256 



SCIENCE. 



[N. S. Vol. XI. No. 268. 



ally free from vegetation ; (5) Phelps Lake, 

 an ephemeral body of water without vege- 

 tation ; (6) Flag Lake, a large swamp, 

 choked with vegetation, rarely drying up in 

 the summer. At times of high water all 

 these localities are submerged. The quan- 

 titative and qualitative examination of the 

 collections, as yet incomplete, indicate the 

 following more or less tentative conclu- 

 sions : 



1. The waters contain a typical fresh- 

 water plankton, that is, one composed of 

 limnetic organisms usually cosmopolitan 

 and identical with or closely related to the 

 plankton organisms of larger bodies of 

 water. It has much in common with the 

 plankton of German streams, and at low 

 ■water is remarkably like that of the Nile 

 at a corresponding stage. The admixture 

 of littoral forms in the open water of the 

 lakes and in the river is surprisingly small 

 in the number of individuals, though pre- 

 senting a considerable range of species. In 

 all about 500 diiferent species have been 

 found, of which at least one-third may be 

 called limnetic. 



2. There is a mai'ked seasonal variation 

 in the amount of the plankton. A spring 

 maximum, usually in May or June, follows 

 the winter minimum and is in turn followed 

 by an apparent mid-summer minimum in 

 August. An autumn maximum of second- 

 ary importance precedes the return to the 

 winter minimum. These statements are 

 based, as usual, upon catches made with 

 the silk net and are subject to the error 

 arising from the leakage of the smaller or- 

 ganisms through the silk. The correction 

 of this error by supplementary methods 

 tends to eliminate the summer minimum 

 and to augment the autumn maximum. 

 The volume of plankton in the spring maxi- 

 mum is from 20 to 50 times that of the 

 winter minimum. These fluctuations are 

 repeated from year to year, but vary in time 

 of appearance, in extent, and in relative 



development in successive years and in 

 different waters. Irregular fluctuations 

 also of great extent may occur in any local- 

 ity especially during the summer and fall. 



3. There is a marked seasonal variation 

 in the organisms composing the plankton. 

 During the winter minimum the chlorophyll 

 bearing forms are relatively few, the plank- 

 ton being composed of a few hibernal and 

 a smaller number of perennial species prin- 

 cipally of Ehizopoda, Copepoda and Eoti- 

 fera. The presence of ice on the surface, 

 and the low temperature of the water (0°- 

 4° C.) do not interfere with the growth and 

 reproduction of this winter plankton. Foul- 

 ing of the water by an excess of sewage 

 under the ice may, however, exterminate 

 the animal life of the water. As the tem- 

 perature rises, the brown flagellates and the 

 diatoms increase in number and subse- 

 quently the green flagellates and other 

 chlorophyll-bearing organisms multiply rap- 

 idly. This is accompanied by a marked 

 development of animal forms, especially 

 Cladocera and Rotifera, resulting in the 

 spring maximum. This culmination is 

 rarelj' due to the excessive development of 

 a single species, but is typically polytonic. 

 A rapid decline of the vernal species, princi- 

 pally Entomostraca and diatoms, produces 

 the midsummer minimum, characterized by 

 the relatively small number of individuals, 

 and the very large number of species, prin- 

 cipally Rotifera and the smaller green 

 flagellates. The autumn maximum ap- 

 pears after the equinoctial rains and is 

 often composed in large part of Synchseta, 

 Synura, and diatoms. This is also the 

 period of maximum numbers of the ciliate 

 Protozoa. The total number of species in 

 the fall plankton is not large and the pre- 

 dominant forms are few. 



No two years present throughout the 

 same sequence of species in the same rela- 

 tive numbers. In some instances, species 

 abundant in one year have not been found 



