448 



THE COMMUNITY 



variation in latitude, to have a uniform en- 

 vironmental background, even in open 

 ocean. Thus the great variety in habitats, 

 with their concomitant variety in environ- 

 mental stratification, is paralleled by an 

 equally great diversity in the details of 

 plankton composition (Bigelow, 1925; Al- 

 len, 1934). 



The uppermost stratum of the sea is 

 termed the photic zone (p. 124). In the 

 photic zone the upper 500 meters of water 

 absorb all the red component of light, while 

 the shorter wavelengths, such as the blue 

 and violet, extend to greater depths (p. 

 125). Below the photic zone the water 



diatoms, dinoflagellates, coccolithophores, 

 and a few species of green algae. The 

 abundant and characteristically diversified 

 zooplankton is also primarily resident in 

 the upper portion of the photic zone. 

 Marine plankton in these first few hundred 

 meters has been examined intensively, and 

 the productivity and variation within this 

 stratum can be studied by consulting the 

 literature (Murray and Hjort, 1912; John- 

 stone, Scott, and Chadwick, 1924; Bigelow, 

 1925; Allen, 1934; Pavillard, 1935; Sver- 

 drup, Johnson, and Fleming, 1942; Coker, 

 1947; with their several bibliographies). 



-J 80,000 



5 60,000 — 



Fig. 155. Amounts of total plankton In the upper 50 meters of the South Atlantic. 



Sverdrup, Johnson, and Fleming.) 



'After 



rapidly becomes less illuminated, until at 

 5578 feet sensitized plates are unaflfected 

 after an exposure of two hours. The aphotic 

 zone below about 1500 meters is essentially 

 dark, although it may be faintly illumi- 

 nated by luminescent fishes or other nekton 

 which can exist under great pressures 

 (Beebe, 1934). The aphotic zone is in- 

 di£Ferently known when contrasted with 

 our information on the photic zone and 

 may be of vast depth, as in some areas of 

 the Pacific Ocean where it embraces a ver- 

 tical layer of 9500 meters. 



The aphotic zone continues to the sea 

 floor, where both active and sessile ben- 

 thos and nekton exist under great pres- 

 sures. This stratum is discussed later under 

 Horizontal Stratification. 



Returning to the upper levels of the 

 gradient, it is clear that the photic zone is 

 not uniformly populated (Fig. 155). Con- 

 servative writers place the lower photo- 

 synthetic limit at 200 meters. This limits 

 the phytoplankton, consisting largely of 



Study of vertical distribution in the sea 

 is best accomplished by examination of a 

 limited taxonomic group. Marine dino- 

 flagellates are excellent material since they 

 are widespread, numerous in species, and 

 characteristic of the photic zone. In this 

 group the genus Ceratium is famihar. 

 Karsten (1907) first suggested that cer- 

 tain marine plankters inhabit the lower 

 strata of the photic zone, and these spe- 

 cies he described as a "shade flora." Niel- 

 sen (1934), reporting on collections made 

 by the Dana, found that about one-third 

 of the species of Ceratium in the southern 

 Pacific inhabit the lower levels of the 

 photic zone and should be called "shade 

 species," while two-thirds inhabit the up- 

 per relatively well-illuminated levels. Niel- 

 sen found that general plankton density 

 affects the vertical distribution of the shade 

 species. In areas where the plankton is 

 rich the shade species live at higher levels, 

 presumably because the abundant plank- 

 ton absorbs so much light that the shade 



