322 M. Butler etal. 



conditions during emergence may have a major effect on the size of 

 chironomid populations in Char Lake. 



Once a cohort has been recruited, the pattern and intensity of 

 mortality over its life span will influence production. Mortality appears to 

 be high among small early instars of C. pilicornis, with a much lower death 

 rate through the fourth instar. Individual cohorts need to be followed 

 through most of their life cycle to confirm this apparent pattern, and to 

 assess the variability of mortality in different ponds and years. The most 

 obvious source of mortality is predation by other chironomid larvae in the 

 family Tanypodinae, which includes four species in the genera Procladius 

 and Derotanypus in these ponds. 



Procladius is the most abundant predator in the community. Its 

 potential importance is illustrated by data from Lake Sniardwy (Kajak 

 and Dusoge 1970). In one experiment 534 Procladius m^ were present 

 with a prey population of 9,760 m "^ In 5 to 9 days, Procladius consumed 

 530 mg of chironomids and 330 mg of benthic crustaceans or about 95 to 

 170 mg day \ Since prey were small, we estimate a weight of 0.3 mg wet 

 weight per individual or 300 to 570 prey day ' for this density of 

 Procladius. Procladius population densities in Pond J in 1972 were similar 

 to those in Lake Sniardwy. If Procladius populations were unusually 

 abundant in a pond or year, their predation on early instars of Chironomus 

 might limit cohort sizes and, consequently, benthic secondary production, 

 for several years. 



Ultimately, the general magnitude of chironomid production may be 

 set by food availability resulting from primary production in the system. 

 In fact, Welch (1973) has proposed that emerging insect biomass is a 

 constant proportion of total primary production in lakes. The biomass 

 emerging from a square meter of Pond J in 1976 averaged about 0.4 g and 

 from Pond G about 0.2 g. Primary production in these ponds, including 

 macrophytes, phytoplankton, and benthic algae, was about 50 g C m"^ 

 yx'\ or about 100 g of dry matter. The resulting ratio of emerging 

 chironomid biomass to primary production was therefore about 0.2 to 

 0.4%, which agrees with the values cited by Welch from other systems. 



The causality of this relationship has not been defined, and there are 

 several variables which preclude its acceptance as anything more than a 

 very general correlation. Emergence of C. pilicornis adults from Pond J in 

 1976 constituted only about 30% of the prepupal cohort predicted to 

 emerge on the basis of larval sampling. This suggests high pupal mortality. 

 The percentage of larvae emerging successfully is unlikely to be constant 

 for different species or in different years. Hence emerging biomass may 

 correlate poorly with secondary production. In addition, since most of the 

 chironomids are detritivores, their food source is somewhat removed from 

 the actual process of primary production. The variations in annual 

 secondary production which could be introduced by differential species 

 composition, by weather influences on recruitment, and by different rates 



