336 ANIMALS IN INLAND WATERS 



limnetic forms. Only one insect, however, is truly limnetic, the so-called 

 phantom larva, Corethra plumicornis (Fig. 102). 



To the truly limnetic animals may be added a number of more or 

 less irregular visitors in the open water. While Bosmina coregoni is 

 limnetic, Bosmina longirostris is permanently so only in the smaller 

 bodies of water, and is only casually limnetic in true lakes. 27 The same 

 is true of a number of other Cladocera and of a few rotifers. 



The amount and composition of the plankton are very variable 

 according to the prevalent environmental conditions in various loca- 

 tions, as well as in the same body of water at various depths and at 

 various times. Very poor ponds or deep, cold lakes often contain only 

 5-10 cc. of animal plankton in 1 cu. m., in contrast with small fertile 

 village ponds which may, exceptionally, contain 1500-2500 cc. 28 On 

 the other hand, the amount of plankton in the Dobersdorf pond 

 (Holstein) varies between 136 and 3977 cc. in 1 cu. m. of water, and 

 in Lake Plon between 13 and 424 cc. during the course of one year. 29 

 The annual yield of plankton in the well-studied Lake Mendota 

 (Wisconsin) is of the order of 5 tons of dry matter per acre; the mean 

 standing crop is approximately 200 pounds of dry stuff per acre. 30 



The plankton of neighboring water basins often shows a consider- 

 ably different character, e.g., in the lakes of Switzerland and the 

 French Jura, 31 or in the different Alpine border lakes; thus all Daphnia 

 and Bosmina are missing in Lake Brienz but their place is taken by 

 Diaphanosoma, which occur in unusual numbers. Unlike that of most 

 other communities, the fresh-water plankton reaches its highest devel- 

 opment, not in the tropics, but in the temperate zones. 



The transparency and coloring of the water are influenced by the 

 amount and composition of the plankton. Very transparent blue water, 

 as, for instance, that in Lake Como, is a sign of scarcity of plankton; 

 the richer the plankton, the more turbid and colored the water. The 

 tinge of the color depends on the composition of the plankton, above 

 all upon the most prevalent algae. Two different types of phyto- 

 plankton can be distinguished in our regions: in one of them the 

 diatoms are most common, in the other the blue-green algae. 32 These 

 two groups of algae seldom have their greatest development in the 

 same lake or at the same time. The optimum for most fresh-water 

 diatoms is at relatively low temperature, about 12°, while the blue- 

 greens thrive best in the higher summer temperatures (19° to 23°). 

 The former are, therefore, most prevalent in the cold northern or 

 Alpine lakes where the blue-green algae, with few exceptions, are 

 almost totally absent; in warmer, deeper lakes they occur only in 

 spring or in late fall or at greater depths. The blue-green algae prevail 



