FRANK E. EGGLETON
DEPARTMENT OF ZOOLOGY AND UNIVERSITY BIOLOGICAL STATION, UNIVERSITY OF MICHIGAN, ANN ARBOR, MICH.
IN the microcosmic economy of a lake, each division of the inanimate habitat, each category of environmental factors, each realm of the biota, forms a structural and functional part of the composite whole. If any one characteristic of lacustrine ecology is more often apparent to the limnologist than any other, it is this interdependency of the physical, chemical, and biological phenomena whose constant interplay weaves a complex design in the fabric of the life of inland waters. The behavior pattern of organisms is everywhere determined by the stimuli which impinge upon them. These stimuli are, in their turn, conditioned by the intrinsic nature of that division of the inanimate habitat within which an organism lives.
Beneath the waters of a lake lies one of the two major categories of lacustrine habitats, the lake floor. There, as elsewhere, the effectiveness with which environmental factors play upon the inhabiting organisms is modified by the particular nature of the immediate environment. Forces inherent in the nature of the substratum itself bend and shape all other forces and thus condition the reactions of the biota. Life in the realm of the benthos is complex, variable, and intricately interwoven with that of the lake as a whole.
Although it is perfectly proper thus to refer to the entire lake floor as one great subdivision of aquatic habitats, it is not to be supposed that there is any marked uniformity of detail in the conditions of existence over the whole basin. Diversity is one of the chief characteristics of fresh- water situations and the benthic type of habitat is no exception to the general rule. Many of the most potent environmental factors vary markedly with depth and this fact has served as the basis for what is undoubtedly the most widely accepted classification of benthic zones.
Several systems have been proposed at various times but within the last decade the one which has come to enjoy nearly universal acceptance divides the lake floor into three major zones, the littoral, sublittoral, and profundal. A fourth major zone, the abyssal, proposed at an earlier date, has found little favor and less application. This fate seems only natural from the fact that its sponsor proposed to limit it to those great depths, as fresh waters go, below 600 meters. It may be that when the two or three lakes in the world known to have such tremendous depths are more adequately studied we shall have need for the term. Until then the mere fact of depth alone seems insufficient reason for applying it. Unless some significant change can be shown to occur in the benthic habitat or the benthic fauna at that or some other depth there seems small justification for setting off the abyssal from the deeper profundal.
Although limnologists have very generally accepted this terminology for the benthic zones, they have been far less in agreement concerning the exact definitions and limitations of the terms. In 1931 the speaker proposed certain definitions which have enjoyed some degree of acceptance. They were stated as follows:
" . . . the littoral zone of the bottom is . . . that region lying between the shore line and, approximately, the lakeward limit of (rooted) aquatic vegetation. The