19221 Allen: Studies on Murine Phytoplankton at La Jolla 341 



Nitzschia seriata CI. is a very prominent diatom in this region. 

 It may be found at any time of year though surface catches indicate 

 that it is typically a winter or spring form. It has been found in 

 large numbers in July and August in vertical tow net catches taken 

 from two hundred meters to surface near Santa Cruz Island and in 

 one or two other localities. This fact leads to the supposition that 

 this as well as other forms of the phytoplankton may flourish at the 

 surface in one season and at a lower level in another season. The 

 largest catch of N. seriata in the four months was on December 22 at 

 8 p.m. The number was 5900 per liter and the average for the month 

 was 830. The maximum occurred in a well developed pulse which 

 began on December 15 and ran ten days. 



As already stated, N. pungens is often not readily distinguishable 

 from X. seriata. Its occurrence in December was coincident with the 

 latter in a pulse that ran the same course. The maximum was reached 

 at 8 a.m. on December 20 in a catch of 7700 per liter and the average 

 for the month was 831. 



The genus Skeletonema is represented in our catches by only one 

 species, Skeletonema costatum Grew Its numbers were few in Sep- 

 tember and it was missing in October. Occurrence in November was 

 very irregular, although it was conspicuoxis in a few catches just after 

 the middle of the month. In December it appeared more regularly 

 in the catches and reached a maximum of 4160 per liter on the 19th 

 at 8 a.m. The average for December catches was 919 per liter. 

 S. costatum is frequently found in our region and is probably an 

 important general constituent of our phytoplankton. 



No other genera of diatoms reached much importance in the four 

 months and there does not seem to be anything in records of their 

 occurrence which is not in essential agreement with those of the 

 genera just discussed. Evidently any peculiar significance which 

 they may have can only be determined by careful statistical analysis. 



One point, however, is clearly shown by detailed studies, i.e., that 

 when there is a pulse of the highly productive forms there is also a 

 pulse or marked increase in numbers of those forms of lower pro- 

 duction and there is also an increase in the number of species and of 

 genera represented. This also seems to be true to a lesser extent of 

 swarms. Such facts naturally lead to the assumption that in a general 

 way ecological conditions in the sea favorable to high production of 

 diatoms affect a large number of forms in the same way. 



