124 



PROBLEMS OF LAKE BIOLOGY 



profuiidal zone extends from the greatest 

 depth . . . np the slope toward shore to a 

 level somewhat above that corresponding 

 to the average npper limit of the hypo- 

 limnion. The snblittoral lies between these 

 two." 



Obviously, under the specifications of 

 these definitions only those lakes which 

 stratify thermally would have a profundal 

 zone and in many shallow, clear-water lakes 

 the whole benthic habitat would lie within 

 the littoral. This is certainly true for tem- 

 perate lakes of the second and third orders 

 respectively. The conditions which typify 

 the profundal are not established in the 

 warm, shallow, well aerated lakes and 

 second order lakes normally stratify. Tem- 

 perate lakes of the first order frequently 

 show thermal stratification with true ther- 

 mocline formation and consequent stagna- 

 tion. Moreover, except perhaps in the very 

 largest lakes of this order, as for example 

 the Great Lakes of this continent, first 

 order lakes quite typically have very short 

 vernal and autumnal circulation periods 

 and may even exhibit incomplete turnover. 

 These facts and others result in the forma- 

 tion of true profundal conditions beneath 

 the deeper waters of first order lakes and 

 the upper limit of the profundal zone is, in 

 such lakes, approximately coincident with 

 the average upper limit of the hypolimnion 

 when thermal stratification does occur. 



The snblittoral is everywhere a transition 

 zone, and being so lacks the positive char- 

 acteristics of the other major zones. It is 

 really a part of Muttkowski's aphytal zone 

 but is, nevertheless, distinctly differenti- 

 ated from the true profundal. Muttkovrski 

 included it as a subdivision of the litto]-al 

 under the term "dysphytal." It is, as that 

 writer said (1918), ''a region of variable 

 extent, a region of heaping refuse. ' ' Within 

 its boundaries, as now understood, may 

 frequently be found a belt-like burial 

 ground for the dead shells from vast mol- 

 lusean populations in the littoral above. 

 This region is called the shell zone. Lund- 

 beck (1926) found it commonly in the 

 north German lakes. Scott discovered it in 

 some of the Indiana lakes. Muttkowski 



described it from Lake Mendota. Miyadi 

 noted its occurrence in certain Japanese 

 lakes. The author has seen it in many 

 small inland lakes and found abundant evi- 

 dence of its existence along the shelf bound- 

 ing much of the shoreward waters of Lake 

 Michigan. 



Where the snblittoral leaves off the pro- 

 fundal begins. From the very nature of 

 conditions it is to be expected that there 

 should be no sharp line of demarcation be- 

 tween the two. Those forces which estab- 

 lish the conditions typical of the profundal 

 are not static. The level at which the ther- 

 mocline stands fluctuates with the advance 

 of the season and with other factors within 

 a season. There is thus a region exposed 

 alternately to conditions of the hypolimnion 

 and to those of the thermocline. Moreover, 

 this is the very stratum of the basin 

 wherein the other conditions of the pro- 

 fundal are apt to be blended with those of 

 shallower regions. The bottom deposits, 

 where a sandy littoral merges into a muddy 

 profundal, for instance, shade almost im- 

 perceptibly from clean sand through 

 muddy sand and sandy mud into the mud 

 of the lower levels. But to attempt to indi- 

 cate exactly where the transition ceases, 

 where true profundal first definitely occurs, 

 is both futile and undesirable. Once well 

 within the profundal, however, one dis- 

 covers a definite combination of circum- 

 stances which characterize the area. The 

 temperature is permanently low; pressures 

 are increasing; dissolved oxygen is lacking 

 throughout some part, often much, of the 

 3'ear ; hydrogen-ion concentration frequently 

 passes into the acid range ; there is little or 

 no light ; and very often there is an accumu- 

 lation of carbon dioxide, methane, and 

 other gases of decomposition. 



Historical Resume 



It has often been asserted that limnology, 

 as a separate, organized division of science, 

 is less than half a century old. Some of its 

 subdivisions naturally are younger still. It 

 is true, of course, that certain general facts 

 concerning the organisms which live in and 

 on the bottom of lakes, at least in the more 



