COMMUNITIES IN STANDING WATERS 



33c 



partly, though to a less extent, forms which demand organic food 

 material. They are so plentiful that in Lake Lunz 160 individuals of 

 the nannoplankton were counted to every 3 of those caught in nets, 

 and in spite of the immense difference in size, the yearly average 

 volume of the nannoplankton is three times as great as that of the 

 larger plankton. 24 Many of the latter animals are restricted partly or 

 entirely to the nannoplankton for their food, and the mass of the 

 latter sometimes controls the number of the former (see Fig. 98) . Large 

 plankton animals, of course, also eat diatoms and other of the larger 

 algae. As the development of plant elements in the plankton is limited, 

 among other factors, by the amount of dissolved nutritive material in 

 the water, this is indirectly also responsible for the total amount of 

 plankton present in a body of water (cf. p. 239) . This is one reason 

 why shallow waters in general are richer in plankton than deep waters; 



6500 



4000-1 



2100 



850 -I 



250 

 30 



11. 18. 25. 4. 1T 19. 26. 2. 10. 17. 24. 31. 7. 14. 21. 29. 5. 14." 



vi. vn. vm. ix. x. 



Fin. 98. — Amounts of rotifers ( ) and of nannoplankton ( ) in waters 



in Saxony, from June to October. Temperature curve ( ). After Dieffenbach. 



small lakes richer than large ones ; above all, why those which lie over 

 fertile ground, over limestone, etc., or receive other plentiful nutrient 

 materials, 25 are richer in plankton than those in relatively sterile sur- 

 roundings such as exist in granitic mountain regions. 



The animals of the open water, compared with their relatives in 

 the littoral regions, are considerably in the minority, a fact which can 

 readily be explained by the peculiar adaptations which a purely pelagic 

 life demands. Of the 66 phyllopods of southeastern Germany, only 11 

 occur as limnetic forms; of the 67 crustaceans of Lake Balaton, only 

 8 are limnetic; and of the 35 rotifers, only 10. Moreover, among the 

 numerous forms of animals and plants in the plankton there are only 

 a few species which are so prevalent that they detennine its character. 

 Of a total of approximately 150 species of plankton organisms of the 

 Danish lakes only a few appear in such numbers that they create a 

 monotonous animal plankton: of animals only the species of Diaptomus 

 among copepods, a number of cladocerans, and a few rotifers. 26 Besides 

 Crustacea and rotifers, the water mites furnish a number of species of 



