivity of offshore areas are still open 

 questions. Nevertheless, the records do show 

 that a number of offshore stations may yield phy- 

 toplankton abundantly. 



As far as the data for these cruises are con- 

 cerned, there is strong evidence that certain 

 offshore areas are more productive, certainly 

 more consistently productive than those near 

 shore. An exception exists in the two stations 

 near Santa Rosa and San Miguel Islands. Still, 

 these two stations lack consistency, one showing 

 extreme abundance in only one of the four times 

 sampled, and the other showing insignificant num- 

 bers in one of the four times sampled. Of other 

 stations within twenty-five miles offshore the 

 two on the northern line showed numbers greater 

 than those at stations more than fifty miles from 

 shore in one out of six times sampled. Nearly 

 the same statement applies to the stations near 

 Los Angeles Harbor and to the two stations near 

 San Diego. 



As far as seasonal differences in production 

 of diatoms is concerned, the evidence from the 

 six cruises seems fairly clear and corresponds 

 very well with the data obtained in most years 

 from daily catches at shore stations. Thus the 

 largest abundance for the year was in April or 

 June at most stations of the cruises. However, 

 station 30 (near San Diego) showed largest num- 

 bers in October, and it is possible that certain 

 other stations would have shown different times 

 of maxima if they had been sampled on all six 

 cruises. Such exceptions are sufficient to show 

 that biological sequences through a year cannot 

 be determined positively without a high degree 

 of continuity of observation. 



As has been suggested in a preceding paragraph, 

 the surface level usually indicates fairly well 

 conditions of abundance or lack of abundance of 

 plankton diatoms at individual stations. How- 

 ever, data from this level are not fully reli- 

 able as indicators of greater or lesser abundance; 

 they may be widely misleading if accepted as in- 



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