of the oligotrophic lakes). But both on submerged slides and agar plates, the kinds of bacteria found in the dystrophic lakes are obviously quite different from those found in the eutrophic-oligotrophic series. On slides, filamentous iron depositing bacteria such as Leptothrix crassa and Actino- myces ferrugineus are abundant, though they are entirely absent from the eutrophic and oligotrophic lakes. On agar plates, there are very few chromogenic bacteria, while spore-forming species are dominant, in cultures from the dystrophic lakes. It will probably be discovered that the differences between dystrophic lakes and the others, so far as bacteria are concerned, are qualitative rather than quantitative.
The numbers of bacteria in the bottom muds serve to classify the lakes roughly, i.e., they are high (on the average) in the eutrophic lakes, low in the oligotrophic ones, and intermediate in the dystrophic ones. The counts of bottom bacteria were made contemporaneously with those of the water, identical plate counting methods were used, and they represent about an equal number of samples. The two sets of data are therefore equally accurate. There is not a very good correlation between the counts for water and for bottom deposits. It follows that the numbers of bacteria in the bottom are influenced by factors other than the growth of bacteria in the overlying water.
Henrici and McCoy ( 1938) made an interesting computation of the relative numbers of bottom and water bacteria of these lakes. By multiplying the average number of bacteria per cubic centimeter in the mud by the length of the column of the sample, and similarly multiplying the average number of bacteria per cubic centimeter in the water by the depth of the lake in centimeters, there were obtained figures which roughly expressed the total numbers of bacteria in the lakes, rather than the concentration only. From these figures, the ratios of total bottom bacteria to total water bacteria were computed. The results for the eutrophic lakes were as follows: Alexander, 5.0; Little John, 1.9; Muskellunge, 0.7; mean, 2.5. For the oligotrophic lakes the following figures were obtained: Trout, 1.8; Weber, 0.3; Crystal, 0.2; mean, 0.7. The ratios for the two dystrophic lakes were very high: Helmet, 17.1; Mary, 6.1; mean, 11.6. These figures, from relatively crude data, suggest that possibly the ratios of bottom bacteria to those in the total volume of water may eventually prove to be one of the characteristics of the different lake types. This is suggestive of Rawson's (1930) findings regarding the relations between total benthic fauna and the volume of lakes.
Summary of distribution in different types of lakes. Data are as yet too incomplete to warrant any general conclusions regarding the distribution of bacteria in different types of lakes. It seems safe to state that bacteria are more abundant in eutrophic lakes than in oligotrophic ones. From the available data, it seems that eutrophic and oligotrophic lakes form a graded series as regards the numbers of bacteria; and that dystrophic lakes are a special case, probably differing more in kinds of bacteria than in numbers. Numbers of bacteria in the bottom deposits do not correlate well with numbers in the water; it is suggested that the ratio of bottom bacteria to water bacteria may eventually prove significant in classifying lakes.
ALSTERBERG G. 1927. Die Sauerstoffsehichtung der Seen. Botan. Notiser, Lund, 255.
BAIER C. 1935. Studien zur Hydrobakteriologie stehenden Binnengewässer. Arch. f. Hydrobiol., 29: 183.
BAVENDAMM W. 1924. Die farblosen und roten Schwefelbakterien. Heft, 2, Pflanzenforschung, Jena.
BEHNING A. 1928. Das Leben der Wolga. Die Binnengewässer, 5, Stuttgart.
BERE R. 1933. Numbers of Bacteria in Inland Lakes of Wisconsin as Shown by the Direct Microscopic Method. Int. Rev. ges. Hydrobiol. u. Hydrogr., 29: 248.
BIRGE E. and JUDAY C. 1922. Inland Lakes of Wisconsin. I. The Plankton, Its Quantity and Chemical Composition. Wis. Geol. and Nat. Hist. Surv., Bull. 64, Sci. Ser. No. 13.
BUCHNER H. 1893. Ueber den Einfluss des Lichtes auf Bakterien und über die Selbst- reinigung der Flüsse. Arch. f. Hydrobiol., 17: 179.