Pelagic Environment 



149 



because plants need sunlight for photosyn- 

 thesis. These plants, although microscopic 

 in size, form a much greater total bulk than 

 the larger bottom-living algae of shallow 

 water; thus they are the chief organisms 

 capable of converting into tissue the in- 

 organic materials present in solution in sea 

 water. As a result, they serve as the ultimate 

 food base of most marine life. Some 

 animals graze directly on phytoplankton 

 as on a pasture of grass. Others, carnivores 

 and scavengers, are one of two steps removed 

 but are still dependent on these plants. 



Two recent books have brought together 

 much information about both phytoplank- 

 ton and zooplankton, but not specifically 

 for southern California. One of these by 

 Davis (1955) contains a useful set of keys 

 and drawings to aid general identifications. 

 The other by Hardy (1956) has excellent 

 photographs of living plankton, made with 

 the aid of an electronic flash, and also many 

 color sketches and interesting bits of infor- 

 mation about plankton collected during the 

 author's many years of work. 



Most of the information about plankton 

 gained during the past has come from col- 

 lections made with plankton nets (conical 

 cloth parachutes) dragged through the water. 

 However, the openings between the meshes 

 are large enough, particularly in a new net, 

 to permit the escape of large numbers of 

 small forms of phytoplankton. A better 

 method of obtaining a representative, al- 

 though smaller, sample is by use of a closing 

 type of bottle which collects all forms except 

 some zooplankton that is capable of escap- 

 ing. After the organisms have settled to the 

 bottom the top water is decanted and the 

 bottom portion further concentrated by 

 centrifuging (Allen, 1939). 



Phytoplankton, classed as yellow-green 

 algae by Sverdrup, Johnson, and Fleming 

 (1942, p. 295), is dominated by diatoms that 

 occur locally off southern California in 

 numbers of more than a milhon cells per 

 liter of sea water. Diatom cells were 

 counted by W. E. Allen for 690 deep-water 

 stations occupied during 23 cruises in the 

 years 1938, 1939, 1940, and 1941 (Sverdrup 

 and Staff, 1942, 1943, 1944, 1947). The 



maximum and average concentrations at 

 several selected depths beyond the shelf 

 are given in Table 9. These yield an over- 

 all average of 39,600 cells per liter in the 

 top 60 meters, with greatest concentrations 

 during spring (Fig. 131). In contrast, the 

 average number of cells in the top 50 meters 

 at 475 stations on the mainland shelf be- 

 tween Point Conception and Mexico between 

 March 1957 and April 1958 was found to 

 be only 1850 cells per liter by Johanna 

 Resig of Hancock Foundation, who used 

 methods similar to those of Allen. The dif- 

 ference in the two studies appears to be a 

 result of shelf versus deep-water environ- 

 ment, because Allen's nearshore stations 

 have lower populations than his offshore 

 stations. In both studies the maximum con- 

 centration of diatoms was commonly en- 

 countered at depths of 20 or 30 meters, and 

 this maximum corresponds to a depth of 

 minimum transparency of the water to 

 hght (Young, 1939). 



Table 9 



Concentration of Diatoms of Open Sea 

 Compiled from Data of W. E. Allen 



Qgp^j^ Number of Cells per liter 



meters Maximum Average 





 20 

 40 

 60 



2,864,000 

 3,284,000 

 2,270,000 

 1,080,000 



57,400 

 55,100 

 34,400 

 11,600 



Diatoms consist of unicellular individuals 

 having a thin-walled shell or frustule com- 

 posed of a type of opal and are commonly 

 linked together into chains or groups. The 

 wide variety of shapes has given rise to per- 

 haps 12,000 named species (Cupp, 1943). 

 Since they have no means of locomotion 

 and yet must stay in the euphotic zone to 

 live, diatoms have developed several methods 

 of delaying their sinking time. Some, such 

 as Coscinosira, Leptocylindrus, Rhizosolenia, 

 and Nitzschia, form long hair-like chains 

 (Fig. 132) which must sink sideways. This 

 tendency toward increasing the water re- 

 sistence is carried much further by other 

 types like Chaetoceras which have many 



