500 



maintain local units of environment within a body of water. 

 Lakes full of vegetation, like Quiver Lake, exhibit greater 

 variations in the local distribution of the plankton than are 

 found in open ones, such as Thompson's Lake. Greater differ- 

 ences in the component organisms also appear. The vegeta- 

 tion thus acts as a barrier, isolating differing assemblages of 

 organisms. Thus, in Quiver Lake in one instance local aggre- 

 gations or swarms of Volvox, of Copepoda, of Oscillaria, and of 

 Melosira were detected in the examination of the local dis- 

 tribution of its plankton. To this isolation by the vegetation 

 may also be attributed the considerable irregularity in the sea- 

 sonal fluctuations of the amount of plankton, which is some- 

 what more evident in the planktographs of Flag Lake (PI. 

 XXXIII. and XXXIV.) and Dogfish Lake (PI. XXX.-XXXII.) 

 than in those of other stations. Such fluctuations, for ex- 

 ample, as those in May, 1896, in Flag Lake, when the plankton 

 fell from 203.52 cm. 3 to 0.72 in 13 days, or the fluctuations in 

 Dogfish Lake in 1895, which do not seem to be correlated with 

 any fluctuating feature of the environment, may be referred in 

 part to the isolation resulting from vegetation and the modifi- 

 cations of food supply and reproduction consequent upon it. 

 The maximum-minimum contrast in Flag Lake was due to an 

 excessive local development of Bosmina followed by its sudden 

 disappearance. The cycle of changes in the succession of life 

 are thus accentuated, and run a more rapid course in the midst 

 of vegetation than they do in the larger unit of environment, 

 the open water, where minor differences are quickly merged by 

 the turmoil of current and waves. 



The plankton catches made in vegetation-rich lakes usually 

 contain a larger proportion of littoral and bottom-loving spe- 

 cies than those from open water. There are the Rhizopoda— 

 often those with the heavier shells — the attached diatoms, cili- 

 ates, and rotifers, together with many bdelloid and Ploiman 

 rotifers not found in open water, the aquatic insects, both 

 adult and larval, the oligochgetes, the smaller mollusks, Hyalella, 

 and Hydra. They materially increase the volume of the 



