Macrobenthos 321 



in any season results from growth by cohorts which are in the fourth 

 instar. Consequently, variation in the strength of these cohorts in different 

 years explains most of the variance in annual production. The cohort 

 recruited in 1970 averaged 1475 fourth instar larvae m~^ in 1975, and 

 contributed 61% of the total population production in that year. Similarly, 

 the 1972 cohort was in the fourth instar in 1977, and its 1261 individuals 

 m~^ produced 66% of the total. The corresponding cohort in 1976 

 (recruited in 1971) had a mean density of only 542 individuals m"^ and 

 produced only 52% of the total. Production by cohorts in earlier instars is 

 almost always lower due to lower individual growth (Figure 7-6) regardless 

 of cohort size; hence the total 1976 C pilicornis production was reduced 

 relative to the other 2 years. 



In reviewing single species estimates of annual production for 

 chironomid larvae. Waters (1977) reports values ranging from as low as 

 0.024 g dry wt m'^ to as high as 161.6 g m^ (for larvae in sewage 

 treatment lagoons). Most values were in the range 0.5 to 3.5 g m ^ The 

 2.3 g m ~ yr~ ^ estimated for Chironomus pilicornis in Pond J is certainly 

 within the range of these values from temperate habitats. Although other 

 species in the pond were not studied in sufficient detail to determine 

 production, this species dominates the biomass of the pond centers due to 

 its large maximum size. Other large species (mainly Tanypodinae) may 

 contribute substantially, but sizes and densities suggest the total is less 

 than double the production of Chironomus. 



Controls over the production of this one species are probably multiple 

 and interactive. The most striking aspect of its biology in this arctic 

 environment is the seven years required for growth to maturity, but 

 production is not limited by the long life cycle. Arctic winters essentially 

 separate the usual two-year life span of cold water Chironomus (e.g., 

 Jonasson 1972) into seven disjunct phases, each with its own cohort. The 

 three cohorts of C. pilicornis which are in the fourth instar at any one time 

 constitute most of the standing stock and contribute most to production. 



Despite buffering factors within a species, life cycle production for 

 different cohorts will depend primarily on their abundances. For the midge 

 community as a whole, differences in species composition can be 

 important. Factors influencing cohort sizes of large species through both 

 recruitment and mortality will have the greatest effect on year-to-year and 

 pond-to-pond variations in production. 



Therefore, major control over production may occur during 

 emergence and reproduction. As weak fliers and synchronous emergers, 

 arctic chironomids are vulnerable to the high winds and cool temperatures 

 which the adults may experience during their short life in the terrestrial 

 environment. Larger species such as Chironomus and Procladius may 

 experience reproductive failure if swarming is inhibited. Smaller species 

 with surface mating habits may be less vulnerable, but also contribute less 

 to benthic faunal production. Welch (1976) has suggested that weather 



