Assemblage-level estimation of nontanypodine chironomid growth and production in a southern Illinois stream

Chironomid midges are abundant and important components of freshwater communities, but estimating their production is problematic because of taxonomic difficulties, short generation times, and overlapping cohorts. We measured growth rates of nontanypodine chironomid larvae from a 3 super(rd)-order stream in southern Illinois, USA, at different temperatures to estimate assemblage-level production using the instantaneous growth method. We collected Chironomini, Tanytarsini, and Orthocladiinae larvae, separated them into small (0-3.9 mm), medium (4-7.9 mm), and large (8-12 mm) size classes, and then reared larvae in a controlled environment for 3 to 7 d at temperatures ranging from 5 to 28 degree C. We measured lengths of individual larvae before and after each trial and estimated biomass using a length-mass regression. Instantaneous growth rates (g) across all size classes ranged from 0.008 mg mg super(-1) d super(-1) at 5 degree C to 0.24 mg mg super(-1) d super(-1) at 22 degree C. Instantaneous growth rates of small larvae increased linearly with temperature between 5 and 22 degree C (r super(2) = 0.99, p = 0.0003). Instantaneous growth rates of medium and large larvae also appeared to increase linearly with temperature (medium: 5-22 degree C, large: 5-25 degree C), although power functions explained a greater proportion of the variance than linear functions (medium: r super(2) = 0.96, p = 0.004, large: r super(2) = 0.77, p = 0.05). We applied these models to data from monthly samples from the same stream and obtained an annual habitat-weighted production estimate of 4.2 g ash-free dry mass m super(-2) y super(-1) and an annual production to biomass ratio of 26. Application of 2 other assemblage-level chironomid growth models developed for headwater streams in the southern Appalachians yielded much different production estimates, illustrating the risks of applying these types of models to systems with different temperature regimes, taxonomic composition, or size spectra. Our temperature-specific models will enable more accurate estimates of chironomid production in central USA streams with thermal regimes and larval midge assemblages similar to those in Big Creek.