502 



THE COMMUNITY 



is the "punti verdi" (Halosphaera viridis) 

 of the Mediterranean fishermen. 



3. Coccospheres or coccoHthophores 

 (Coccolithoporidae) are poorly known 

 since they are so minute in size that they 

 must be collected by centrifuge. Although 

 they pass through the finest nets, they are 

 considered as constituting a large propor- 

 tion of marine phytoplankton. Their bodies 

 contain calcareous plates or processes typi- 

 cal of the abyssal Globigerina ooze. They 

 are widely distributed, are sparse to absent 

 from polar seas, and are especially charac- 

 teristic of tropical and subtropical seas. 

 (The interested reader will find Calkins, 

 1926; Coker, 1947; Kudo, 1931; and Rus- 

 sell and Yonge, 1928, of service in their 

 further study.) 



Fig. 166. Interrelation of the seasonal cycle 

 in abundance of diatoms, light intensity, 

 phosphates, and nitrates in the open North 

 Atlantic. (From Park, Allee, and Shelf ord, 

 after Russell and Yonge.) 



4. Peridinians (Dinophyceae, Dino- 

 flagellata) are abundant in the photic zone, 

 important in photosynthesis and contain 

 such well-known genera as Ceratium, dis- 

 cussed previously (p. 448). Their important 

 place in the marine food web has been 

 much studied by Bohm (1931), Graham 

 (1941), Gran (1912), Jorgensen (1920), 

 Kofoid and Swezy (1921), Nielsen (1934) 

 and Peters (1934). 



5. Diatoms (Bacillarophyceae) proba- 

 bly are the most important taxonomic group 

 in the marine photic zone from the point 

 of view of carbohydrate anabohsm in the 

 vast marine community. They have been 

 choS'Pn for especial mention. The area to 



be discussed is the open North Atlantic, 

 where the characteristics of the annual 

 population cycle have been shown to be 

 expressions of a sensitive response to the 

 operating factor complex of the photic zone 

 (Russell and Yonge, 1928). For conven- 

 ience of presentation, this diatom cycle will 

 be discussed with reference to the four 

 seasons, as indicated in Figure 166. 



Winter 



The surface water is as cold as or colder 

 than the aphotic layers; there is no region 

 of temperature transition; hght intensity is 

 minimal (1000 to 2000 foot-candles or 

 less.) Under these conditions the inorganic 

 nitrates and phosphates, which have been 

 produced through the bacterial industry in 

 the intermediate layers, now diflFuse up- 

 ward, under partial pressures, until the 

 distribution of these salts is relatively uni- 

 form. This accumulation of raw protein 

 precursors is possible since there is insufii- 

 cient light for large-scale diatom photo- 

 synthesis; consequently the diatom popu- 

 lation is minimal. 



Spring 



By March or April the upper layer of 

 the photic zone warms up. This warming 

 process is progressive, and by May or June 

 a transitional temperature zone (partial dis- 

 continuity layer) forms at between 10 and 

 20 meters (p. 94). Light intensity in- 

 creases rapidly. This rise in illumination 

 after the renewal of needed mineral nutri- 

 ents in the surface waters makes possible 

 the dramatic reproduction of diatoms 

 known as the "spring pulse." This vernal 

 increase is chiefly responsible for the an- 

 nual yield of diatoms, and is indirectly re- 

 sponsible for the great productivity of the 

 sea. For example, in the English Channel 

 oflF Plymouth, the annual diatom crop is 

 5.5 tons (wet weight) per acre of sea sur- 

 face. This is a minimal weight figure. The 

 vernal pulse accounts for one-third to one- 

 half of this total and is of high significance 

 in the food web of the marine community. 

 The salts accumulated through the winter 

 months are suflBcient for the diatom metab- 

 oUsm. The light intensity is high enough 

 (6000 to 7000 foot-candles) at the surface 

 of the water for several hours in the middle 

 of the day to allow diatom photosynthesis. 



