| Abstract |
- 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 (DNJT),
phosphatase activity (PHOS), and extractable ammonium (EA) remained stable over a
minimum of 11 hours of storage at 5°C. Mineralizable N (NMIN), respiration (RESP)
rates, and [beta]-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). Decreases
in C:N, CTOT, NTOT and organic P (PORG) were mirrored by reciprocal increases in
FBOM RESP, BGLC, PHOS and NFIX, all relative indicators of microbial activity. The
lower C:N and higher DNIT, RESP, BGLC, PHOS, and NMJN observed in YS FBOM
compared with OG suggests higher quality FBOM and faster decomposition rates in YS
FBOM. 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 gOM1 at
high and low elevations respectively, a difference (p<=O.O5) probably due to the young
geologic age of parent material at high elevation. The data from this study suggest a
potential link between headwater forest management and stream food web dynamics as
mediated by shifts in FBOM.
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