320 M. Butler etal. 



Mortality was estimated from the decrease in density of species-size 

 classes (assumed to be cohorts) corrected for emergence losses. This 

 density change was converted to biomass by a length-weight relationship. 



In spite of some serious problems with Bierle's method of calculation 

 (see below), these estimates probably reflect the actual relative secondary 

 benthic production to be expected in these ponds. Production in Pond J 

 exceeded that in Ponds D and B by factors greater than 3 and 4, 

 respectively. The relatively large number and size of emerging adults and 

 large weight gain of larvae during the summer contributed the most to the 

 high value estimated for Pond J. Such differences may be primarily 

 determined by between-pond variation in populations of large species. 

 Emergence trapping and qualitative larval collections in a number of 

 ponds in 1975-1977 support this explanation, as some ponds contain good 

 populations of larger species like Chironomus while others are dominated 

 by smaller species. 



Production estimates such as these are subject to a number of errors. 

 When a population takes several years to mature, as is true with all of 

 these chironomids, it is not correct to attribute the total biomass emerging 

 during a season to annual production for that year, for some of this 

 biomass was produced during previous years. Some fraction of the 

 biomass increment over a season will emerge in a future year and would 

 thus be counted twice. Production lost to mortality before emergence can 

 only be calculated when cohorts can be consistently recognized, clearly a 

 difficult problem when up to seven cohorts of one species can coexist. 

 Finally, Bierle's estimates of adult weights appears to have been too high, 

 judging from later measurements in the same ponds. 



Data collected in 1975-1977 on the population dynamics of 

 Chironomus pilicornis larvae in the center of Pond J (data will be 

 presented in M. Butler's Ph.D. thesis, University of Michigan ) were used 

 to calculate production of this species by the increment summation 

 method. Here, the mean number of larvae in a given cohort during one 

 summer is multiplied by the change in weight of an average individual over 

 that summer. These cohort production values are summed for all 

 coexisting cohorts (seven in this case) to provide a value for annual 

 production by the entire species population. If only development, and not 

 actual feeding and growth, occurs during the final summer (see Life Cycles 

 above), emerging adult biomass during a season includes no actual 

 production from that year. 



Annual production estimates of 2.77, 1.42, and 1.80 g dry wt m~^ 

 were determined for Chironomus pilicornis in Pond J during 1975, 1976, 

 and 1977 respectively, based on samples from two dates each year. Since 

 sampling intervals were different in different years, these values have been 

 adjusted to a standard growing season of 92 days (approximately June 15 

 through September 15). The mean value is 2.00 g dry wt m "^ (about 1.0 g 

 C m~^ yr~'), with a range ±38% over the 3 years. Most of the production 



