LIFE HISTORIES AND GROWTH OF LOTIC CHIRONOMIDS (DIPTERA, CHIRONOMIDAE)

Biweekly benthic samples were collected for 1 yr in a third-order midwestern U.S. stream to determine chironomid life histories and growth patterns for the 14 numericaIly dominant taxa. Benthic densities during the year ranged from 7,500/m2 in October to 90,000/m2 in May. Seasonally, densities were highest in spring (49,600/m2) and lowest in summer (15,100/m2). Most Orthocladiinae and Diamesinae had multivoltine life cycles, whereas all Chironominae bad either univoltine or asynchronous patterns. Low summer water temperatures, due to groundwater input, may explain how cold-adapted Diamesinae could complete more than one generation per year. LarvaI growth patterns (mean individual biomass versus time) of all 25 cohorts could be classified into four types of growth curves. Overall, biomass accumulation of third and fourth instars accounted for 95-99% of the maximal larval biomass. Nine cohorts had maximal growth rates during spring and autumn, whereas six cohorts had maximal growth during winter. Despite low water temperatures, the increased availability of high-quality autochthonous food resources during winter and spring may account for high growth rates during these two seasons. Combining growth and development data for all cohorts revealed an annual pattern of continuous species replacements and sequential periods of maximal growth. The recognition of these patterns is valuable in understanding the temporal dynamics of this diverse assemblage of stream insects.