Fine benthic matter (FBOM) dynamics in low-order mountain streams: (1) methods evaluation and (2) the effects of stand age, season, and elevation on FBOM nutrient availability and microbiological characteristics

Allochthonous litter inputs are a primary source of organic matter in low-order forested streams. A major component of this litter moves through small streams as fine particulate organic matter (FPOM). Litter decomposition has been well studied, but few studies have examined benthic FPOM (FBOM) dynamics. The purpose of this study was to investigate, (1) how FBOM is controlled by headwater vegetation, elevation and seasons, (2) the links between organic matter inputs and FBOM substrate quality and (3) the relationships between FBOM substrate quality and microdecomposer activity. In preparation for this study, the stability of various microbial and chemical characteristics of FBOM during storage and analysis was determined. Denitrification potential (DNIT), phosphatase activity (PHOS), and extractable ammonium (EA) remained stable over aminimum of 11 hours of storage at 5°C. Mineralizable N (NMIN), respiration (RESP) rates, and B-glucosidase (BGLC) activity all decreased within 12 hours of collection. Results varied for nitrogen fixation (NFIX). In response to these results, our laboratory protocol was altered to accomodate analyses with varying levels of stability. Following this protocol, FBOM was sampled and analyzed over a 10 month period from 14 first order streams in the Cascade Mountains of western Oregon. Streams ran through forests in three successional age classes: old-growth forest (OG) dominated by Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla), and young regenerating stands (YS) either 10 years old, with a large riparian herbaceous component,or 30 years old, surrounded by deciduous trees such as red alder (Alnus rubra). Decreasesin C:N, CTOT, NTOT and organic P (PORG) were mirrored by reciprocal increases inFBOM RESP, BGLC, PHOS and NFIX, all relative indicators of microbial activity. Thelower C:N and higher DNIT, RESP, BGLC, PHOS, and NMIN observed in YS FBOM compared with OG suggests higher quality FBOM and faster decomposition rates in YSFBOM. Seasonal trends showed a major autumn deflection in FBOM C:N ratios and microbial activities, a likely result of increased leaf inputs following an early fall storm. Significantly lower C:N at high elevation (1220-1280 m) suggested the presence of more herbaceous vegetation and alder in high elevation riparian zones. Lower NTOT and CTOT and elevated DNIT, NFIX, RESP, PHOS at low elevation (580-800 m) suggested greater decomposition rates at low elevations. PORG was 2.2 and 3.6 mg P gOM-1 at high and low elevations respectively, a difference (p