332 University of California Publications in Zoology [Vol.22 



In most catches the living material consisted almost entirely of 

 diatoms and dinoflagellates. There were times, however, when 

 copepods. rotifers, and other very small animal forms were caught in 

 considerable numbers. More rarely a few forms of various other 

 kinds of microscopic plants were included. 



According to a policy already determined after experience with 

 phytoplankton from offshore waters, attention was confined to diatoms 

 and dinoflagellates because their small size, large numbers, rapidity 

 of growth, facility of multiplication, wide distribution, cosmopolitan 

 character, and synthetic powers give them superior value as indicators 

 of environmental conditions, mark them a.s the most favorable objects 

 for quantitative study, and distinguish them as the most easily and 

 continuously accessible of all marine organisms. 



The distributional interrelationships of the two groups show many 

 points of interest for which there are as yet no adequate grounds of 

 discussion. This lack need not, however, invalidate tentative state- 

 ments concerning them. 



First, in making microscopic examination of the material I gained 

 the impression that when diatoms were exceedingly abundant, dino- 

 flagellates were rare, and vice versa. Hence I have earlier called 

 attention to my belief in the possibility that the two groups are to 

 some extent mutually exclusive of each other. It is still an open 

 (piestion whether this amounts to anything more than the commonly 

 observed fact that excessive abundance of one group of organisms 

 tends to reduce or exclude certain other groups. In fact, it is quite 

 possible that many cases of apparent reduction of one under 

 dominance of the other were merely apparent because dilutions for 

 counting the abundant form rendered the other inconspicuous. 



Both groups show rhythms and pulses of production which are 

 more or less evident in each month of the year although, of course, 

 the species concerned in the pulses vary according to season. It is 

 rather difficult to define a pulse, but for present purposes I am 

 arbitrarily assigning the term to a marked increase in numbers which 

 extends over a period of three or more days before decreasing to or 

 near the numbers found at its beginning. Of pulses in this arbitrary 

 sense there were five for diatoms and four for dinoflagellates in the 

 last four months of 1919 at our pier. The diatom pulses were un- 

 evenly distributed, two occurring in September, none in October, one 

 in November, and two in December. The dinoflagellate pulses had 

 similar distribution by months except that there was only one in 



