The Plankton Algae of the Palisades Interstate Park iii 



Chroococciis lim^eticus (PL 3, Fig. i), Sphaerocystis Schroeteri 

 (PL 7, Fig. 2), Trichodesmium lacustre (PL 4, Fig. 3), various 

 Anabaena species (PL 4, Figs. 4-6), the Micractinieae (PL 9), 

 or certain of the Desmidiaceae (Pis. 13-22) may be cited as examples 

 of eulimnetic organisms. In all of these, flattened colonies, curving 

 of the filament, gelatinous bristles and the like, are adaptations for 

 the free-floating life. These peculiarities of pelagic algae have been 

 discussed by W. & G. S. West ('09) and Lemmermann (04) and 

 the conclusion is reached that a character like curving of the filament 

 does not occur whenever an organism is transferred to pelagic con- 

 ditions but is found only in those filamentous species that have 

 been living under pelagic conditions for some time. On the other 

 hand, in the case of the tycholimnetic algae, of which Tetraedron 

 mitficum (PL 8, Fig. 4), Scenedesnuis ohliquus (PL 11, Fig. 6), 

 and Closterium Kuetzingii (PL 13, Fig. 7) may be cited as exam- 

 ples, we have algae which are not adapted to the free-floating life, 

 and which do not multiply when they are accidentally carried to 

 mid-lake. The reason for certain algae living in mid-lake and others 

 finding it an unfavorable environment is probably in the greater 

 buoyancy of the eulimnetic algae. The problem of buoyancy is the 

 chief one for the pelagic organism, and this problem is all the 

 more complicated since the viscosity of the water varies at different 

 times of the year, and the organism must meet these changes in 

 viscosity. Ostwald ('02, '03) has discussed at length the physical- 

 chemical changes of water at different temperatures ; while Wesen- 

 berg-Lund has observed the changes of length of cell in colonial 

 diatoms to meet this varying- environmental factor. 



The facultative planktonts are best represented by several of the 

 Pediastrum (PL 12), Scenedesntus (PL 11), and Merismopedia 

 (PL 3, Figs. 4-6) species. These algae are frequently found in 

 collections from pools and ditches, but are also found in sufficient 

 quantity in plankton collections to show that they reproduce under 

 pelagic conditions. 



Although students of the phytoplankton recognize its distinctive 

 nature they are at considerable variance on the question of its rela- 

 tion to the size and shape of a lake. Zacharias ('98) has proposed 

 the term heleoplankton to describe the plankton community of a 

 pond in contradistinction to the limnoplankton of a lake. He states 

 that certain small bodies of water have a rather constant eulimnetic 

 flora whereas the same species are only of sporadic occurrence in 

 large bodies of water. G. S. West ('16) also recognizes the heleo- 

 plankton formation, citing the Micractinieae as examples of algae 

 that are of more frequent occurrence in ponds than in lakes. In 

 certain regions of North America, the distinction between a limno- 

 plankton and heleoplankton flora is quite sharp, but in the area 

 under consideration these distinctions are not evident. 



The physical distinction between a " pond " and a " lake " cannot 

 be made with certainty. There have been several attempts to 

 characterize bodies of water upon morphometric bases, but if such 

 distinctions are to be made they must rest upon the basis of depth. 



