and can be expressed only at night (Parker 1902). 

 An alternative explanation is that zooplankton ac- 

 tively orient to a band of optimum light intensity and 

 move up and down at different times to avoid light 

 of too great or too little intensity (Gushing 1951, 

 Hardy and Bainbridge 1954). 



These diel movements are most widespread 

 among Cladocera and Copepoda, but other species 

 are also involved. One of the most interesting cases 

 is the dipteran larva Chaobonis pnnctipennis that 

 rests on the lake bottom during the daylight hours 

 but is often teeming in the surface waters at night. 

 It appears that the buoyancy of this larva varies with 



the size of its two pairs of air-sacs (Damant 1924). 

 There are a few rotifers, Mysis among the Mala- 

 costraca, and Ceratiinn among the Mastigophora, in 

 which vertical day and night movements have been 

 demonstrated (Pennak 1944). 



Seasonal distribution 



The different species of plankton vary in their 

 response to seasonal changes in the physical and chem- 

 ical nature of the water, in number of generations per 

 year, and in time of occurrence. Accordingly there 

 is a marked seasonal variation in total numbers dur- 



ZOOPLANKTON PER LITER 



25 50 75 



5000 4000 3000 2000 



1000 1000 2000 

 ALGAE PER LITER 



3000 4000 5000 6000 



FIG. 6-6 Vert 

 oligotrophlc I 

 Note that the horizontal 



distribution of net plankton (left) in an 



and (right) in a eutrophic lake, Wisconsin. 



different for the two lakes, and 



for the algae as compared with the looplankton. The cross- 

 hatched horizontal belts show the region of the thermocline 

 (from Birge and Juday 191 I). 



68 Habitats, communities, succession 



