The lake with its plant and animal inhabitants presents a sufficiently unified bio- type that it may, with certain reservations, be termed a microcosm. If we consider it as a living unit we may also characterize its complex internal exchanges as metabolism. The biota in question and its activities are to a large extent controlled by physical and chemical influences both internal and external to the lake. It is thus desirable that we should begin with a consideration of these primary physical and chemical factors.
In reviewing so broad a field it has been necessary to select factors which are considered more fundamental and the emphasis is on their interrelations rather than specific effects. The subject of light and he direct influence of various factors on organisms in the lake will be considered in other papers of the series.
THE INTERRELATION OF FACTORS
In approaching the question of physical and chemical factors influencing the metabolism of a lake we might ask certain significant questions. What are these factors? Which of them are most important? Can their separate effects be recognized? How are they interrelated?
In answer to the first question, there are many factors and listing them would serve no useful purpose at the moment. Their relative importance and separate effects will be demonstrated in some measure in our later discussions. With respect to the last question the factors are interrelated in very many ways. The logical relations are so complex that we have been led to construct the accompanying chart (Fig. 1), as a graphic aid to our discussion.
Without considering the chart in detail you will note in a general way how, starting with geographic location, we have indicated various chains and complexes of factors which have a part in determining the trophic nature and biological productivity of the lake. While the scheme is elaborate it is still not complete. No attempt has been made to indicate the relative importance of particular factors, although something of the sort might be possible with respect to individual lakes rather than for lakes in general. Possibly each investigator should construct his own chart. In any case we think it desirable to visualize something of the maze of relations before entering on the separate discussion of factors as we shall now proceed to do.
THE MORPHOLOGY OF THE LAKE BASIN
The effects of the physical dimensions of a lake on its population and metabolism is a matter of common observation. The fundamental nature of this relation was expressed by Thienemann in 1927 when he demonstrated that the nature of the lake basin and especially its mean depth, was a most important factor in determining lake type. It should be noted, however, that the effects of morphology are often indirect, involving secondary factors such as temperature, oxygen, and the production of nitrogenous materials. The effects may also be greatly modified by the geological nature of the surroundings.
The mean depth of a lake is probably, as Thienemann believes, its most significant dimension. After examining data from many lakes he concluded that most eutrophic lakes had mean depths of less than 18 meters and oligotrophic more; also that the volume of the epilimnion was greater than the hypolimnion in eutrophic and smaller in oligotrophic lakes. In spite of . . .