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 14 1st-order streams in the Cascade Mountains of western Oregon to investigate 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. Streams drained forests in 3 successional age classes: old-growth forest dominated by Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla), and young regenerating stands, either 10 y old with a large riparian 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, beta-glucosidase and phosphatase activities, and acetylene reduction, all of which are relative indicators of microbial activity. Lower C:N and higher denitrification potentials, respiration rates, beta-glucosidase and phosphatase activities, and mineralizable N were observed in young stands compared to old growth, suggesting higher quality FBOM and faster decomposition rates in young stands. An exception to this trend was acetylene reduction, which was greater in FBOM from old-growth streams. Significantly lower C:N at high elevations (1220-1280 m) versus low elevations (580-800 m) suggested the presence of more herbaceous vegetation and alder in high-elevation riparian zones. Lower total N and total C, and elevated denitrification potentials, acetylene reduction, respiration rates, and phosphatase activity at low elevations (580-800 m) suggested greater decomposition rates at low elevations. Organic P was 3.6 and 2.2 mg P/g organic matter at high and low elevations, respectively, a significant difference probably resulting from the young geologic age of parent material at high elevations. Data from this study suggest a potential link, mediated by shifts in FBOM, between headwater forest management and dynamics of stream food webs.