Cohort structure and voltinism in two profundal Chironomus populations
Chironomus larvae are the largest and most abundant profundal invertebrates in many mesotrophic to eutrophic lakes, where they may dominate benthic biomass and production (JONASSON 1972, 1979, TUDORANCEA et al. 1979, UUTALA 1981, BEATTIE 1982, GRAHAM \& BURNS 1983). Regardless of the method used, estimating secondary production requires knowledge of voltinism (WATERS 1979) the cohort production interval (CPI) (BENKE 1984), as life cycle duration: and recruitment frequency have major influences on the rate of production. Such information is easily obtained when discrete cohorts can be identified, but many chironomid populations lack the synchrony of development necessary to resolve cohorts and track them through time. Most profunqal Chironomus are reported to have univoltine life histories (TuDORANCEA et al. 1979, UUTALA 1981, BEATTIE 1982, GRAHAM \& BURNs 1983), but several cases of semivoltine development have been documented (JONASSON 1972, LINDEGAARD \& JONASSON 1979, CARTER 1980). BUTLER (1987) recognized a three-year life cycle for Chironomus cucini in the profundal of Crystal Lake, Wisconsin by identifying two immature cohorts in samples collected immediately following the annual emergence. In this paper we present a detailed cohort analysis documenting a three-year developmental period for this species in two northern Wisconsin lakes, through resolution of a three-cohort population structure in three consecutive years. We discuss problems associated with detection of this merovoltine life history, and consider the limnological conditions that cause such slow development yet permit establishment of a multicohort population.