The recognition of secondary production as a means of quantifying the influence of stream-dwelling invertebrates on ecosystem

Methods and growth chambers are described which permit in situ estimates of the growth rates of chironomid larvae inhabiting litter accumulations in lotic habitats. Instantaneous growth rates (IGRs) for larvae of different sizes ranged from 0.01 to 0.24 mg mg-l ash-free dry mass (AFDM) d-l at stream temperatures of 2.9”15.1”C. IGRs were significantly and linearly related to temperature. Regression equations relating IGRs and temperature, combined with fieldderived data for chironomid standing stock and stream temperature, enabled calculation of the production of chironomids inhabiting litter accumulations (15-g DM litter bags) in a temperate mountain stream. The annual production per litter bag was about 224 mg AFDM yr-1 and the annual P:B was 42, indicating rapid turnover of chironomid biomass. The annual production of chironomids exceeded the mean standing crop biomass of all macroinvertebrates by 4.6 x . The recognition of secondary production as a means of quantifying the influence of stream-dwelling invertebrates on ecosystem processes (Benke and Wallace 1980; Parker and Voshell 1983; Fisher and Gray 1983) has resulted in the compilation of a considerable volume of literature (Benke 1984). Most secondary production estimates have been restricted to relatively large organisms with semi-, bi-, or univoltine life cycles which are amenable to the size-frequency and various cohort methods of measurement (Waters 1977; Benke 1984). However, benthic invertebrates with low standing stock biomass, short generation times, and overlapping cohorts (e.g. Chironomidae) may have high annual P : B ratios (P : B = production : 2 standing stock biomass) and may contribute substantially to total community secondary production (Mackey 1977a; Fisher and Gray 1983; Benke et al. 1984). Such organisms are often overlooked I This work was supported in part by grants from the National Science Foundation, the Georgia Power Company, and the Southeastern Forest Experiment Station, U.S. Forest Service. or ignored in production studies due to difficulties in recognizing specific cohorts or estimating the cohort production interval (CPI of Benke 1979). Shortcut methods, such as the estimation of secondary production as the product of standing stock biomass and some P : B ratio (often the empirically derived 3.5 to 5: Waters 1977; Benke 1984) or the uncorrected size-frequency method (Benke 1979; Benke et al. 1984), have been applied to entire benthic invertebrate communities and almost certainly have resulted in significant underestimation (e.g. Fisher and Likens 1973; Fisher 1977; Webster 1983). In these studies, quantification of the influence of macroinvertebrates on the processing of organic matter was attempted. However, the low production estimates utilized may have influenced the final conclusions, thus contributing to our uncertainty of the role of these organisms. Production was probably most underestimated among those groups with short CPIs, low standing stocks, and overlapping cohorts. Among chironomids inhabiting temperate mountain streams, P: B values exceeding 30 or 40 may be general (Waters 1979; Benke 1984). Clearly, methodologies that are not extremely labor-intensive yet allow realistic estimates of the production of this group are needed. We describe a simple field-based method for obtaining community level estimates of chironomid growth rates. These data, combined with temperature and standing stock, will allow a reasonable estimate of secondary production to be calculated by the instantaneous growth method (Waters 1977; Benke 1984). Our method is based on the assumption that changes in weight of groups of chironomid larvae of mixed taxa can be used to estimate an average growth rate reflective of an entire community. The influences of fast growing taxa should generally be balanced by slow growing taxa.