THE DISTRIBUTION OF BACTERIA IN LAKES 



45 



sinkin (1934) for quantitative studies of 

 the distribution of lalve bacteria. Tlie 

 method consists in submerging microscope 

 slides in the lake for a time. Bacteria 

 attach themselves to the glass and grow 

 there. The slides are removed, dried, fixed, 

 stained, and examined under the micro- 

 scope. The bacteria may be readily counted, 

 and morphologically distinctive species may 

 be determined separately. 



Such a method has great advantages over 

 the others. It takes into account only those 

 bacteria which are actually growing in the 

 w^ater, ignoring those that are dead or 

 which have merely been washed into the 

 lake from the surrounding land. It brings 

 into the picture the large numbers of mor- 

 phologically peculiar water bacteria which 

 have not been so far cultivated in the 

 laboratory. It requires no elaborate appa- 

 ratus, and is therefore well adapted to field 

 use. 



I have described elsewhere (Henrici 

 1936) the use of this method as a quantita- 

 tive procedure and have discussed its accur- 

 acy. Briefly, it has been found that fairly 

 reliable results may be obtained if counts 

 are made from two pairs of 50 x 75 mm 

 slides suspended at each station, one pair 

 to be removed after a short period of im- 

 mersion, the second after a longer period. 

 The duration of exposure must be roughly 

 adjusted to the productivity of the station, 

 which may be readily determined after a 

 little experience. The bacteria are counted 

 in each of 50 fields on each of the four 

 slides, the counts for each slide are con- 

 verted to numbers of bacteria per square 

 millimeter per day of immersion, and the 

 results from the four slides are averaged. 



The microscopic counts of bacteria grow- 

 ing on submerged slides is limited in accur- 

 acy due to the highly varied distribution of 

 the bacteria on the slides. Irregularity of 

 distribution is greater with low counts. 

 With very high counts errors arise due to 

 the difficulty in distinguishing all of the 

 cells; such counts are likely to be too low 

 because cells have been overlooked. Sepa- 

 rate counts which show small difterences 

 should be scrutinized from a statistical 

 standpoint before conclusions are drawn. 



Where a number of observations from tlie 

 same habitat are averaged, the results of 

 course are more accurate in proportion to 

 the number of observations. In the data to 

 be presented in the following pages, stand- 

 ard deviations are not presented though 

 they have been calculated and are on file. 

 In general it may be stated that a mere 

 doubling of numbers in individual observa- 

 tions is not likely to be significant, that a 

 five-fold difference is almost certainly sig- 

 nificant. It is my impression that counts 

 of bacteria from submerged slides have a 

 degree of useful accuracy greater than is 

 indicated by statistical analyses based upon 

 the distribution of the bacteria on the in- 

 dividual slides. Thus the curve shown in 

 Fig. 3 is regular and agrees well with the 

 otlier curves, even though in some instances 

 the differences between two successive 

 observations are not statisticallj" significant. 



The submerged slide method has a dis- 

 advantage in that it collects only those 

 bacteria which can grow attached to sub- 

 merged surfaces. There is, however, grow- 

 ing evidence that this is a characteristic of 

 a large portion of the water bacteria. I 

 have found that in a broad way there is a 

 good correlation between the results from 

 submerged slides and from plate counts, 

 although in individual observations such a 

 correlation may be lacking. If large num- 

 bers of observations at various stations are 

 studied, there is found a fairly close cor- 

 relation between the two sets of counts ; for 

 100 pairs of counts the coefficient of corre- 

 lation was found to be 0.625. 



ZoBell and Allen (1933) found that 24 

 of 73 strains of marine bacteria isolated in 

 pure cultures were capable of attaching 

 themselves to and growing upon submerged 

 slides in the laboratory. Hotchkiss and 

 Waksman (1936) noted a correlation be- 

 tween counts of bacteria on submerged 

 slides, and plate counts, in laboratory ex- 

 periments with sea water; constants could 

 be determined from experimental data 

 which permitted the calculation of the 

 numbers to be expected from the slide 

 counts, if the plate counts are known. Kus- 

 netzow (1937) submerged slides for a 

 short time and then made cultures of the 



