496 



of vegetation on the distribution of the plankton. Only the quan- 

 titative data are at present available, and the results are con- 

 flicting. In some cases the plankton is greater in the vegeta- 

 tion than in the adjacent open water ; in others the reverse is 

 true. These examinations were made at times of unstable 

 river levels, and the movements of water consequent thereupon 

 make any satisfactory analysis difficult. The general conclu- 

 sion that lakes full of vegetation (Quiver) are everywhere 

 poor in plankton, while those relatively free from it (Thomp- 

 son's and Matanzas) support generally a more abundant plank- 

 ton is in all cases upheld by these examinations. 



This poverty of the plankton in vegetation-rich lakes was 

 one of the surprises of our investigations, and, so far as I have 

 been able to ascertain, it contradicts the general expectation 

 among observers of aquatic life. It has its parallel in the pau- 

 city of life in tropical forests and among the pines and red- 

 woods of the Sierras. It is fundamentally a problem of nutri- 

 tion, and inheres in the utilization of the available food supply 

 by a single type, or a few types, of plants which do not them- 

 selves in turn afford support for an abundant or varied animal 

 life. 



Wherever the depth of the water, the currents, the winds, 

 or other factors, prevent the development of a submerged 

 aquatic flora, the nutrient materials for plant growth^the oxy- 

 gen, the carbon dioxid, the nitrates, phosphates, sulphates, and 

 carbonates dissolved in the water — are utilized by the phyto- 

 plankton, which, in turn, supports the zooplankton. The en- 

 tire production of such a lake takes the form of plankton and, 

 in turn, of those larger species, insect larvae, mollusks, and fish, 

 which are directly or indirectly supported by it. When, on the 

 other hand, the conditions are such that a submerged non- 

 rooted aquatic flora obtains possession of a lake, — as, for exam- 

 ple, CerafopJii/lluin and Elodea in Quiver Lake, — these nutrient 

 materials are appropriated by it to the great reduction, even 

 practical exclusion, of the phytoplankton. In the struggle 

 which must ensue between the phytoplankton and the sub- 



