531 



data from the silt-laden planktons of Spoon River. The leak- 

 age affects primarily the smaller flagellates, ciliates, and dia- 

 toms. For reasons previously given, the leakage introduces 

 proportionately a greater source of error in the Spoon River 

 data, and, in so far as it is operative, tends to increase the con- 

 trast between the two streams. From data now at hand it is 

 not possible to estimate the extent of this error, but it should 

 be borne in mind in considering the data in the table concern- 

 ing these groups especially. In spite, however, of these sources 

 of error, some points of contrast are so striking that certain 

 conclusions seem justified. 



There is a marked contrast in the total population. The 

 ratio of the average number of planktonts in a cubic meter 

 of Spoon River water to that in a like quantity from the main 

 stream is 750,429 to 28,283,295, or 1 to 38 — a ratio much greater 

 than that found in the volumetric comparisons. It is due to 

 the relative preponderance of small planktonts, especially the 

 limnetic diatoms, in the collections from the main stream, as 

 will be seen on comparison of the ratios of the other groups 

 listed in the table. If the low-water period of 1897 be excluded 

 from both averages the contrast between the two streams will 

 be very much heightened. The data from these grand averages 

 of all the coincident collections in the two streams are thus 

 strongly confirmatory of the contrast in productiveness dis- 

 closed by the volumetric comparisons, and of the fact that 

 the access of water from Spoon River causes a diminution in 

 the number of organisms per cubic meter of water in the 

 main stream. 



The comparison of the representations of the various 

 groups in the collections tabulated, throws some light on the 

 qualitative differences in the plankton of the two streams. 



The algae, other than diatoms, as in the case of all groups 

 except the insect larvae, are present in smaller numbers in 

 Spoon River, the total ratio being 1 to 1.4 — abnormally low be- 

 cause of a single unusual collection in the autumn of 1897- 

 The ratio in 1898 and the latter part of 1896, 1 to 10, is proba- 



