Cohort structure, biomass, and production of a merovoltine Chironomus population in a Wisconsin bog lake
A three-year life cycle was determined for a population of Chironomus tenuistylus, the predominant benthic macroinvertebrate in a small, dystrophic lake in northern Wisconsin, by analyzing larval size and developmental stage distributions on eight dates over a two-year period. The coexistence of three discrete cohorts could be recognized clearly only during the short September emergence period. At other seasons, we could not separate two cohorts within the final instar due to overlapping larval sizes and similar developmental states. Frequent hypoxic or anoxic conditions at the bottom of this sheltered, poorly mixed bog lake apparently limited Chironomus growth and development to brief intervals, prolonging the life cycle and leading to a multi-cohort population structure. Most growth, development, and emergence occurred in the fall when peak annual temperatures coincided with highest levels of dissolved oxygen. Biomass showed relatively low temporal variation (1.9-7.2 g/m$^\textrm2$) over the two years, and annual production was estimated at 4-5 g/m$^\textrm2$ by cohort and P/B̄ methods. Failure to recognize the long cohort production interval and three-cohort structure of this population could result in substantial errors if production were estimated with non-cohort methods.