556 



THE COMMUNITY 



that still other factors may sometimes influence 

 the reactions." 



Table 49 gives a condensed summary of 

 certain species of zooplankton, the extent of 

 their vertical movements, speed of descent 

 in the downward movement, and speed of 

 ascent in the upward movement of these 

 migrations. Worthington's careful work 

 (1931) on the Lake Lucerne community, 

 Switzerland, serves to emphasize the differ- 

 ential response of several species popula- 

 tions to the same community factor com- 

 plex. 



©plankton are periodic changes in the posi- 

 tional organization, which affect the com- 

 munity as a whole or in part, depending 

 upon its size. 



An indirect effect on stratification by 

 vertical movements is described by Welch 

 (1935) in lake communities. Winds that 

 arise at night, when the zooplankton is at 

 or near the lake surface, drift the plankters 

 into shoal water, and many of them are 

 unable to return to the lake depths at 

 dawn. They are trapped in the surface 

 stratum, and may not return to deeper 



Table 49. Vertical Movements of Lake Lucerne Zooplankton (Modified from Worthington, 



1981) 



As the table shows, no generality will 

 cover the details of the vertical migrations 

 of all species of zooplankton, unless it be 

 that the majority have such movements, 

 the details of which vary with species and 

 age. Reaction differences between young 

 and adult organisms of the same species 

 preclude generalizations about the pattern 

 for a given species population, except 

 where such differences are known. Some 

 organisms make vertical movements only 

 when juvenile. Thus the larvae of a fly, 

 Corethra punctipennis, spend the day in 

 the profundal mud of lake communities, 

 and rise to the surface at night (Juday, 

 1921), where they prey upon infusorians, 

 entomostracans, and small larvae. 



Vertical movements of animals affect the 

 stratal organization of the community. As 

 noted in Figure 195, the upward nocturnal, 

 and downward diurnal, movements of zo- 



strata until there is a change in the weath- 

 er. Meanwhile, the stratal organization is 

 temporarily deranged. 



Again, there may be a concentration of 

 plankton at the center of the lake surface, 

 with a diminution of plankton in the pe- 

 ripheral water. This, too, may be an indirect 

 result of vertical movements in the lake 

 community. Welch (1935, p. 218) offers 

 an explanation: 



"Plankters which migrate vertically descend 

 with the onset of dawn, those nearer the shore 

 arriving at the bottom at a higher level, and, 

 following down the slope of the basin in order 

 to reach deeper water, tend to concentrate in 

 the depths; on the next trip to the surface, they 

 rise vertically from the profundal area, thus 

 producing at the surface a greater concentration 

 above the deep parts than above the peripheral 

 regions." 



