ENZYMATIC MODELS FOR ESTIMATING DECOMPOSITION RATES OF PARTICULATE DETRITUS

Particulate organic matter (POM) is a major vector for carbon and nutr ients in stream ecosystems. The decomposition process by which this ma terial enters the food web has been difficult to study directly becaus e in-situ degradation rates for POM are difficult to obtain, particula rly for fine particles. We evaluated a new statistical method that est imates POM mass loss rates from indices of microbial enzyme activity. Litter bags containing benthic POM in three size ranges-Fine (F) 0.063 -0.25, Medium (M) 0.25-1 and Coarse (C) 1-4 mm - were placed in a eutr ophic woodland stream in June 1992. Over a 6-mo period, samples were c ollected and analyzed for mass loss and the activities of seven extrac ellular enzymes involved in the degradation of holocellulose, chitin, and polyphenolic compounds. In-situ POM samples were collected concurr ently and assayed for the same suite of enzyme activities. In all case s, mass loss was related to cumulative enzyme activity; however, appar ent enzymatic degradation efficiencies were generally lower for MPOM a nd FPOM (=MFPOM) than for CPOM by factors of 1.5-7. Separate regressio n models, relating mass loss to average cumulative enzyme activity, we re developed for CPOM and MFPOM and used to estimate in-situ decomposi tion rates. Mean model-estimated turnover times for in-situ CPOM, MPOM , and FPOM were 127, 145, and 185 d, respectively, compared with value s of 143, 312, and 244 d determined for size-sorted POM confined in li tter bags. The application of enzymatic decomposition models may circu mvent some of the methodological problems that have limited the study of POM dynamics in lotic ecosystems.