uncolored lakes, i.e., those with little allochthonous materials, the percentage was as high as 23. We may point to this as a demonstration of dystrophy as a disturbing element in the metabolism of lakes. It cuts across the normal gradation from eutrophy to oliogotrophy and thus should not be considered as indicating a third lake type. It has been suggested that these organic materials of allochthonous origin not only fail to support production but they may actively inhibit production by adsorbing inorganic materials, e.g., phosphorus, of nutritive importance.
The comparative constancy of dissolved organic materials in lakes is in striking contrast with the high variability of the same elements in their inorganic state. Birge and Juday ( 1927) found little correlation between amounts of the two groups of substances except in lakes where both were present in very small quantities. A part of this constancy may be apparent rather than real since the quantities of organic materials are much larger than the inorganic and the metabolic exchanges make lesser inroads into this large capital.
The immediate temptation is to hope that the dissolved organic matter will provide an index to productivity. This could only be considered in lakes where the dystrophic factor is known to be at a minimum. Even in these cases we still face the objection that productivity must be determined not by the amount of organic capital but by the rate of turnover. Our final statement is that which Birge and Juday have made on various occasions. We do not know the function of the dissolved organic matter in the biological economy of the lake.
Our discussion began with an attempt to organize the physical and chemical factors in the metabolism of a lake and a chart (Fig. 1) is presented to suggest these interrelations. As a second step we have considered a number of these factors summarizing and attempting to evaluate present knowledge in this field. In doing so we
Oben: Milieu eines Biotops
a) zur Zelt der Voltzirkulation,
b) während der Sommerstagnation, α) Epilimnische Kurve,
β) Hypolimnfsche Kurve.
Mitie: Valeazspektrum eines autotrophen planktischen Organismus. -- Unten: Beide Spektra (Milieu während der Sommerstagnation)
kombiniert. -- Weiteres s. Text!
FIG. 6 . The ecological total spectrum of Grote ( 1934).