Nutrient and microbiological characteristics of fine benthic organic matter in mountain streams

Fine benthic organic matter (FBOM) was collected over a 10-mo period from 1 4 1st-order streams in the Cascade Mountains of western Oregon to investiga te 1) relationships between FBOM substrate quality and microbial activity, 2) links between organic matter sources and FBOM substrate quality, and 3) how FBOM is influenced by riparian vegetation, elevation, and season. Strea ms drained forests in 3 successional age classes: old-growth forest dominat ed by Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga hetero phylla), and young regenerating stands, either 10 y old with a large ripari an herbaceous component, or 30 y old and surrounded by deciduous trees such as red alder (Alnus rubra). Seasonal trends showed a major autumn depression in carbon:nitrogen ratios (C:N) and an increase in microbial activities, a likely result of increased leaf inputs after an early fall storm. Decreases in C:N, total C, total N, and organic P were correlated with reciprocal increases in respiration, be ta-glucosidase and phosphatase activities, and acetylene reduction, all of which are relative indicators of microbial activity. Lower C:N and higher d enitrification potentials, respiration rates, beta-glucosidase and phosphat ase activities, and mineralizable N were observed in young stands compared to old growth, suggesting higher quality FBOM and faster decomposition rate s in young stands. An exception to this trend was acetylene reduction, whic h was greater in FBOM from old-growth streams. Significantly lower C:N at h igh elevations (1220-1280 m) versus low elevations (580-800 m) suggested th e presence of more herbaceous vegetation and alder in high-elevation ripari an zones. Lower total N and total C, and elevated denitrification potential s, acetylene reduction, respiration rates, and phosphatase activity at low elevations (580-800 m) suggested greater decomposition rates at low elevati ons. Organic P was 3.6 and 2.2 mg P/g organic matter at high and low elevat ions, respectively, a significant difference probably resulting from the yo ung geologic age of parent material at high elevations. Data from this stud y suggest a potential link, mediated by shifts in FBOM, between headwater f orest management and dynamics of stream food webs.