54 IOWA STUDIES IN NATURAL HISTORY 



only a very fragmentary idea of the bottom fauna since 

 they are limited to two depths and since they were made at 

 a time when the insect larvae are at their minimum in va- 

 riety and number. A proper study of the bottom popula- 

 tion would require hundreds of observations; these should 

 cover all depths, from the shoreline to the deepest water in 

 various parts of the lake, as well as all seasons of the year. 

 Such a study would involve the expenditure of much time 

 and labor but the results would undoubtedly amply justify 

 the undertaking. 



Only three groups of bottom dwellers were found in any 

 abundance. The Chironomus larva apparently belongs to 

 the tentans group and all of the specimens were either full 

 grown or nearly so, varying in length from 21 mm. to 25 

 mm. Protenthes was not noted at 21 m. but a few were 

 found at 33 m. ; these specimens were substantially full 

 grown larvae also. The Oligochaeta included representa- 

 tives of Limnodrilus and Tubifex. In addition to these some 

 ostracods and nematodes were noted at both depths. 



The chironomids were more abundant at 21 m. than at 

 33 m. while the reverse was true of the Oligochaeta. 



The weight of the forms shown in table 27 were deter- 

 mined and the amount of organic matter per square meter 

 of bottom is indicated in the table in grams. The Oligo- 

 chaeta varied greatly in size so that it was necessary to 

 pick out and weigh all of the specimens in a single haul of 

 the dredge in order to get an average. While the total 

 number of individuals was much larger at 33 m. than at 

 21 m., the reverse was true on the basis of the amount of 

 dry organic material * the larger number of Oligochaeta at 

 the former depth did not compensate for the larger bulk of 

 the chironomids at the latter depth. 



Stated in larger units this bottom fauna yielded 109.6 kg. 

 of dry organic matter per hectare, or 97.6 pounds per acre, 

 at 21 m., and 65.1 kg. per hectare, or 58.0 pounds per acre, 

 at 33 m. The latter figure is substantially the same as the 

 average yield of the total plankton. .(See p. 51). These 

 figures, like those for the total plankton, do not represent 

 the annual production, but they indicate the number of 



