128 



PROBLEMS OF LAKE BIOLOGY 



Interrelationships of the Benthic 

 Fauna 



The benthic fauna constitutes one of the 

 major links in the fundamental food chain 

 of a lake. Although lakes vary greatly 

 among themselves they also exhibit many 

 characteristics in common with certain 

 others. It is thus possible to place them in 

 definite classes. Further, certain benthic 

 organisms tend to be associated with certain 

 kinds of lakes. Recognizing these facts, 

 several limnologists, Europeans especially, 

 have attempted to classify lakes on a basis 

 of their typical or predominant benthic ani- 

 mals. Usually these have been profundal 

 forms and we consequently read of "Co- 

 rethra lakes," "Tanypus lakes," "plu- 

 mosus lakes, " ' ' Tubif ex-Pontoporeia lakes, ' ' 

 and several others. The validity of these 

 schemes, advanced by various Scandi- 

 navian, German, and Russian workers is 

 probably sound for the north European 

 lakes. Whether they, or some modification 

 of them, will be found to fit American lakes 

 remains to be seen. Little attempt has been 

 made to apply these criteria to waters in 

 the Western Hemisphere. Certain Japa- 

 nese workers seem to have found the types 

 applicable in some measure to the lakes of 

 their islands. 



Previously, in this paper, mention has 

 been made of the intimate relation between 

 type of substratum and type of fauna in- 

 habiting it. There are several categories of 

 important factors in the ecology of bottom 

 animals, for example, general type of lake, 

 major region of the lake floor, physico- 

 chemical nature of the superimposed water, 

 and the like, but within each of these major 

 realms of environmental influence the type 

 of bottom is in its own right a dominant 

 factor. It is true of course that the mere 

 presence of a flat rock outcrop within the 

 profundal zone of a deep lake will not pro- 

 duce a faunal complex in any way similar 

 to a flat rock fauna from the littoral of that 

 or another lake. But a muddy bottom in a 

 shallow bay of one eutrophic, temperate 

 zone lake will tend to support a fauna very 

 similar to that inhabiting a like situation 

 in another lake of the same general type. 



It is always necessary to remember in this 

 connection that not only may some other 

 condition supersede character of the sub- 

 stratum as a dominant factor, but that the 

 reverse may also be true and type of bottom 

 become the determining influence in other- 

 wise similar habitats. We know all too 

 little as yet concerning the exact effect of 

 different kinds of bottoms and we lack well 

 standardized and distinctive criteria for 

 judging and recording the exact nature of 

 the bottom materials. A moment's reflec- 

 tion will call to anyone's mind the wide 

 diversity in the intrinsic nature of such a 

 substance as sand, or flat rock, or nnid. 

 From what we know now it seems probable 

 that from the standpoint of selective action 

 on the fauna the major types of substratum 

 may be listed as flat rock, boulders, rock 

 and gravel, gravel, sand, clay, and what for 

 want of more discriminatory terms we are 

 compelled to lump into one great hetero- 

 geneous class and call simply mud. 



Many students of the subject, the author 

 included, have written of the pronounced 

 effect of the physico-chemical factors of the 

 superimposed water upon the life and ac- 

 tivities of the benthic animals. All these 

 influences fall into two major classes : those 

 which are inherent in the nature of water 

 itself and those others which act through 

 the water as a medium. In the first cate- 

 gorv are such characteristics of water as 

 weight, which results in pressure, relative 

 incompressibility, density, low viscosity, 

 transparency, buoyancy, and others. Within 

 the second class will fall such factors as 

 temperature, light, current; dissolved sub- 

 stances, gaseous, solid, organic and inor- 

 ganic; turbidity, color and the like. 



Each calendar year is divided, for the 

 inhabitants of a lake, into four linmological 

 seasons: spring turn-over, summer stratifi- 

 cation, autumn turn-over, and winter stag- 

 nation. The annual cycle of this chain of 

 events profoundly affects the organisms 

 which live in and on the bottom. This is 

 especially true, of course, in the second 

 order lakes within the higher latitudes of 

 the temperate zone and the lower ones of 

 the polar zone. Likewise it is of extreme 



