COMMUNITY ORGANIZATION: PERIODISM 



555 



and Fleming, 1942). Similarly, such a 

 sharp theiTnocline may limit the lower 

 component in some species populations; for 

 example, the copepod, Metridia lucens, as- 

 cends to the thermocUne, but not through 

 it, in the Gulf of Maine, where the thermo- 

 cUne has an approximate gradient of 7° C. 

 in 10 meters, and another copepod, Centro- 

 pages typicus, inhabits the upper stratum 

 of warm water (10 to 20 meters in thick- 

 ness), but does not pass through the ther- 



logical state which causes migrations even in 

 the absence of light (Esterly, 1917). 



"Since an animal is constantly subjected to 

 various and simultaneous stimuli, some of 

 fluctuating intensity, the nature of the move- 

 ment will depend upon the combination at any 

 one time, and it is not always clear which is 

 the dominating stimulus." 



Pelagic copepods and chaetognaths have 

 been studied especially in these marine ver- 

 tical movements, but such movements are 



Fig. 195. Diagram of vertical diel migration of three species of marine plankters in the North 



Atlantic. (After Russell and Yonge. ) 



mocline, into water of from 8° to 5° C. 

 (Clarke, 1933). 



Consequently, the normal range in depth 

 of the vertical movement, and the time of 

 the movement, vary wdth the species re- 

 sponse to many physical and biological fac- 

 tors. This complexity is summed by Sver- 

 drup, Johnson, and Fleming (1942, p. 

 837): 



"Not only do difiFerent species react differ- 

 ently, but different stages of development, and 

 the sexes also, have their own characteristic be- 

 havior with respect to diurnal [diel] response 

 associated with hght. A further complication 

 is encountered in that the degree of response is 

 also seasonal, indicating changes in physio- 

 logical state (Russell, 1928). Species of Acartia 

 give some indication of a diurnal [diel] physio- 



not restricted to species inhabiting the up- 

 per layers of water. Thus, certain sergasid 

 prawns have a maximum population den- 

 sity at between the sea surface and 200 

 meters at night, but during the day move 

 down to between 600 and 800 meters 

 (Welsh, Chace, and Nunnemacher, 1937). 

 Diel vertical movements of plankton in 

 fresh-water communities show the same 

 general pattern as that discussed for the 

 marine community, and the problem is 

 summarized by Welch (1935, p. 232) : 



"Not one factor but several factors interact 

 in the production of some, and probably all, 

 forms of diurnal [diel] movement. The prin- 

 cipal factors involved appear to be light, tem- 

 perature, food and gravity. It is also possible 



